Title of Invention	"PORTABLE LASER PANEL FOR PROJECTILE SPEED MEASUREMENT SUITABLE FOR INDOOR AND OUTDOOR RANGES"
Abstract	Among different know techniques of projectile velocity measurement, systems based on optical screens are most accurate and versatile. All prior arts use single or multiple sources to construct a fixed predetermined screen size. In case of large screen systems, the precision of a measurement depends upon the zone through which a projectile has passed. All prior arts are not equally effective under indoor and outdoor ambient lighting conditions. The present invention has proposed a unique design concept and construction of an apparatus to solve the above problems. Application of a pair of Laser diodes, beam expander collimator, a set of 45°-90°-45° optical prisms, suitably located photodetector and signal processing circuit permit modular construction, uniform intensity screens insensitive to the level and fluctuations of ambient light and provision for alteration of screen size and inter-screen distance. The method and the apparatus find wide applications in forensic studies, projectiles manufacturing, toy manufacturers, golf training centre, physiological studies, birds movement studies and other areas where characterising a projectile in terms of its velocity is required.

Title of Invention

"PORTABLE LASER PANEL FOR PROJECTILE SPEED MEASUREMENT SUITABLE FOR INDOOR AND OUTDOOR RANGES"

Abstract

Among different know techniques of projectile velocity measurement, systems based on optical screens are most accurate and versatile. All prior arts use single or multiple sources to construct a fixed predetermined screen size. In case of large screen systems, the precision of a measurement depends upon the zone through which a projectile has passed. All prior arts are not equally effective under indoor and outdoor ambient lighting conditions. The present invention has proposed a unique design concept and construction of an apparatus to solve the above problems. Application of a pair of Laser diodes, beam expander collimator, a set of 45°-90°-45° optical prisms, suitably located photodetector and signal processing circuit permit modular construction, uniform intensity screens insensitive to the level and fluctuations of ambient light and provision for alteration of screen size and inter-screen distance. The method and the apparatus find wide applications in forensic studies, projectiles manufacturing, toy manufacturers, golf training centre, physiological studies, birds movement studies and other areas where characterising a projectile in terms of its velocity is required.

Full Text	Laser Based Apparatus for Projectile Velocity Measurement Field of Invention This invention relates to the development of an improved computer controlled system for opto-electronically measuring of the time of flight of any small caliber projectile between two optical screens separated by a known distance and computing its velocity. The system involves a pair of optical screens generated by laser sources, beam expander-collimators and prism assemblies using their retroreflecting property; focusing lenses; photodetectors and associated electronic circuitry; timer card; necessary hardware and software for computer interface; system operating and software. In the present invention retroreflecting prisms create large optical screen from an expanded collimated input laser beam which is focused by a lens on to a photodetector at the output end of the screen, The high speed electronic circuit generates two pulses to start and stop a clock controlled at the time entry and exit of the projectile through and from the entry and exit screens respectively. A computer controls entire system operation. The present invention has useful applications where the measurement of the velocity of a dynamic object is required e.g. forensic studies, police training centers, projectile characterization by the manufacturers and users, golf ball velocity, physiological studies like checking the throwing power of a human arm, study of birds' movements, and similar areas. Background an Prior Art to the Invention The method to determine the velocity of a projectile moving at a high velocity is based on (i) the measurement of its Time of Flight (ToF) over a known distance; (ii) measurement induced of electromotive force generated due to the passage of a ferromagnetic projectile through a magnetic field; and (iii) Doppler frequency shift caused by the moving projectile. A fast moving projectile while passing through a steady field, serving as a screen, perturbs the field energy distribution. The time of occurrence of the perturbation is measured by a suitable sensor clock combination. When the projectile passes through two such screens in sequence, the time interval between the two perturbations gives the ToF between the two screens. The known screen distance divided by ToF is the required velocity of the projectile. All or arts to measure projectile velocity are based on this basic principle. These vary in terms of the type and construction (including shape and size) of the screen and sensing methods, the apparatus geometry, the physical properties used to make the screen and matching sensor. The physical quantities employed to make the screens whose perturbation time is measured can be broadly classified into the following categories: • Pressure/Acoustics • Magnetic • Electrical • Doppler Frequency Shift • Radiation Field Analysis of Prior Art The following is list of patent numbers closest to the present invention: US 6020594, US 3215932, US 4239962, US 4845690, US 4574238, JP 60024453, EP 0126423 (A) & (B), US 4272189, US 4155647, US 557733, US 3487226, US 64414747, US 3918061, US 3918061, US 5806848, DE 2610708, US 4649528, US 6388438, DE 3404953, FR 2567651 Pressure/Acoustics Patent Nos. US 4649528 and DE 4106040 fall under this category. Herein two air pressure sensors are placed at a known distance along the direction of travel of the projectile. The pressure wave traveling with a high velocity projectile triggers the sensors sequentially when the projectile is in their vicinity. Each pressure sensor generates an electrical pulse. A timer circuit measures the time interval between two such pulses generated by the two sensors. Drawbacks of pressure/acoustic wave based speed/velocity measurement are: • The traveling air/acoustics pressure wave depends upon the projectile mass, geometry and its velocity. The location of the projectile to impart sufficient pressure needed to actuate the sensor is likely to be different for various projectiles, introducing large and unknown variations in the flight path for which ToF is measured leading to unreliable apparatus readings. • Small projectiles of low mass and low velocity may not trigger sensors. • In case of a projectile traveling at subsonic speed, pressure/acoustics wave created by the detonation of the explosives during propulsion i.e. at the time firing, may actuate the sensors before the waves associated with the projectile does so. • External disturbances may also lead to erroneous results. Magnetic Patent Nos. US 3215932, US 4342961, EP 0035802 (A) & (B), GB 2200215 and CH 693248 fall under this category. Such types create a magnetic or pulsating electromagnetic field by employing magnets/electromagnets or current carrying coils. Projectile made of magnetic or electrically conducting material changes the field distribution, which is sensed. The common drawbacks of the above apparatuses are: • The principle does not work for electrically nonconducting projectiles like plastic or ceramic projectiles. • Magnetic field screen based systems do not sense a projectile made of nonmagnetic material. • The apparatuses are to be mounted on the projectile firing mechanism, eg. a gun. These can not be used to measure velocity of projectile propelled from a distance. • Critical axis alignment is essential every time the projectile propelling system is to be changed. • These have limited accuracy. Electrical The screen of an apparatus under this category is made by a sheet or thin plate of an insulating material sandwiched between two sheets of a conducting material like metal foil and a potential difference is applied between the two conducting surfaces. But no current flows due to the presence of the insulating layer in between. When a projectile pierces this sandwich arrangement short circuit takes place and a shorting pulse is generated. ToF is the time interval of i.v shorting pulses. The drawbacks of such systems are: • Every time a projectile passes i Dies are created in the screen after score usage screens are to be replaced. • Every time the screens are replaced it is to be ensured that the distance between the screens remains same throughout the screen area to avoid creeping in of the distance values. • Susceptible to electromagnetic interference/electromagnetic current (EMI/EMC) disturbances • The principle does not work for electrically nonconducting projectiles like plastic or ceramic bullets. Doppler Frequency Shift Patent Nos. US 3918061, US 4283989, DE 3404953 are based on Doppler Frequency Shift measurement. A modulated microwave or millimeter wave beam is directed towards the moving target, the reflected beam is frequency shifted by the amount depending upon the projectile speed. The drawbacks are: • Since only a fraction of incident beam is scattered by a projectile towards the sensor, for enhanced sensitivity, probe beam size is small, effectively reducing the active area. • Microwave or millimeter waves offer limited resolution, these apparatuses are suitable for large projectiles or moving objects. Radiation Field Screens based on infrared (IR) or optical radiation fields offer very high resolution and are highly sensitive while being immune to EMI/EMC. A number of patents describe optical screen using either visible or near IR wave lengths. The variations are in the type of source(s) used, construction of the screen and detection mechanism. The following are the references of the apparatuses based on optical screen. Patent Nos. US 3487226, US 4155647, US 4239962, US 4272189, US 4574238 and its family EP 01264 (A) & (B) and JP 60024453 (A), US 4845690, US 557733, US 6020594, US 64414747; Publications: "Shooting Chrony@ Brochure, Shooting Chrony Inc., N. Tonawanda, New York, Jan 1996, pagel; "Oehler Ballistics Chronographs", Brochure, M/s Oehler Research Inc., TX, Jan 1996, pages 1-7 & 28; "Infrared Light Screen LS 19i3/A2", catalogue, M/s Drello, Germany; "Projectile Measuring System Type 758", Catalogue, M/s MS Instruments PLC, UK. In all the above referred systems, either single or multiple sources are used emitting either visible or IR radiation. Some systems have used IR emitting Laser Diodes also. The light emitted from the source(s) is expanded by employing refracting type collimator(s)/beam expander(s) using cylindrical lenses or reflecting type collimator using a parabola (US 64414747). The expanded beam of light, visible or IR, forms the screen. In one family of patents (US 4574238, JP 60024453, EP 0126423 (A) & (B)), from a single collimated beam a number of beams are generated by using prism made of double refracting material or diffraction grating. The expanded beam is collected by single or multiple lenses or by parabolic reflector and focused on single or multiple detectors(s). In some cases sun light has also been used to make the light screen. A rectangular slit is used to define the cross sectional area of each screens. The common drawbacks of all existing apparatuses using visible or IR based optical screen are: • The systems employing sunlight source cannot be used in indoors. • The precision and accuracy of systems employing sun light source vary depending upon the intensity fluctuations of sunlight. • The intensity of light varies from point to point in the active area of the screen leading to non uniform sensitivity at various points. The precision and accuracy of the measurement depends upon the projectile path i.e. at what point it crosses both in and out screens. • The effective screen area for high precision measurement is small and as such apparatuses are to be placed near the projectile propellant system or else due to dispersion or deviation from linear path the projectile may not cross the effective screen area. • The screen areas of the systems using parabola for beam expansion are determined by the aperture of the parabola. Up scaling of the screen size requires replacement of expensive parabola. • The screen does have any regular geometry make it difficult to aim and direct the projectile through the most sensitive area of the screen. • In multi source systems, failure of any source during an experiment will not be noticeable, when a number of projectiles are fired in rapid succession. • Major changes are necessary to increase the screen size like changing the parabolas used for expanding and focusing, adding of more source elements and detectors which need additional power supplies. Such systems cannot be upscaled in user premises. • Performance of the systems is affected by the ambient light. • Can not measure the velocities of slow moving projectiles. The systems with small screen area necessarily to be kept very near to the origin of the projectile while systems with large screen area needs careful alignment during installation and can not moved from indoor to outdoor frequently. The present invention circumvents the drawbacks of the existing prior arts by providing a system to measure the velocity of a projectile moving at a high or slow speed incorporating a pair of optical screens, which can be easily up scaled or down scaled or inter-screen distance can be changed; each screen being constructed from a single source with same and uniform sensitivity throughout the screen area at the same time provides a system which does not require any expensive aspheric optical components like parabola, cylindrical lenses etc. doing away with the requirement of elaborate alignment during installation and can be 'picked and placed' for usage in indoor or outdoor or any other desired place. Objects of the Invention The main object of the invention is to provide an improved computer controlled apparatus to measure the velocity of various projectiles including those fired from small caliber guns without the drawbacks of the prior arts using laser diode, beam expander-collimator, reflecting prisms in plurality, collector lens, photodetector, signal processing electronics circuitry, computer hardware and software with interfaces for system operation, computation and display with built-in diagnostics. Another objective of the present invention is to provide an apparatus to measure velocity of projectiles, with two optical screens, each having single source and single photodetector with modular design such that by simply changing the columns housing a set of prism assemblies, the screen size can be changed. One more objective of the present invention is to provide a system to measure velocity of projectiles, which employ laser diode, beam-expander collimator and multiple reflecting prism assemblies to create a pair of screen in such a manner that the sensitivity, precision and accuracy of each screen to sense the crossing of a projectile do not depend upon the zone where the projectile interrupts. Still another objective of the present invention is to provide a system to measure the velocity of projectiles, in which light source, beam expander-collimator, multiple reflecting prisms for both the screens do not need any alignment during installation and as such can be picked and placed for usage at any location as frequently as the user desires. Yet another objective of the present invention is to provide a system to measure the velocity of projectiles, employing a thin rectangular expanded-collimated laser beam to improve sensitivity. Yet one more objective of the present invention is to provide a system to measure the velocity of projectiles, with a modular design that the distance between the columns and the distance between two screens are maintained by a set of spacer bars such that screen size and inter-screen distance can be increased or decreased by simply changing the spacer bars and moving the sliding carriers on the rails respectively. Still another objective of the present invention is provide a system to measure velocity of projectiles, wherein different light sensitive devices are so mounted that these are shielded from stray light and apparatus performance is not affected by ambient lighting conditions and can be used in indoor or outdoor environment without any need of any modification/alteration/improvement. Yet another objective of the present invention is to provide a system to measure velocity of projectiles, which employs standard laser diode sources, standard fast responding photodetectors, standard high slew rate amplifiers to measure the velocity of projectile. Still another objective of the present invention is provide a system which provides simple to operate user settings to offset the deterioration in performance of the laser diodes and photodetectors due to aging, the provision also helps to offset low frequency drifts. Still another objective of the present invention is to provide a system capable of measuring the velocities of slow moving projectiles as well. Still one more objective of the present invention is to provide a system with velocity measurement capability for a plurality of projectiles. Summary of Invention The present invention relates to optical screen time of flight (ToF) measurement based apparatus to measure of velocities of projectiles traveling at high speeds by measuring its time of flight between a pair of optical screens. In each screen, light from a laser diode, emitting visible or infrared radiation, is expanded and collimated by a beam-expander collimator. A set of similar 45°-90°-45° glass prisms, used in reflection mode, generates a large area screen by reflecting the beam back and forth at the same time shifting the beam laterally by an amount equal to the half the length of the hypotenuse of each prism, between the two columns housing the prisms. The separation between these two columns defines the screen width and the prism size together with the number of prisms defines the height of the screen. After the final passage the beam is focused as a small patch on the active area of a photodetector by a collector lens. The photodetector output is proportional to the incident energy. The output is amplified, conditioned and processed. In absence of any projectile, the electronic signal remains in steady state. A projectile during it journey partially or fully blocks the collimated beam and the photodetector signal level falls. The reduction in input energy on to the photodetector due to the obscuration produced by a projectile in the first screen is sensed and used to start an electronic clock. A second screen, similar to the first one, is employed to generate a pulse to stop the clock. Computer resident software divides the stored value of the distance between the two screens by the measured time of flight and displays the results or prints hard copies as desired. The entire system operation including initialization is computer controlled and can be implemented remotely. The lasers, photodetectors and electronics circuitry are power up by an electronic control box, which also houses signal conditioning and processing electronics. A hardware module interfaces electronic control box with a computer fitted with a timer card. Brief Description of the Accompanying Drawings In the present invention, a number of drawings are given to illustrate the apparatus in detailed. However, the invention is not limited to the precise arrangements and instrumentalities. Fig.1 Block Diagram of a velocity measurement apparatus, which shows the subsystems namely, opto-electronic subsystem comprising of a pair of laser screens; electronic subsystem; and a computer; Fig.2 A Schematics of the apparatus showing different modules/components of the subsystems. Laser- beam expander collimators, prism assemblies housed in mechanical columns, rails, sliding carriers, spacer bars, base plate are used to construct the two laser screens of the opto-electronic subsystem. ON/ OFF switch, power supplies indicators, LCD displays, potentiometers knobs, reset button, connection cables and computer interface etc. of the electronic subsystem and computer are also shown in the Fig.2; Fig.3 A prism assembly and components showing three dimensional view, the side view of a prism and two plates. The figure also shows the front and side view of the prism assembly; Fig.4 A laser screen of the apparatus showing the various modules/components namely laser-beam expander collimator, prism assemblies housed in mechanical columns, beams traveling back and forth between prism assemblies and collector lens-photodetector and cross-section view of a projectile; Fig.5 A Photodetector Modules of the apparatus showing a collector lens and photodetector in a barrel. The figure also shows a collimated beam focused by a lens on the active area of the photodetector and ambient light rays; Fig.6 An isometric view of mechanical columns and rail assembly which shows various mechanical components/modules like mechanical columns, spacer bars, rails, sliding carriers etc.; Fig.7 Apparatus with covers showing two independent covers, one for each laser screen assembly and other subsystems of the apparatus, Fig.8 A block diagram of electronic circuit of the apparatus showing the transfer of signals from the photodetectors to the computer through the various components shown in the form of block: preamplifiers, subtractors, potentiometers, buffers & drivers, opto-isolators, monoshots, flip-flop and a gating pulse etc.; and Fig.9 A electronic circuit diagram of the apparatus which shows all the components as shown in Fig.8 but with detailed circuitry. Accordingly, the present invention provides a method and also an apparatus to measure velocity of projectiles traveling at high or low speeds, the said method consisting of use of two optical screens constructed by using two laser diodes, two beam expander collimators, a set of prisms and a pair of collector lens-photodetector modules; and the said apparatus comprising of a pair laser screens, each screen constructed by incorporating a laser diode, a beam expander-collimator, a set prisms, a collector lens photodetector module, signal processing unit with opto-isolator, system power supply and interfaced computer installed with a firmware to operate the apparatus. In another embodiment of the present invention, the method incorporates a set of similar 45°-90°-45° prisms to make screens having uniform light energy distribution throughout the screen area so that performance of the apparatus is independent of location of the point where any projectile crosses the screens. In one more embodiment of the present invention, the method permits modular design and the apparatus permits on site up scaling or down scaling the screen size and permits on site changing of inter-screen distance. In one more embodiment of the present invention, different opto-electronics and optical components are so housed that only negligible amount of ambient light falls on the photodetector active area so that the present invention can be deployed outdoor or indoor without the need of any modification or calibration. In yet one more embodiment of the present invention, a suitable cover is provided to further minimize the interference of ambient or stray light. In still one more embodiment of the present invention, all mechanical components and the hypotenuse faces of the prisms are given suitable coating to attenuate stray reflections. In yet one more embodiment of the present invention, apparatus design allows standard components and devices to be used. In yet another embodiment of the present invention the design and construction make the apparatus portable. In still another embodiment of the present invention, the apparatus is capable of measuring velocities of slow moving projectiles also. In still one more embodiment of the present invention, the apparatus provides an array of LEDs and a pair of LCD displays to online monitor the health and tuning of different modules. In still one more embodiment of the present invention, the apparatus can used to measure the velocity of projectiles in plurality. Having given the principle of measuring the velocity of any slow or fast moving projectile using two laser screens, each incorporating single laser, a set of prisms and associated optical and opto-electronic components, we now provide schematic design of the apparatus. Detailed Description of the Invention All Figures are not to the scale. Fig.1 shows the Apparatus Block Diagram of different subsystems of the invention. Opto-electronic subsystem 1 consists of a pair of similar laser screen assemblies, 1a and 1b, kept at known distance. Electronic subsystem 2 is in the form of a closed box, shielded from EMI/EMC consisting of power supplies for opto-electronic subsystem, all electronic circuits and signal processing circuitry and Computer 3. The subsystem 2 operates either with 220V AC and 50HZ or alternately with 24V, 300mA battery. A timer card and user's friendly software are installed in the computer 3 for recording the Time of Flight (ToF), calculation of the velocity of the projectile, entering of various data to operate the apparatus and preparation of the test reports. Fig.2 shows the apparatus schematics without cover. The laser screen assembly 1a consists of a laser- beam expander collimator, called source module, 4a; lens-photodetector called photodetector module, 5a; mechanical columns 9a and 10a house two sets of prism assemblies. Similarly the laser screen assembly 1b consists of source module 4b and photodetector module 5b and mechanical columns 9b and 10b house another two sets of prism assemblies. One end of the source module 4a is rigidly mounted to the base of mechanical column 9a and other to the support bracket 11a, which is mounted on the base pate 14 and detector 5b is rigidly mounted to the top of 9a. The prism assemblies 7a, 7b, 8a and 8b have a set of 45°-90°-45° prisms 30 each. 10a is another mechanical column similar to 9a, except that it does not have any source module or photodetector module. The mechanical columns 9a and 10a are so mounted that they are parallel and also the hypotenuse surfaces of all prisms housed in 9a and 10a are parallel. With this geometry, expanded and collimated laser beam gets reflected back and forth between a pair of opposite facing prisms and at the same time gets displaced in the direction of the column height after each passage through a prism till the beam falls on to the photodetector module 5a. This creates a laser screen 12a whose dimensions are governed by the distance between 9a and 10a and also the height of the column 9a or 10a. Dimensions of all the four columns 9a, 10a, 9b and 10b are same. Similarly items corresponding to the second laser screen 12b are depicted by 4b, 5b, 9b, 10b and 11b. All the columns 9a, 10a, 9b and 10b are rigidly mounted on respective sliding carriers. A pair of sliding carriers 13 are fitted to each of two rails 15 which can be locked at any distance along the path of the projectile with the help of a pair of knobs 14 and the sliding carriers mounted on the rails are separated by a pair of spacer bars 16. The rails are fitted rigidly on a base plate 17a supported on a number of vibration isolation pads17b. Spacer bars 16 define screen width while height of mechanical column 9a (columns 9a, 10a, 9b and 10b are of same dimensions) defines the height of the screen and the separation between the two screens depends on the positions of the sliding carriages on the rail. The Modular design permits easy change of the distance between the laser screens and alteration of the screen size. Electrical connector 18 is attached to the base plate17 provides connection to the source modules 4a & 4b through laser power cables 19a & 19b respectively and to the photodetector modules 5a & 5b through photodetector I/O signal cables 20a & 20b respectively. The cable 21 connects the connector 18 to the electronic subsystem 2. The electronic subsystem comprises of an ON/OFF switch 22 to supply power to various electronic modules/circuits and signal processing circuitry, power supplies indicators 23, for the display of the status of the laser screen assembly 1a, a potentiometer knob 24a for subtracting a variable DC voltage from the output signal of 5a and the resultant voltage in an arbitrary scale is shown by a LCD display 25a. Similarly a potentiometer knob 24b and a LCD display 25b are used for setting of the laser screen assembly 1b. A reset switch 26 is used to make the instrument ready for operation, which is indicated by an indicator 27. A computer interface 28 connects the opto-electronic subsystem 2 and the computer 3. All the sub systems are placed on any workbench or nearly flat structure 29. All the mechanical components/modules are painted dull black to avoid any ambient or stray light entering in the photodetector active area. To start the operation of apparatus the following procedure isfollowed: The knobs 24a and 24b are adjusted so that the LCD displays 25a and 25b lie within a specified range. Through a user friendly software, loaded in the computer 3 all the system operations like initialization of the apparatus, measurement and recording of experimental data, calculation and production of hard and soft copies of the results. A projectile 30, whose velocity is to be measured, while crossing the laser screen 12a partially or fully obscures the laser light beam. This produces a change in the intensity of the light falling on the photodetector module 5a. The out put signal of 5a is processed by processing circuitry and used to start a clock installed in the computer. Similarly when projectile obscures the screen 12b it creates a signal which stops the clock. The projectile path is shown by 31. The time between the start and stop of the clock is called the Time of Flight (ToF). The velocity of the projectile is calculated by the computer 3 which is the ratio of the distance between the screen 12a & 12b and the measured ToF. The measured velocity data is displayed on the monitor and recorded by the computer. The apparatus can be used in both manual and automatic modes. The software gives suitable command to reset and to initialize the apparatus either in automatic or one-at-a-time mode. A reset button 26 provided on the electronic subsystem 2 is for manual resetting. Fig. 3 illustrates the prism assembly and components of the apparatus. The prism assembly 7a, as shown in the Fig. 3a (front-view) and Fig. 3b (side cross-section -view), comprises of a set of 45°-90°-45° prisms and two plates 31a and 33b made of plate glass as shown in Fig. 3f. In the prism assembly 7a, prisms are mounted between the plates 33a and 33b in such a way that their hypotenuse surfaces are coplanar and parallel to each other. Each prism 32 as shown in Fig. 3c (3 Dimensional-View) & 3d (Side- View) is made of low dispersion optical glass or plastic or similar homogenous isotropic transparent dielectric material. The length and width of the hypotenuse face of the prism defines the cross section of the expanded collimated laser beam used for construction of the laser screens. All the edges of each prism are chamfered to avoid chipping and hypotenuse surface is coated with antireflection coating to enhance the transmission. The other prism assemblies are similar to prism assembly 7a. The laser screen assembly 12a of the apparatus as shown in Fig.4, consists of two prism assemblies 7a and 8a which are fitted rigidly in the mechanical columns 9a and 10a respectively. Both the columns 9a and 10a are similar except that source module 4a is attached to the base and photodetector module 5a to the top of the mechanical column 9a. The prism assembly 8a has one prism more than the prism assembly 7a as shown in the Fig.4. The prism assemblies 7a and 8a are so mounted that hypotenuse surfaces of the prisms of assembly 7a and 8a are parallel. The optical output of the source module 4a is a collimated beam such that its size is equal to half of the size of the hypotenuse of a prism 30 and its thickness may have any value between 1mm to few mm. The source module comprises of a standard laser diode emitting visible or near infra red radiation of suitable power which may vary from 1mW to 10mW depending upon the size of the laser screen and a beam expander collimator (Galilean Configuration) fabricated using spherical lenses. The collimated light beam 34a (travelling from right to left) from the source module 4a falls orthogonal on the lower half of the first prism of the column 10a, the beam gets reflected within the prism and emerges from upper half of the prism as a collimated beam 32b (travelling from left to right). Consequently the beam 34b proceeds towards the first prism of the column 9a and which in turn sends the beam as beam 32a towards the second prism of the column 10a. In this way the collimated beam is reflected back and forth between the prisms of the columns 9a and 10a in such a way that all the beams 34a and 34b are parallel and separation between adjacent beams is always less than or equal to 0.5mm. Finally emergent beam from the last prism of the column 10a falls on the photodetector module 5a wherein a collector lens focuses the collimated beam on the active area of the photodetector. In this way, a laser screen 12a is constructed. Cross - section view of the projectile is shown by 30. Sensitivity, precision and accuracy of the screen is same throughout the area of the screen. The laser screen 12b can also be constructed in the same way using the source module 4b, the photodetector module 5b, mechanical columns 9b & 10b, prism assemblies 7b & 8b. The photodetector module 5a shown in the Fig.5 has a collector-lens 35a and a photodetector 36a with an active area 37a, which are house in a barrel 38a attached to the mechanical column 9a. The collimated beam emerging from the last prism of the column 10a focuses on the active area 37a which produces an electric signal processed by electronic circuitry. The inner and outer surfaces of the barrel 38a are dull black coated to prevent any ambient light or stray light, as shown by ray 39, entering in the active area 37a of the photodetector 36a. The collector-lens 35a is placed with respect to the column 9a at such a distance that any ambient or stray light namely 40 after passing the collector-lens 35a do not fall on the active area 37a avoiding the noise signal. The photodetector module 5b is similar to 5a. Fig.6 shows the isometric view of mechanical columns and rail assembly. Fig.7 shows the complete apparatus with covers on the laser screen assemblies. Covers 41a and 41b made of metal or any hard plastic sheet, painted black inside, cover the laser screen assemblies 1a and 1b respectively. The covers 41 work as baffle to prevent ambient light or stray light entering the photodetector modules which makes the apparatus suitable for indoor and outdoor ambient lighting conditions. The performance of the apparatus is not affected by ambient light conditions. Fig.8 shows the bock diagram of the electronic circuit. The obscuration of the laser screen 12a by a projectile 30 produces a decrease in the energy falling on the photodetector module 5a, which in turn creates a momentary decrease in the output DC current of the photodetector. To make the laser screen sensitive to even the smallest projectile of the size 5mm, very high speed operational amplifiers constituting different signal conditioning stages are used after the photodetector module 5a. 5a is used in photocoducting mode with reverse bias. High frequency components in the output signal of the photodetector increase exponentially with the speed of the projectile whereas the amplitude of the signal decreases which decreases the signal to noise ratio (S/N). High speed opamps inherently prone to noise, require careful circuit layout design, special grounding and shielding techniques to minimise the noise. Keeping in view the above points, preamplifier and signal conditioning circuits are designed. A preamplifier 40a constitutes a current to voltage conversion stage followed by an inverting amplifier stage. A variable DC reference voltage using a potentiometer 24a is subtracted by the subtractor 43a from the signal. Output of 43a is shown by an LCD display 25a. An optical - isolator 45a driven by a buffer 44a is used to transfer the output signal of 43a to the input of a mono shot circuit 46a to trigger it. The analogue and digital circuits are connected through optical-isolator to minimise the noise. The above circuit is used for the laser assembly 1a. Similar circuit is employed for the laser screen assembly 1b with same numbering except the subscript 'a' is replaced by 'b' as shown in the fig.8. The circuitries related to 1a and 1b are called start and stop channels respectively. Whenever a projectile produces an interruption in the laser screen 12a which is connected to the start channel, Q output of the flip-flop 47 is set which enables the gate of the timing counter. Similarly an interruption in the laser screen 12b which is connected to the stop channel resets Q output of 47 which disables the gate of the timer counter. Gating pulse 48 is TTL compatible which is fed to a standard timer card 49 fitted in the computer. 5 MHz clock option of the timer card has been selected for measurement of ToF. Fig. 9 shows electronic circuit diagram. Photodetector module 5a with a photodiode of rise time 12 nanoseconds is used in photoconductive mode with the reverse bias. With increase in reverse bias, rise time decreases and dark current increases. Reverse bias is optimized to 14V to get high speed with minimum dark current. The preamplifier 42a comprises of two op-amps having high speed, fast settling time and a C2 compensation capacitor as shown in the fig.8. The first op-amp converts current to voltage while the second op-amp amplifies the signal. A variable DC reference voltage using a potentiometer 24a is subtracted by the subtractor 43a from the input signal to offsets the long term stability and also provides a reference signal for the subsequent circuit. Thus an initial condition is established by subtracting a DC voltage varied through a 10 turn potentiometer R6 mounted on front panel. The output signal of photodiode 5a decreases for a period for which projectile interrupted the laser screen. Change in output level of 5a depends on the projectile size but its duration depends on velocity and length. Actual voltage level and change in voltage are not much important as long as interruption is detected. Still variations should be within specified range so that detection is recorded. A high slew rate, fast settling time op-amp is used in the subtractor 43a. Output value of signal is displayed on an LCD 25a fitted on front panel of the electronic subsystem 2. For operating the system having 3mW laser diode fitted in the source module, initially LCD reading is adjusted within a range 0.9 to 1.2V. Output of 44a which is high speed transistor acting as driver to opto-isolator at ground initially. On interruption in the laser screen 12a, output of 43a decreases and as soon as it goes below 0.7V approximately, 42a is switched on. Opto-isolator 45a gives a momentary pulse which is related to momentary change in the light energy at photodetector at the time of interruption of the laser screen by projectile and triggers the monoshot making signal TTL compatible. Rising edge of output pulse of monoshot sets the D flip-flop 47 high. Similarly, rising edge of monoshot of stop channel resets the D type flip-flop. The reset switch 26 gives initial ready condition which is further ensured by on state of the indicator 27. The gating pulse 48 is fed to the timer card 49 is plugged in the computer 3 with some special features such as software re-setting, software arming and disarming of the counter etc. The computer is provided with a user friendly software with graphical user interface developed in Visual C++. The software has provisions of inputting the variable fields like, date, test number, name of the user and operator, type of projectile and distance between laser screens through key board which are shown on monitor screen. User friendly messages at all operational stages of the system are displayed on monitor screen during the test and after measurement, velocity data is also shown on the screen. Apart from the above, the software also has provisions of calculation, data storage and test reports preparation. As per the requirement of the test, distance field data shown on the screen can easily be changed through keyboard. Different components namely rails, spacer bars, slide carriers and mechanical columns of the system are so fitted that these can be easily dissembled and reassembled without the rigorous alignment during installation. Modular design of the system permits that it can be shifted from one place to other place easily and also permits field replacement/repair of faulty components/modules. The apparatus has provision for measurement of velocity of a projectile in the range 1 m/sec to 2500 m/sec which can be divided into three ranges namely, slow, medium or fast. In case of projectiles whose velocities change rapidly, the screens have to be placed nearer to each other, the apparatus has provision to do so. Following is the mathematical analysis of the working of the present invention: During the measurement of the velocity of the projectile it has to pass through two similar laser screens placed at a known distance apart. Time of Flight (ToF) of the projectile between the two laser screens is recorded by an electronic clock. Velocity of the projectile under test is the ratio of the distance between the screens and ToF and the error in the measurement of the velocity depends on these two factors. The velocity measurement based ToF is governed by following equations: (Equation Removed) The error in the computed velocity and the measurement errors of distance and time are related by, (Equation Removed) where; v : Actual value of the velocity X : Distance between the laser screens t : Time of Flight (ToF) dt and dx are the errors of measurement of ToF and distance between two screens respectively and which in turn produces dv error in the measurement of velocity according to the equation (2). For example error contribution in measurement of the velocity of a projectile with nominal velocity (v) 1000 meter/sec is as follows: Distance between the screens (x) : 1000 mm Time of Flight (t) : 10"3sec Distance measurement error (dx) : 0.5 mm Time of Flight error (dt) : 0.5X10"6 sec Velocity measurement error (dv) : 500mm or 0.5 meter Velocity measurement error in percentage : 0.1% The invention is described in detail in the examples given below which are provided by the way of illustration and, therefore, should not be considered to limit the present invention in any manner. During interpretation of the measured velocity data given under the examples It may kept in mind that projectile velocity measurement is of destructive type as recycling of the same projectile to be propelled with the same velocity second time is not possible and also as no two projectiles are identical in shape, size and weight, every projectile has a unique velocity different from all others including the same of similar kind. All these trials have been conducted with each laser screen of size 250mmX250mm, distance between the screens 1000 mm and few of the results are shown in the examples. Example 1 For experimental testing of the short term stability of the system, the displays on both the LCDs have been set to the fixed known value of 1.00. The apparatus has been left undisturbed and displays have been noted after every fifteen minutes for 6 hours. It has been observed that neither displayed value nor their difference changed by more than 5% and 5% respectively. The experiment has been repeated several times over a period of one week, the observation being the same every time. This indicates that the apparatus is stable over a day to yield consistent and precise results. Example 2 For experimental testing of the long term stability of the system, the displays on both the LCDs have been set to the fixed known value of 1.00. The apparatus has been switched off at end of each day and switched on next day for a week. It has been observed that all display values have remained within the permissible limits. The results indicate that once the apparatus is fine tuned it need not retuned frequently. Example 3 For experimental testing of the apparatus, velocities of slow and fast moving projectiles have been measured. A steel ball of 3 mm diameter thrown by human arm using different muscular power for each throw, lead pellet fired from air pistol and a projectile fired from a high velocity rifle have been used as test objects. Measured values are given in Table I. The results show that the apparatus is capable of measuring the velocity of both slow and fast moving projectiles. Example 4 For experimental testing of the apparatus, the velocities of a number of projectiles of different calibers have been measured. The results are given in Table II. The experiment proves that the system yields precise measured values. Example 5 For experimental testing, the apparatus has been used to measure the velocities of different projectiles under (i) outdoor conditions with about 100000 lux, (ii) indoor with artificial flood lights producing about 2000 lux directed towards the system, and (iii) darkroom conditions with all lights switched off except those forming the part of theapparatus. Results have shown that the apparatus yields consistent values. Example 6 For experimental testing of precision of the apparatus, after conducting one round tests it has been dismantled and dismantled parts have been transported to another lab situated at a distance 25 km. The system has been reassembled in the new location and tested again. During reassembly no special toolings and rigorous alignment have been required , while the reassembled apparatus has given consistent results. This shows that the apparatus can be easily dismantled and reassembled without any special tool and hence portable. Advantages of the Present Invention The main advantages of the present invention are: 1. It can measure the velocity of the both slow moving and fast moving projectiles, the measurable velocity range being 1 meter/second to 2500 meter/second. 2. For the same apparatus, laser screen can be modified to any preferred size whose height may be in the range of 100 mm to 1500 mm provided that the dimension is an integral multiple of the base length of a prism while the width may be in the range of 100 mm to 1500 mm without changing any other part of the apparatus except the columns and spacer bars. 3. The laser screen assemblies, comprising of laser diodes, a set of prism assemblies and photodetectors are housed in field replaceable mechanical columns enabling easy upscaling/downscaling of the screens. 4. For the same apparatus, the distance between the two laser screens can be adjusted to any value between 100 mm to 1000 mm by the operator in predefined discrete steps during the usage of the apparatus depending upon the projectile type and its anticipated velocity without changing any part of the apparatus. 5. Only one light source is used for each laser screen assembly irrespective of the screen size lying within the specified range. 6. The light level is uniform throughout the laser screen area making all zones equally sensitive such that the projectile need not be made to pass through any preferred zone. 7. The method is equally applicable for indoor and outdoor ambient lighting conditions and the same apparatus can used under any level of ambient light or background light without any modification/alteration. 8. Opto-isolators are used to effectively reduce the noise created during analogue to digital conversion of the signals at the same time simplifying the noise reduction circuitry. Table I (Table Removed) Table II (Table Removed) We Claim: 1. A method of measuring the velocity of a projectile whose velocity may vary from 1 meter per second to 2500 meter per second using a pair of optical screens, each screen constructed by utilizing a laser diode, expanded-collimator to expand and collimate the laser beam, reflecting and refracting properties of prisms, a collector lens to focus the laser beam onto any standard high speed photodetector having high sensitivity such that any projectile passing through two screens, separated by a distance varying from 100mm to 1000 mm, in succession is sensed by the photodetectors and the signal generated by the photodetectors being used to start and stop a clock to measure the Time of Flight (ToF) of the projectile whose velocity is computed by dividing the known distance between the screens by ToF. 2. A method as claimed in claim 1, wherein narrow output beam from a standard laser diode of suitable power depending upon the screen size and may vary from 1 mW to 10 mW, emitting visible or near infra red radiation, is expanded and collimated by an optical beam expander-collimator constructed using spherical lenses; the beam being expanded to such an extent that center to edge intensity variation in the beam due to its Gaussian profile is small enough to enable the photodetector to sense the obscuration of the incident beam created by a projectile of minimum diameter of 5 mm traveling at a velocity lying in the range of 1 meter per second to 2500 meter per second while it crosses any part of the beam. 3. A method as claimed in claims 1-2, wherein a number of similar 45°-90°-45° prisms made of low dispersion optical glass, plastic or similar homogeneous isotropic transparent dielectric material is used to construct an optical screen. 4. A method as claimed in 1-3, wherein the length and width of the hypotenuse face of prism defines the beam cross section, the width varies from 1 mm to few millimeters converting the incident cross section of the expanded-collimated laser beam into a rectangle in order to make ToF measurement more precise and doing away with the need of cylindrical lenses. 5. A method as claimed in claims 1-4, wherein the size of the hypotenuse face of each prism is twice the size of the expanded-collimated laser beam and wherein, the orthogonally incident collimated laser beam on one half of the hypotenuse face is refracted undeviated inside the prism and gets reflected by the two other sides of the prism and finally emerges out of the prism as a parallel beam traveling in opposite direction to the incident beam but displaced such that the incident and emerging beams from a continuous curtain of laser light. 6. A method as claimed in claims 1-5, wherein similar prisms used in plurality are mounted rigidly on two mechanical columns of same height with the hypotenuse faces of all prisms housed in the same column are coplanar and parallel to each other and also at right angles to the collimated laser beam and wherein, one of the columns houses the laser diode and beam expander-collimator at one end and a collector lens photodetector combination at the other end, the diameter of the collector lens being few millimeters larger than the expanded collimated laser beam and mounted at a suitable depth in side the column to cut off any light save the laser beam being received by the photodetector, while the other column houses only prisms; the two columns are erected rigidly in such a fashion that the hypotenuse faces of the prisms housed in one column faces and are parallel to the same on the other column and also the prisms in one column are so mounted that these are displaced by half the hypotenuse length measured along the array of prisms; these two types of columns form one set. 7. A method as claimed in claims 1-6, wherein a laser screen is created by using a set of columns housing prism assemblies such that, for the same apparatus, laser screen height can be modified to any value between 100 mm to 1500 mm in steps of integral multiple of prism hypotenuse length by simply replacing the columns, the width can be modified to any value in the range of 100 mm to 1500 mm by replacing the spacer bars and for the same apparatus the distance between two screens can be set to any value in the range of 100 mm to 1000 mm by the operator without replacing any part or component. 8. A method as claimed in claims 1-7, wherein sensitivity of each screen to accurately and precisely determine the time of arrival of the projectile in the screen is same through out the screen area since the same light beam, after back and forth reflection and transmission, reaches the photodetector module, fall in output of the photodetector due to the obscuration by a projectile is independent of the projectile position within the defined screen area. 9. A method as claimed in claims 1-8, whBrein the columns with prealigned prisms are mounted rigidly on a frame and columns the frames and other relevant parts are modular in design and construction such that the laser screens it is possible to easily dismantle, reassemble and/or up scale/down scale and/or change the inter-screen distances on site without the need of rigorous alignment during erection. 10.A method as claimed in claims 1-9, wherein laser screens can be mounted on vibration isolation pads and covered with metal or any hard plastic cover painted black inside to work as baffle to prevent any ambient light or stray light from being incident on the collector lenses to enable the method being applicable for indoor and outdoor ambient lighting conditions. 11. A method as claimed claims 1-10, wherein the photodetector output of each screen forms a part of a signal processing channel comprising of a preamplifier and amplifier, variable reference voltage generator for subtraction from the inverted signal, opto-isolator; and wherein the processed signal from two channels are used to start and stop a clock interfaced with a remote computer; and system design permits the use of standard components and devices. 12. A laser based apparatus for projectile velocity measurement comprising of two laser screen assemblies, elebtronic sub-system, firmware and mechanical mounts with cover; each screen assembly comprising of: • a pair of first and second column, • a laser diode producing visible or near infra red light signal, • a beam expander-collimator producing expanded and collimated light, • a set of transmitting and reflecting prisms receiving collimated light from the beam expander-collimator, and • a photodetector module having a photodetector and a collector lens focusing the collimated light signal received from the set of prisms on to the photodetector, electronic sub-system comprising of: • power supply unit, • opto-electronic and electronic signal processing unit, • LED and LCD displays, • potentiometer, and • computer interface hardware, firmware comprising of: • user interface with computer software • system operating software, and • computational software, mechanical mounts with cover comprising of: • rails, sliding carriers mounting the columns and the spacer bars, • light weight base plate • vibration isolation pads supporting the base plate, and • cover 13. The apparatus as claimed in claim 12, wherein the first column houses the source module, prism assembly and photodetector module. 14. The apparatus as claimed in claim 12, wherein the second column houses only the prism assembly. 15. The apparatus as claimed in claim 12, wherein laser diode power varies in the range of 1 mW to 10 mW depending upon the screen size and end application. 16. The apparatus as claimed in claim 12, wherein the laser diode emit radiation lying in the range of visible to near infra red region of electromagnetic spectrum. 17. The apparatus as claimed in claim 12, wherein laser diode is mounted in a holder coupled with beam expander-collimator having a provision of easy replacement and quick alignment. 18. The apparatus as claimed in claim 12, wherein all the prisms are 45°-90°-45° type and are identical within standard tolerances. 19. The apparatus as claimed in claims 12 and 18, wherein sharp corners of each prism are ground by a fraction of a millimeter avoiding the chance chipping. 20. The apparatus as claimed in claims 12 and 18-19, wherein hypotenuse face of each prism is coated with thin film antireflection coating corresponding to the laser diode wavelength used reducing the loss of collimated beam power on successive reflections. 21. The apparatus as claimed in claims 12,18-20, wherein all the prisms are made of low dispersion glass or any other optical grade transparent homogenous isotropic dielectric. 22. The apparatus as claimed in claims 12 and 18-21, wherein the length of hypotenuse of a prism is twice the diameter of the collimated laser beam while width varies from 1mm to few millimeters depending upon the sensitivity of the photodetector used. 23. The apparatus as claimed in claims 12, wherein each prism assembly is constructed in a manner that the hypotenuse faces of all prisms in the assembly are parallel and coplanar. 24. The apparatus as claimed in claims 12 and 23, wherein the 45° corner of each prism in each prism assembly is in contact with the similar corner of the prism adjacent to it. 25. The apparatus as claimed in claims 12, 23-24, wherein the prisms in a prism assembly are so arranged that their corresponding ground faces are coplanar. 26. The apparatus as claimed in claims 12, 23-25, wherein one of the ground faces of all prisms in a prism assembly are glued to a parallel rectangular glass plate by suitable strain free glue while the other ground faces of all prisms in the prism assembly are glued to another parallel rectangular glass plate by suitable strain free glue such that prealigned prisms are held together rigidly sandwiched between two glass plates. 27. The apparatus as claimed in claims 12, 23-26, wherein for each screen, each prism assembly formed by sandwiching between two glass plates is housed inside the first column such that the hypotenuse faces of the prism array are parallel to the column. 28. The apparatus as claimed in claims 12, 23-27, wherein for each screen, each prism assembly formed by sandwiching between two glass plates is housed inside the second column such that the hypotenuse faces of the prism array are parallel to the column. 29. The apparatus as claimed in claims 12, 23-28, wherein all the prism assemblies in all the respective columns are rigidly held in place yet can be easily taken out for replacement by tightening or loosening a set of screws. 30.The apparatus as claimed in claims 12, 27-29, wherein the first and second columns of each screen are so mounted that the hypotenuse faces of the prism assembly in the first column are parallel to and face the corresponding surfaces of the prism assembly in the second column. 31. The apparatus as claimed in claims 12, 27-30, wherein all the mechanical columns together with prism assemblies housed inside, can be easily and rigidly fixed to a frame, inter-column distances being maintained by a set of field replaceable spacer bars, in such a manner that the columns and space-bars can be taken out of the sliding carriers for replacement by larger or smaller columns to enable on site upscaling or downscaling the screen size by changing the screen width varying from 100mm to 1500 mm and the screen height varying from 100 mm to 1500 mm in steps of an integral multiple of the hypotenuse length of a prism and/or changing of the inter-screen distances in the range of 100 mm to 1000 mm. 32. The apparatus as claimed in claim 12, wherein all the internal walls of all mechanical parts housing optical and electro-optical components or devices are painted dull black minimizing the optical noise due to internal reflections of stray light from the inner walls of mounts and houses. 33. The apparatus as claimed in claim 12, wherein the photodetector module is mounted in a recess in the first column in each laser screen assembly blocking any light other than expanded collimated laser beam making the apparatus performance independent of ambient light. 34. The apparatus as claimed in claim 12, wherein, in each laser screen assembly, a continuous curtain of laser light of rectangular geometry is created by the back and forth reflection of the same laser beam enabled by the transmission and reflection properties of a 45°-90°-45° prism when it receives collimated laser beam incident normally on one half of the hypotenuse face. 35. The apparatus as claimed in claims 12, and 34, wherein, in each laser screen assembly, a continuous curtain of laser light of rectangular geometry is created by the displacement of the laser beam and its emergence from the other half of the hypotenuse face using transmission and reflection properties of a 45°-90°-45° prism when it receives collimated laser beam incident normally on one half of the hypotenuse face. 36. The apparatus as claimed in claim 12, wherein an electronic power supply module supplies required stabilized power supplies to laser diodes, photodetectors, electronic circuitry, display devices and other opto-electronic, electronic devices, components and circuits incorporated in the apparatus. 37. The apparatus as claimed in claims 12 and 36, wherein the power supply module is powered either by mains supply or 24 V, 300 mA battery. 38. The apparatus as claimed in claim 12, wherein a dual channel signal processing unit is provided such that first channel processes the signal received from the photodetector module of the first screen and the second channel processes the signal received from the photodetector module of the second screen. 39. The apparatus as claimed in claims 12 and 38, wherein in each channel the received signal from each photodetector module is converted from current to voltage. 40. The apparatus as claimed in claims 12, 38 and 39, wherein the signal processing unit inverts and amplifies the input signals. 41. The apparatus as claimed in claims 12, 38-40, wherein in each channel a variable dc signal is generated and subtracted from the inverted amplified signal to offset drifts affecting long term stability. 42. The apparatus as claimed in claims 12, 38-41, wherein in each channel a reference signal is generated after subtraction. 43. The apparatus as claimed in claims 12, 38-42, wherein the variable voltage in each channel is provided to fine tune the apparatus for higher precision and also to offset low frequency drifts in laser power outputs and aging effects of the laser diodes and the photodetectors. 44. The apparatus as claimed in claims 12, 38-43, wherein a potentiometer is provided in each channel for manual adjustment of the voltages generated for subtraction. 45. The apparatus as claimed in claims 12, 38-44, wherein a Liquid Crystal Device (LCD) displays the signal level in each channel after subtraction. 46. The apparatus as claimed in claims 12, 38-45, wherein it is possible, using the two potentiometers, to manually and easily adjust the data, corresponding to the first and second laser screens as displayed on the two LCDs to within 30% of a fixed known value to offset the drift produced by electronic circuitry improving the sensitivity of measurement, the fixed value being same for both the laser screens. 47. The apparatus as claimed in claims 12 and 38-46, wherein two manual potentiometers can be adjusted easily to bring the two LCD displays very close to each other improving apparatus precision; the permissible limit of the difference being 10%. 48. The apparatus as claimed in claims 12 and 38, wherein an opto-isolator is provided in each channel in the signal processing unit to eliminate line interference and other cross channel noise. 49. The apparatus as claimed in claims 12 and 38, wherein, in the first channel, a flipflop after receiving signal from a monoshort starts a clock whenever a projectile interrupts the first laser screen. 50. The apparatus as claimed in claims 12, 38 and 49, wherein, in the second channel, the flipflop after receiving a signal from a monoshort stops the running clock whenever the projectile interrupts the second laser curtain. 51. The apparatus as claimed in claims 12 and 38, wherein reset circuit permits automatic as well as manual resetting through a reset button in the signal processing unit to initialize and reset the apparatus. 52.The apparatus as claimed in claim 12, wherein apparatus incorporates standard laser diode, optical components, standard high speed opto-electronic and electronic devices and is adequately shielded from EMI/EMC. 53. The apparatus as claimed in claim 12, wherein a computer interface hardware is provided enabling remote apparatus by any standard IBM PC or equivalent. 54. The apparatus as claimed in claim 12 and 53, wherein a firmware provides user friendly software for data entry needed for apparatus operation, controls the apparatus operation, computes the desired velocity from the time interval between the starting and stopping of the clock, stores data and formats these to give soft and hard copies. 55. The apparatus as claimed in claims 12, 53 and 54, wherein the firmware is installed in an IBM PC and creates a data base of results obtained by the apparatus, the amount of data stored depending upon the storage capacity of the computer hard disk. 56. The apparatus as claimed in claims 12 and 33, wherein a cover made of thin metal or any hard plastic and painted/anodised dull black on all surfaces is provided to ensure that the energy of any stray light falling on the active area of each photodetector is negligibly small compared to the energy of the focused laser beam falling on the same area of the same photodetector. 57. The apparatus as claimed in claim 12, wherein all the mechanical parts and the chassis housing the electronics are dully painted or anodized eliminating atmospheric corrosion. 58. The apparatus as claimed in claim 12, wherein an array of LEDs permits on- line health monitoring of the apparatus. 59. Laser Based Apparatus for Projectile Velocity Measurement substantially as herein described with reference to the examples and drawings accompanying this specification.

Full Text

Laser Based Apparatus for Projectile Velocity Measurement
Field of Invention
This invention relates to the development of an improved computer controlled system
for opto-electronically measuring of the time of flight of any small caliber projectile
between two optical screens separated by a known distance and computing its velocity.
The system involves a pair of optical screens generated by laser sources, beam
expander-collimators and prism assemblies using their retroreflecting property; focusing
lenses; photodetectors and associated electronic circuitry; timer card; necessary
hardware and software for computer interface; system operating and software. In the
present invention retroreflecting prisms create large optical screen from an expanded
collimated input laser beam which is focused by a lens on to a photodetector at the
output end of the screen, The high speed electronic circuit generates two pulses to start
and stop a clock controlled at the time entry and exit of the projectile through and from
the entry and exit screens respectively. A computer controls entire system operation.
The present invention has useful applications where the measurement of the velocity of
a dynamic object is required e.g. forensic studies, police training centers, projectile
characterization by the manufacturers and users, golf ball velocity, physiological studies
like checking the throwing power of a human arm, study of birds' movements, and
similar areas.
Background an Prior Art to the Invention
The method to determine the velocity of a projectile moving at a high velocity is based on (i) the measurement of its Time of Flight (ToF) over a known distance; (ii) measurement induced of electromotive force generated due to the passage of a ferromagnetic projectile through a magnetic field; and (iii) Doppler frequency shift caused by the moving projectile. A fast moving projectile while passing through a steady field, serving as a screen, perturbs the field energy distribution.
The time of occurrence of the perturbation is measured by a suitable sensor clock combination. When the projectile passes through two such screens in sequence, the time interval between the two perturbations gives the ToF between the two screens.
The known screen distance divided by ToF is the required velocity of the projectile. All
or arts to measure projectile velocity are based on this basic principle. These vary in terms of the type and construction (including shape and size) of the screen and sensing methods, the apparatus geometry, the physical properties used to make the screen and matching sensor. The physical quantities employed to make the screens whose perturbation time is measured can be broadly classified into the following categories:
• Pressure/Acoustics
• Magnetic
• Electrical
• Doppler Frequency Shift
• Radiation Field Analysis of Prior Art
The following is list of patent numbers closest to the present invention:
US 6020594, US 3215932, US 4239962, US 4845690, US 4574238, JP 60024453, EP
0126423 (A) & (B), US 4272189, US 4155647, US 557733, US 3487226, US 64414747,
US 3918061, US 3918061, US 5806848, DE 2610708, US 4649528, US 6388438, DE
3404953, FR 2567651
Pressure/Acoustics
Patent Nos. US 4649528 and DE 4106040 fall under this category. Herein two air
pressure sensors are placed at a known distance along the direction of travel of the
projectile. The pressure wave traveling with a high velocity projectile triggers the
sensors sequentially when the projectile is in their vicinity. Each pressure sensor
generates an electrical pulse. A timer circuit measures the time interval between two
such pulses generated by the two sensors.
Drawbacks of pressure/acoustic wave based speed/velocity measurement are:
• The traveling air/acoustics pressure wave depends upon the projectile mass, geometry and its velocity. The location of the projectile to impart sufficient pressure needed to actuate the sensor is likely to be different for various projectiles, introducing large and unknown variations in the flight path for which ToF is measured leading to unreliable apparatus readings.
• Small projectiles of low mass and low velocity may not trigger sensors.
• In case of a projectile traveling at subsonic speed, pressure/acoustics wave created by the detonation of the explosives during propulsion i.e. at the time

firing, may actuate the sensors before the waves associated with the projectile does so.
• External disturbances may also lead to erroneous results.
Magnetic
Patent Nos. US 3215932, US 4342961, EP 0035802 (A) & (B), GB 2200215 and CH 693248 fall under this category. Such types create a magnetic or pulsating electromagnetic field by employing magnets/electromagnets or current carrying coils. Projectile made of magnetic or electrically conducting material changes the field distribution, which is sensed. The common drawbacks of the above apparatuses are:
• The principle does not work for electrically nonconducting projectiles like plastic or ceramic projectiles.
• Magnetic field screen based systems do not sense a projectile made of nonmagnetic material.
• The apparatuses are to be mounted on the projectile firing mechanism, eg. a gun. These can not be used to measure velocity of projectile propelled from a distance.
• Critical axis alignment is essential every time the projectile propelling system is to be changed.
• These have limited accuracy.
Electrical
The screen of an apparatus under this category is made by a sheet or thin plate of an insulating material sandwiched between two sheets of a conducting material like metal foil and a potential difference is applied between the two conducting surfaces. But no current flows due to the presence of the insulating layer in between. When a projectile pierces this sandwich arrangement short circuit takes place and a shorting pulse is generated. ToF is the time interval of i.v shorting pulses. The drawbacks of such systems are:
• Every time a projectile passes i Dies are created in the screen after score
usage screens are to be replaced.

• Every time the screens are replaced it is to be ensured that the distance between the screens remains same throughout the screen area to avoid creeping in of the distance values.
• Susceptible to electromagnetic interference/electromagnetic current (EMI/EMC) disturbances
• The principle does not work for electrically nonconducting projectiles like plastic or ceramic bullets.
Doppler Frequency Shift
Patent Nos. US 3918061, US 4283989, DE 3404953 are based on Doppler Frequency Shift measurement. A modulated microwave or millimeter wave beam is directed towards the moving target, the reflected beam is frequency shifted by the amount depending upon the projectile speed. The drawbacks are:
• Since only a fraction of incident beam is scattered by a projectile towards the sensor, for enhanced sensitivity, probe beam size is small, effectively reducing the active area.
• Microwave or millimeter waves offer limited resolution, these apparatuses are suitable for large projectiles or moving objects.
Radiation Field
Screens based on infrared (IR) or optical radiation fields offer very high resolution and are highly sensitive while being immune to EMI/EMC. A number of patents describe optical screen using either visible or near IR wave lengths. The variations are in the type of source(s) used, construction of the screen and detection mechanism. The following are the references of the apparatuses based on optical screen. Patent Nos. US 3487226, US 4155647, US 4239962, US 4272189, US 4574238 and its family EP 01264 (A) & (B) and JP 60024453 (A), US 4845690, US 557733, US 6020594, US 64414747; Publications: "Shooting Chrony@ Brochure, Shooting Chrony Inc., N. Tonawanda, New York, Jan 1996, pagel; "Oehler Ballistics Chronographs", Brochure, M/s Oehler Research Inc., TX, Jan 1996, pages 1-7 & 28; "Infrared Light Screen LS 19i3/A2", catalogue, M/s Drello, Germany; "Projectile Measuring System Type 758", Catalogue, M/s MS Instruments PLC, UK.

In all the above referred systems, either single or multiple sources are used emitting either visible or IR radiation. Some systems have used IR emitting Laser Diodes also. The light emitted from the source(s) is expanded by employing refracting type collimator(s)/beam expander(s) using cylindrical lenses or reflecting type collimator using a parabola (US 64414747). The expanded beam of light, visible or IR, forms the screen. In one family of patents (US 4574238, JP 60024453, EP 0126423 (A) & (B)), from a single collimated beam a number of beams are generated by using prism made of double refracting material or diffraction grating. The expanded beam is collected by single or multiple lenses or by parabolic reflector and focused on single or multiple detectors(s). In some cases sun light has also been used to make the light screen. A rectangular slit is used to define the cross sectional area of each screens.
The common drawbacks of all existing apparatuses using visible or IR based optical screen are:
• The systems employing sunlight source cannot be used in indoors.
• The precision and accuracy of systems employing sun light source vary depending upon the intensity fluctuations of sunlight.
• The intensity of light varies from point to point in the active area of the screen leading to non uniform sensitivity at various points. The precision and accuracy of the measurement depends upon the projectile path i.e. at what point it crosses both in and out screens.
• The effective screen area for high precision measurement is small and as such apparatuses are to be placed near the projectile propellant system or else due to dispersion or deviation from linear path the projectile may not cross the effective screen area.
• The screen areas of the systems using parabola for beam expansion are determined by the aperture of the parabola. Up scaling of the screen size requires replacement of expensive parabola.
• The screen does have any regular geometry make it difficult to aim and direct the projectile through the most sensitive area of the screen.
• In multi source systems, failure of any source during an experiment will not be noticeable, when a number of projectiles are fired in rapid succession.

• Major changes are necessary to increase the screen size like changing the parabolas used for expanding and focusing, adding of more source elements and detectors which need additional power supplies. Such systems cannot be upscaled in user premises.
• Performance of the systems is affected by the ambient light.
• Can not measure the velocities of slow moving projectiles.
The systems with small screen area necessarily to be kept very near to the origin of the projectile while systems with large screen area needs careful alignment during installation and can not moved from indoor to outdoor frequently. The present invention circumvents the drawbacks of the existing prior arts by providing a system to measure the velocity of a projectile moving at a high or slow speed incorporating a pair of optical screens, which can be easily up scaled or down scaled or inter-screen distance can be changed; each screen being constructed from a single source with same and uniform sensitivity throughout the screen area at the same time provides a system which does not require any expensive aspheric optical components like parabola, cylindrical lenses etc. doing away with the requirement of elaborate alignment during installation and can be 'picked and placed' for usage in indoor or outdoor or any other desired place.
Objects of the Invention
The main object of the invention is to provide an improved computer controlled apparatus to measure the velocity of various projectiles including those fired from small caliber guns without the drawbacks of the prior arts using laser diode, beam expander-collimator, reflecting prisms in plurality, collector lens, photodetector, signal processing electronics circuitry, computer hardware and software with interfaces for system operation, computation and display with built-in diagnostics.
Another objective of the present invention is to provide an apparatus to measure velocity of projectiles, with two optical screens, each having single source and single photodetector with modular design such that by simply changing the columns housing a set of prism assemblies, the screen size can be changed.

One more objective of the present invention is to provide a system to measure velocity
of projectiles, which employ laser diode, beam-expander collimator and multiple
reflecting prism assemblies to create a pair of screen in such a manner that the
sensitivity, precision and accuracy of each screen to sense the crossing of a projectile
do not depend upon the zone where the projectile interrupts.
Still another objective of the present invention is to provide a system to measure the
velocity of projectiles, in which light source, beam expander-collimator, multiple
reflecting prisms for both the screens do not need any alignment during installation and
as such can be picked and placed for usage at any location as frequently as the user
desires.
Yet another objective of the present invention is to provide a system to measure the
velocity of projectiles, employing a thin rectangular expanded-collimated laser beam to
improve sensitivity.
Yet one more objective of the present invention is to provide a system to measure the
velocity of projectiles, with a modular design that the distance between the columns and
the distance between two screens are maintained by a set of spacer bars such that
screen size and inter-screen distance can be increased or decreased by simply
changing the spacer bars and moving the sliding carriers on the rails respectively.
Still another objective of the present invention is provide a system to measure velocity
of projectiles, wherein different light sensitive devices are so mounted that these are
shielded from stray light and apparatus performance is not affected by ambient lighting
conditions and can be used in indoor or outdoor environment without any need of any
modification/alteration/improvement.
Yet another objective of the present invention is to provide a system to measure velocity
of projectiles, which employs standard laser diode sources, standard fast responding
photodetectors, standard high slew rate amplifiers to measure the velocity of projectile.
Still another objective of the present invention is provide a system which provides
simple to operate user settings to offset the deterioration in performance of the laser
diodes and photodetectors due to aging, the provision also helps to offset low frequency
drifts.
Still another objective of the present invention is to provide a system capable of
measuring the velocities of slow moving projectiles as well.

Still one more objective of the present invention is to provide a system with velocity measurement capability for a plurality of projectiles.
Summary of Invention
The present invention relates to optical screen time of flight (ToF) measurement based apparatus to measure of velocities of projectiles traveling at high speeds by measuring its time of flight between a pair of optical screens. In each screen, light from a laser diode, emitting visible or infrared radiation, is expanded and collimated by a beam-expander collimator. A set of similar 45°-90°-45° glass prisms, used in reflection mode, generates a large area screen by reflecting the beam back and forth at the same time shifting the beam laterally by an amount equal to the half the length of the hypotenuse of each prism, between the two columns housing the prisms. The separation between these two columns defines the screen width and the prism size together with the number of prisms defines the height of the screen. After the final passage the beam is focused as a small patch on the active area of a photodetector by a collector lens. The photodetector output is proportional to the incident energy. The output is amplified, conditioned and processed. In absence of any projectile, the electronic signal remains in steady state. A projectile during it journey partially or fully blocks the collimated beam and the photodetector signal level falls. The reduction in input energy on to the photodetector due to the obscuration produced by a projectile in the first screen is sensed and used to start an electronic clock. A second screen, similar to the first one, is employed to generate a pulse to stop the clock. Computer resident software divides the stored value of the distance between the two screens by the measured time of flight and displays the results or prints hard copies as desired. The entire system operation including initialization is computer controlled and can be implemented remotely. The lasers, photodetectors and electronics circuitry are power up by an electronic control box, which also houses signal conditioning and processing electronics. A hardware module interfaces electronic control box with a computer fitted with a timer card. Brief Description of the Accompanying Drawings
In the present invention, a number of drawings are given to illustrate the apparatus in detailed. However, the invention is not limited to the precise arrangements and instrumentalities.

Fig.1 Block Diagram of a velocity measurement apparatus, which shows the subsystems namely, opto-electronic subsystem comprising of a pair of laser screens; electronic subsystem; and a computer;
Fig.2 A Schematics of the apparatus showing different modules/components of the subsystems. Laser- beam expander collimators, prism assemblies housed in mechanical columns, rails, sliding carriers, spacer bars, base plate are used to construct the two laser screens of the opto-electronic subsystem. ON/ OFF switch, power supplies indicators, LCD displays, potentiometers knobs, reset button, connection cables and computer interface etc. of the electronic subsystem and computer are also shown in the Fig.2;
Fig.3 A prism assembly and components showing three dimensional view, the side view of a prism and two plates. The figure also shows the front and side view of the prism assembly;
Fig.4 A laser screen of the apparatus showing the various modules/components namely laser-beam expander collimator, prism assemblies housed in mechanical columns, beams traveling back and forth between prism assemblies and collector lens-photodetector and cross-section view of a projectile;
Fig.5 A Photodetector Modules of the apparatus showing a collector lens and photodetector in a barrel. The figure also shows a collimated beam focused by a lens on the active area of the photodetector and ambient light rays;
Fig.6 An isometric view of mechanical columns and rail assembly which shows various mechanical components/modules like mechanical columns, spacer bars, rails, sliding carriers etc.;
Fig.7 Apparatus with covers showing two independent covers, one for each laser screen assembly and other subsystems of the apparatus,
Fig.8 A block diagram of electronic circuit of the apparatus showing the transfer of signals from the photodetectors to the computer through the various components shown in the form of block: preamplifiers, subtractors, potentiometers, buffers & drivers, opto-isolators, monoshots, flip-flop and a gating pulse etc.; and
Fig.9 A electronic circuit diagram of the apparatus which shows all the components as shown in Fig.8 but with detailed circuitry.
Accordingly, the present invention provides a method and also an apparatus to measure velocity of projectiles traveling at high or low speeds, the said method consisting of use

of two optical screens constructed by using two laser diodes, two beam expander collimators, a set of prisms and a pair of collector lens-photodetector modules; and the said apparatus comprising of a pair laser screens, each screen constructed by incorporating a laser diode, a beam expander-collimator, a set prisms, a collector lens photodetector module, signal processing unit with opto-isolator, system power supply and interfaced computer installed with a firmware to operate the apparatus. In another embodiment of the present invention, the method incorporates a set of similar 45°-90°-45° prisms to make screens having uniform light energy distribution throughout the screen area so that performance of the apparatus is independent of location of the point where any projectile crosses the screens.
In one more embodiment of the present invention, the method permits modular design and the apparatus permits on site up scaling or down scaling the screen size and permits on site changing of inter-screen distance.
In one more embodiment of the present invention, different opto-electronics and optical components are so housed that only negligible amount of ambient light falls on the photodetector active area so that the present invention can be deployed outdoor or indoor without the need of any modification or calibration.
In yet one more embodiment of the present invention, a suitable cover is provided to further minimize the interference of ambient or stray light.
In still one more embodiment of the present invention, all mechanical components and the hypotenuse faces of the prisms are given suitable coating to attenuate stray reflections.
In yet one more embodiment of the present invention, apparatus design allows standard components and devices to be used.
In yet another embodiment of the present invention the design and construction make the apparatus portable.
In still another embodiment of the present invention, the apparatus is capable of measuring velocities of slow moving projectiles also.
In still one more embodiment of the present invention, the apparatus provides an array of LEDs and a pair of LCD displays to online monitor the health and tuning of different modules.
In still one more embodiment of the present invention, the apparatus can used to measure the velocity of projectiles in plurality.

Having given the principle of measuring the velocity of any slow or fast moving projectile using two laser screens, each incorporating single laser, a set of prisms and associated optical and opto-electronic components, we now provide schematic design of the apparatus.
Detailed Description of the Invention
All Figures are not to the scale. Fig.1 shows the Apparatus Block Diagram of different subsystems of the invention. Opto-electronic subsystem 1 consists of a pair of similar laser screen assemblies, 1a and 1b, kept at known distance. Electronic subsystem 2 is in the form of a closed box, shielded from EMI/EMC consisting of power supplies for opto-electronic subsystem, all electronic circuits and signal processing circuitry and Computer 3.
The subsystem 2 operates either with 220V AC and 50HZ or alternately with 24V, 300mA battery. A timer card and user's friendly software are installed in the computer 3 for recording the Time of Flight (ToF), calculation of the velocity of the projectile, entering of various data to operate the apparatus and preparation of the test reports. Fig.2 shows the apparatus schematics without cover. The laser screen assembly 1a consists of a laser- beam expander collimator, called source module, 4a; lens-photodetector called photodetector module, 5a; mechanical columns 9a and 10a house two sets of prism assemblies. Similarly the laser screen assembly 1b consists of source module 4b and photodetector module 5b and mechanical columns 9b and 10b house another two sets of prism assemblies. One end of the source module 4a is rigidly mounted to the base of mechanical column 9a and other to the support bracket 11a, which is mounted on the base pate 14 and detector 5b is rigidly mounted to the top of 9a. The prism assemblies 7a, 7b, 8a and 8b have a set of 45°-90°-45° prisms 30 each. 10a is another mechanical column similar to 9a, except that it does not have any source module or photodetector module. The mechanical columns 9a and 10a are so mounted that they are parallel and also the hypotenuse surfaces of all prisms housed in 9a and 10a are parallel. With this geometry, expanded and collimated laser beam gets reflected back and forth between a pair of opposite facing prisms and at the

same time gets displaced in the direction of the column height after each passage through a prism till the beam falls on to the photodetector module 5a. This creates a laser screen 12a whose dimensions are governed by the distance between 9a and 10a and also the height of the column 9a or 10a. Dimensions of all the four columns 9a, 10a, 9b and 10b are same.
Similarly items corresponding to the second laser screen 12b are depicted by 4b, 5b, 9b, 10b and 11b. All the columns 9a, 10a, 9b and 10b are rigidly mounted on respective sliding carriers. A pair of sliding carriers 13 are fitted to each of two rails 15 which can be locked at any distance along the path of the projectile with the help of a pair of knobs 14 and the sliding carriers mounted on the rails are separated by a pair of spacer bars 16. The rails are fitted rigidly on a base plate 17a supported on a number of vibration isolation pads17b. Spacer bars 16 define screen width while height of mechanical column 9a (columns 9a, 10a, 9b and 10b are of same dimensions) defines the height of the screen and the separation between the two screens depends on the positions of the sliding carriages on the rail. The Modular design permits easy change of the distance between the laser screens and alteration of the screen size.
Electrical connector 18 is attached to the base plate17 provides connection to the source modules 4a & 4b through laser power cables 19a & 19b respectively and to the photodetector modules 5a & 5b through photodetector I/O signal cables 20a & 20b respectively. The cable 21 connects the connector 18 to the electronic subsystem 2. The electronic subsystem comprises of an ON/OFF switch 22 to supply power to various electronic modules/circuits and signal processing circuitry, power supplies indicators 23, for the display of the status of the laser screen assembly 1a, a potentiometer knob 24a for subtracting a variable DC voltage from the output signal of 5a and the resultant voltage in an arbitrary scale is shown by a LCD display 25a. Similarly a potentiometer knob 24b and a LCD display 25b are used for setting of the laser screen assembly 1b.
A reset switch 26 is used to make the instrument ready for operation, which is indicated by an indicator 27. A computer interface 28 connects the opto-electronic subsystem 2 and the computer 3. All the sub systems are placed on any workbench or nearly flat

structure 29. All the mechanical components/modules are painted dull black to avoid any ambient or stray light entering in the photodetector active area.
To start the operation of apparatus the following procedure isfollowed: The knobs 24a and 24b are adjusted so that the LCD displays 25a and 25b lie within a specified range. Through a user friendly software, loaded in the computer 3 all the system operations like initialization of the apparatus, measurement and recording of experimental data, calculation and production of hard and soft copies of the results. A projectile 30, whose velocity is to be measured, while crossing the laser screen 12a partially or fully obscures the laser light beam. This produces a change in the intensity of the light falling on the photodetector module 5a. The out put signal of 5a is processed by processing circuitry and used to start a clock installed in the computer. Similarly when projectile obscures the screen 12b it creates a signal which stops the clock. The projectile path is shown by 31. The time between the start and stop of the clock is called the Time of Flight (ToF). The velocity of the projectile is calculated by the computer 3 which is the ratio of the distance between the screen 12a & 12b and the measured ToF. The measured velocity data is displayed on the monitor and recorded by the computer. The apparatus can be used in both manual and automatic modes. The software gives suitable command to reset and to initialize the apparatus either in automatic or one-at-a-time mode. A reset button 26 provided on the electronic subsystem 2 is for manual resetting.
Fig. 3 illustrates the prism assembly and components of the apparatus. The prism assembly 7a, as shown in the Fig. 3a (front-view) and Fig. 3b (side cross-section -view), comprises of a set of 45°-90°-45° prisms and two plates 31a and 33b made of plate glass as shown in Fig. 3f. In the prism assembly 7a, prisms are mounted between the plates 33a and 33b in such a way that their hypotenuse surfaces are coplanar and parallel to each other. Each prism 32 as shown in Fig. 3c (3 Dimensional-View) & 3d (Side- View) is made of low dispersion optical glass or plastic or similar homogenous isotropic transparent dielectric material.
The length and width of the hypotenuse face of the prism defines the cross section of the expanded collimated laser beam used for construction of the laser screens. All the

edges of each prism are chamfered to avoid chipping and hypotenuse surface is coated with antireflection coating to enhance the transmission. The other prism assemblies are similar to prism assembly 7a.
The laser screen assembly 12a of the apparatus as shown in Fig.4, consists of two prism assemblies 7a and 8a which are fitted rigidly in the mechanical columns 9a and 10a respectively. Both the columns 9a and 10a are similar except that source module 4a is attached to the base and photodetector module 5a to the top of the mechanical column 9a. The prism assembly 8a has one prism more than the prism assembly 7a as shown in the Fig.4. The prism assemblies 7a and 8a are so mounted that hypotenuse surfaces of the prisms of assembly 7a and 8a are parallel.
The optical output of the source module 4a is a collimated beam such that its size is equal to half of the size of the hypotenuse of a prism 30 and its thickness may have any value between 1mm to few mm. The source module comprises of a standard laser diode emitting visible or near infra red radiation of suitable power which may vary from 1mW to 10mW depending upon the size of the laser screen and a beam expander collimator (Galilean Configuration) fabricated using spherical lenses. The collimated light beam 34a (travelling from right to left) from the source module 4a falls orthogonal on the lower half of the first prism of the column 10a, the beam gets reflected within the prism and emerges from upper half of the prism as a collimated beam 32b (travelling from left to right). Consequently the beam 34b proceeds towards the first prism of the column 9a and which in turn sends the beam as beam 32a towards the second prism of the column 10a. In this way the collimated beam is reflected back and forth between the prisms of the columns 9a and 10a in such a way that all the beams 34a and 34b are parallel and separation between adjacent beams is always less than or equal to 0.5mm. Finally emergent beam from the last prism of the column 10a falls on the photodetector module 5a wherein a collector lens focuses the collimated beam on the active area of the photodetector. In this way, a laser screen 12a is constructed. Cross - section view of the projectile is shown by 30. Sensitivity, precision and accuracy of the screen is same throughout the area of the screen. The laser screen 12b can also be constructed in the same way using the source module 4b, the photodetector module 5b, mechanical columns 9b & 10b, prism assemblies 7b & 8b.
The photodetector module 5a shown in the Fig.5 has a collector-lens 35a and a photodetector 36a with an active area 37a, which are house in a barrel 38a attached to

the mechanical column 9a. The collimated beam emerging from the last prism of the column 10a focuses on the active area 37a which produces an electric signal processed by electronic circuitry. The inner and outer surfaces of the barrel 38a are dull black coated to prevent any ambient light or stray light, as shown by ray 39, entering in the active area 37a of the photodetector 36a. The collector-lens 35a is placed with respect to the column 9a at such a distance that any ambient or stray light namely 40 after passing the collector-lens 35a do not fall on the active area 37a avoiding the noise signal. The photodetector module 5b is similar to 5a.
Fig.6 shows the isometric view of mechanical columns and rail assembly. Fig.7 shows the complete apparatus with covers on the laser screen assemblies. Covers 41a and 41b made of metal or any hard plastic sheet, painted black inside, cover the laser screen assemblies 1a and 1b respectively. The covers 41 work as baffle to prevent ambient light or stray light entering the photodetector modules which makes the apparatus suitable for indoor and outdoor ambient lighting conditions. The performance of the apparatus is not affected by ambient light conditions.
Fig.8 shows the bock diagram of the electronic circuit. The obscuration of the laser screen 12a by a projectile 30 produces a decrease in the energy falling on the photodetector module 5a, which in turn creates a momentary decrease in the output DC current of the photodetector. To make the laser screen sensitive to even the smallest projectile of the size 5mm, very high speed operational amplifiers constituting different signal conditioning stages are used after the photodetector module 5a. 5a is used in photocoducting mode with reverse bias. High frequency components in the output signal of the photodetector increase exponentially with the speed of the projectile whereas the amplitude of the signal decreases which decreases the signal to noise ratio (S/N). High speed opamps inherently prone to noise, require careful circuit layout design, special grounding and shielding techniques to minimise the noise. Keeping in view the above points, preamplifier and signal conditioning circuits are designed. A preamplifier 40a constitutes a current to voltage conversion stage followed by an inverting amplifier stage. A variable DC reference voltage using a potentiometer 24a is subtracted by the subtractor 43a from the signal. Output of 43a is shown by an LCD display 25a. An optical - isolator 45a driven by a buffer 44a is used to transfer the output signal of 43a to the input of a mono shot circuit 46a to trigger it. The analogue and digital circuits are connected through optical-isolator to

minimise the noise. The above circuit is used for the laser assembly 1a. Similar circuit is employed for the laser screen assembly 1b with same numbering except the subscript 'a' is replaced by 'b' as shown in the fig.8. The circuitries related to 1a and 1b are called start and stop channels respectively. Whenever a projectile produces an interruption in the laser screen 12a which is connected to the start channel, Q output of the flip-flop 47 is set which enables the gate of the timing counter. Similarly an interruption in the laser screen 12b which is connected to the stop channel resets Q output of 47 which disables the gate of the timer counter. Gating pulse 48 is TTL compatible which is fed to a standard timer card 49 fitted in the computer. 5 MHz clock option of the timer card has been selected for measurement of ToF. Fig. 9 shows electronic circuit diagram. Photodetector module 5a with a photodiode of rise time 12 nanoseconds is used in photoconductive mode with the reverse bias. With increase in reverse bias, rise time decreases and dark current increases. Reverse bias is optimized to 14V to get high speed with minimum dark current. The preamplifier 42a comprises of two op-amps having high speed, fast settling time and a C2 compensation capacitor as shown in the fig.8. The first op-amp converts current to voltage while the second op-amp amplifies the signal. A variable DC reference voltage using a potentiometer 24a is subtracted by the subtractor 43a from the input signal to offsets the long term stability and also provides a reference signal for the subsequent circuit. Thus an initial condition is established by subtracting a DC voltage varied through a 10 turn potentiometer R6 mounted on front panel. The output signal of photodiode 5a decreases for a period for which projectile interrupted the laser screen. Change in output level of 5a depends on the projectile size but its duration depends on velocity and length. Actual voltage level and change in voltage are not much important as long as interruption is detected. Still variations should be within specified range so that detection is recorded. A high slew rate, fast settling time op-amp is used in the subtractor 43a. Output value of signal is displayed on an LCD 25a fitted on front panel of the electronic subsystem 2. For operating the system having 3mW laser diode fitted in the source module, initially LCD reading is adjusted within a range 0.9 to 1.2V. Output of 44a which is high speed transistor acting as driver to opto-isolator at ground initially. On interruption in the laser screen 12a, output of 43a decreases and as soon as it goes below 0.7V approximately, 42a is switched on. Opto-isolator 45a gives a momentary pulse which is related to momentary change in the light energy at photodetector at the

time of interruption of the laser screen by projectile and triggers the monoshot making signal TTL compatible. Rising edge of output pulse of monoshot sets the D flip-flop 47 high. Similarly, rising edge of monoshot of stop channel resets the D type flip-flop. The reset switch 26 gives initial ready condition which is further ensured by on state of the indicator 27. The gating pulse 48 is fed to the timer card 49 is plugged in the computer 3 with some special features such as software re-setting, software arming and disarming of the counter etc. The computer is provided with a user friendly software with graphical user interface developed in Visual C++. The software has provisions of inputting the variable fields like, date, test number, name of the user and operator, type of projectile and distance between laser screens through key board which are shown on monitor screen. User friendly messages at all operational stages of the system are displayed on monitor screen during the test and after measurement, velocity data is also shown on the screen. Apart from the above, the software also has provisions of calculation, data storage and test reports preparation. As per the requirement of the test, distance field data shown on the screen can easily be changed through keyboard. Different components namely rails, spacer bars, slide carriers and mechanical columns of the system are so fitted that these can be easily dissembled and reassembled without the rigorous alignment during installation. Modular design of the system permits that it can be shifted from one place to other place easily and also permits field replacement/repair of faulty components/modules.
The apparatus has provision for measurement of velocity of a projectile in the range 1
m/sec to 2500 m/sec which can be divided into three ranges namely, slow, medium or
fast. In case of projectiles whose velocities change rapidly, the screens have to be
placed nearer to each other, the apparatus has provision to do so.
Following is the mathematical analysis of the working of the present invention:
During the measurement of the velocity of the projectile it has to pass through two
similar laser screens placed at a known distance apart. Time of Flight (ToF) of the
projectile between the two laser screens is recorded by an electronic clock. Velocity of
the projectile under test is the ratio of the distance between the screens and ToF and
the error in the measurement of the velocity depends on these two factors.
The velocity measurement based ToF is governed by following equations:
(Equation Removed)
The error in the computed velocity and the measurement errors of distance and time are
related by,
(Equation Removed)
where;
v : Actual value of the velocity
X : Distance between the laser screens
t : Time of Flight (ToF)
dt and dx are the errors of measurement of ToF and distance between two screens respectively and which in turn produces dv error in the measurement of velocity according to the equation (2).
For example error contribution in measurement of the velocity of a projectile with nominal velocity (v) 1000 meter/sec is as follows:
Distance between the screens (x) : 1000 mm
Time of Flight (t) : 10"3sec
Distance measurement error (dx) : 0.5 mm
Time of Flight error (dt) : 0.5X10"6 sec
Velocity measurement error (dv) : 500mm or 0.5 meter
Velocity measurement error in percentage : 0.1% The invention is described in detail in the examples given below which are provided by the way of illustration and, therefore, should not be considered to limit the present invention in any manner.
During interpretation of the measured velocity data given under the examples It may kept in mind that projectile velocity measurement is of destructive type as recycling of the same projectile to be propelled with the same velocity second time is not possible and also as no two projectiles are identical in shape, size and weight, every projectile has a unique velocity different from all others including the same of similar kind. All these trials have been conducted with each laser screen of size 250mmX250mm, distance between the screens 1000 mm and few of the results are shown in the examples.

Example 1 For experimental testing of the short term stability of the system, the displays on both the LCDs have been set to the fixed known value of 1.00. The apparatus has been left undisturbed and displays have been noted after every fifteen
minutes for 6 hours. It has been observed that neither displayed value nor their difference changed by more than 5% and 5% respectively. The experiment has been repeated several times over a period of one week, the observation being the same every time. This indicates that the apparatus is stable over a day to yield consistent and precise results.
Example 2 For experimental testing of the long term stability of the system, the displays on both the LCDs have been set to the fixed known value of 1.00. The apparatus has been switched off at end of each day and switched on next day for a week. It has been observed that all display values have remained within the permissible limits. The results indicate that once the apparatus is fine tuned it need not retuned frequently.
Example 3 For experimental testing of the apparatus, velocities of slow and fast moving projectiles have been measured. A steel ball of 3 mm diameter thrown by human arm using different muscular power for each throw, lead pellet fired from air pistol and a projectile fired from a high velocity rifle have been used as test objects. Measured values are given in Table I. The results show that the apparatus is capable of measuring the velocity of both slow and fast moving projectiles.
Example 4 For experimental testing of the apparatus, the velocities of a number of projectiles of different calibers have been measured. The results are given in Table II. The experiment proves that the system yields precise measured values.
Example 5 For experimental testing, the apparatus has been used to measure the velocities of different projectiles under (i) outdoor conditions with about 100000 lux, (ii) indoor with artificial flood lights producing about 2000 lux directed towards the system, and (iii) darkroom conditions with all lights switched off except those forming the part of theapparatus. Results have shown that the apparatus yields consistent values.

Example 6
For experimental testing of precision of the apparatus, after conducting one round tests it has been dismantled and dismantled parts have been transported to another lab situated at a distance 25 km. The system has been reassembled in the new location and tested again. During reassembly no special toolings and rigorous alignment have been required , while the reassembled apparatus has given consistent results. This shows that the apparatus can be easily dismantled and reassembled without any special tool and hence portable. Advantages of the Present Invention
The main advantages of the present invention are:
1. It can measure the velocity of the both slow moving and fast moving projectiles, the measurable velocity range being 1 meter/second to 2500 meter/second.
2. For the same apparatus, laser screen can be modified to any preferred size whose height may be in the range of 100 mm to 1500 mm provided that the dimension is an integral multiple of the base length of a prism while the width may be in the range of 100 mm to 1500 mm without changing any other part of the apparatus except the columns and spacer bars.
3. The laser screen assemblies, comprising of laser diodes, a set of prism assemblies and photodetectors are housed in field replaceable mechanical columns enabling easy upscaling/downscaling of the screens.
4. For the same apparatus, the distance between the two laser screens can be adjusted to any value between 100 mm to 1000 mm by the operator in predefined discrete steps during the usage of the apparatus depending upon the projectile type and its anticipated velocity without changing any part of the apparatus.
5. Only one light source is used for each laser screen assembly irrespective of the screen size lying within the specified range.
6. The light level is uniform throughout the laser screen area making all zones equally sensitive such that the projectile need not be made to pass through any preferred zone.
7. The method is equally applicable for indoor and outdoor ambient lighting conditions and the same apparatus can used under any level of ambient light or background light without any modification/alteration.
8. Opto-isolators are used to effectively reduce the noise created during analogue to digital conversion of the signals at the same time simplifying the noise reduction circuitry.

Table I
(Table Removed)
Table II
(Table Removed)

We Claim:
1. A method of measuring the velocity of a projectile whose velocity may vary from 1 meter per second to 2500 meter per second using a pair of optical screens, each screen constructed by utilizing a laser diode, expanded-collimator to expand and collimate the laser beam, reflecting and refracting properties of prisms, a collector lens to focus the laser beam onto any standard high speed photodetector having high sensitivity such that any projectile passing through two screens, separated by a distance varying from 100mm to 1000 mm, in succession is sensed by the photodetectors and the signal generated by the photodetectors being used to start and stop a clock to measure the Time of Flight (ToF) of the projectile whose velocity is computed by dividing the known distance between the screens by ToF.
2. A method as claimed in claim 1, wherein narrow output beam from a standard laser diode of suitable power depending upon the screen size and may vary from 1 mW to 10 mW, emitting visible or near infra red radiation, is expanded and collimated by an optical beam expander-collimator constructed using spherical lenses; the beam being expanded to such an extent that center to edge intensity variation in the beam due to its Gaussian profile is small enough to enable the photodetector to sense the obscuration of the incident beam created by a projectile of minimum diameter of 5 mm traveling at a velocity lying in the range of 1 meter per second to 2500 meter per second while it crosses any part of the beam.
3. A method as claimed in claims 1-2, wherein a number of similar 45°-90°-45° prisms made of low dispersion optical glass, plastic or similar homogeneous isotropic transparent dielectric material is used to construct an optical screen.
4. A method as claimed in 1-3, wherein the length and width of the hypotenuse face of prism defines the beam cross section, the width varies from 1 mm to few millimeters converting the incident cross section of the expanded-collimated laser beam into a rectangle in order to make ToF measurement more precise and doing away with the need of cylindrical lenses.
5. A method as claimed in claims 1-4, wherein the size of the hypotenuse face of each prism is twice the size of the expanded-collimated laser beam and wherein, the orthogonally incident collimated laser beam on one half of the hypotenuse face is refracted undeviated inside the prism and gets reflected by the two other sides of the prism and finally emerges out of the prism as a parallel beam traveling in

opposite direction to the incident beam but displaced such that the incident and emerging beams from a continuous curtain of laser light.
6. A method as claimed in claims 1-5, wherein similar prisms used in plurality are mounted rigidly on two mechanical columns of same height with the hypotenuse faces of all prisms housed in the same column are coplanar and parallel to each other and also at right angles to the collimated laser beam and wherein, one of the columns houses the laser diode and beam expander-collimator at one end and a collector lens photodetector combination at the other end, the diameter of the collector lens being few millimeters larger than the expanded collimated laser beam and mounted at a suitable depth in side the column to cut off any light save the laser beam being received by the photodetector, while the other column houses only prisms; the two columns are erected rigidly in such a fashion that the hypotenuse faces of the prisms housed in one column faces and are parallel to the same on the other column and also the prisms in one column are so mounted that these are displaced by half the hypotenuse length measured along the array of prisms; these two types of columns form one set.
7. A method as claimed in claims 1-6, wherein a laser screen is created by using a set of columns housing prism assemblies such that, for the same apparatus, laser screen height can be modified to any value between 100 mm to 1500 mm in steps of integral multiple of prism hypotenuse length by simply replacing the columns, the width can be modified to any value in the range of 100 mm to 1500 mm by replacing the spacer bars and for the same apparatus the distance between two screens can be set to any value in the range of 100 mm to 1000 mm by the operator without replacing any part or component.
8. A method as claimed in claims 1-7, wherein sensitivity of each screen to accurately and precisely determine the time of arrival of the projectile in the screen is same through out the screen area since the same light beam, after back and forth reflection and transmission, reaches the photodetector module, fall in output of the photodetector due to the obscuration by a projectile is independent of the projectile position within the defined screen area.
9. A method as claimed in claims 1-8, whBrein the columns with prealigned prisms are mounted rigidly on a frame and columns the frames and other relevant parts are modular in design and construction such that the laser screens it is possible to

easily dismantle, reassemble and/or up scale/down scale and/or change the inter-screen distances on site without the need of rigorous alignment during erection. 10.A method as claimed in claims 1-9, wherein laser screens can be mounted on vibration isolation pads and covered with metal or any hard plastic cover painted black inside to work as baffle to prevent any ambient light or stray light from being incident on the collector lenses to enable the method being applicable for indoor and outdoor ambient lighting conditions.
11. A method as claimed claims 1-10, wherein the photodetector output of each
screen forms a part of a signal processing channel comprising of a preamplifier and
amplifier, variable reference voltage generator for subtraction from the inverted
signal, opto-isolator; and wherein the processed signal from two channels are used
to start and stop a clock interfaced with a remote computer; and system design
permits the use of standard components and devices.
12. A laser based apparatus for projectile velocity measurement comprising of two
laser screen assemblies, elebtronic sub-system, firmware and mechanical mounts
with cover; each screen assembly comprising of:
• a pair of first and second column,
• a laser diode producing visible or near infra red light signal,
• a beam expander-collimator producing expanded and collimated light,
• a set of transmitting and reflecting prisms receiving collimated light from the beam expander-collimator, and
• a photodetector module having a photodetector and a collector lens focusing the collimated light signal received from the set of prisms on to the photodetector,
electronic sub-system comprising of:
• power supply unit,
• opto-electronic and electronic signal processing unit,
• LED and LCD displays,
• potentiometer, and
• computer interface hardware, firmware comprising of:
• user interface with computer software

• system operating software, and
• computational software, mechanical mounts with cover comprising of:
• rails, sliding carriers mounting the columns and the spacer bars,
• light weight base plate
• vibration isolation pads supporting the base plate, and
• cover

13. The apparatus as claimed in claim 12, wherein the first column houses the source module, prism assembly and photodetector module.
14. The apparatus as claimed in claim 12, wherein the second column houses only the prism assembly.
15. The apparatus as claimed in claim 12, wherein laser diode power varies in the range of 1 mW to 10 mW depending upon the screen size and end application.
16. The apparatus as claimed in claim 12, wherein the laser diode emit radiation lying in the range of visible to near infra red region of electromagnetic spectrum.
17. The apparatus as claimed in claim 12, wherein laser diode is mounted in a holder coupled with beam expander-collimator having a provision of easy replacement and quick alignment.
18. The apparatus as claimed in claim 12, wherein all the prisms are 45°-90°-45° type and are identical within standard tolerances.
19. The apparatus as claimed in claims 12 and 18, wherein sharp corners of each prism are ground by a fraction of a millimeter avoiding the chance chipping.
20. The apparatus as claimed in claims 12 and 18-19, wherein hypotenuse face of each prism is coated with thin film antireflection coating corresponding to the laser diode wavelength used reducing the loss of collimated beam power on successive reflections.
21. The apparatus as claimed in claims 12,18-20, wherein all the prisms are made of low dispersion glass or any other optical grade transparent homogenous isotropic dielectric.
22. The apparatus as claimed in claims 12 and 18-21, wherein the length of hypotenuse of a prism is twice the diameter of the collimated laser beam while width varies from 1mm to few millimeters depending upon the sensitivity of the
photodetector used.

23. The apparatus as claimed in claims 12, wherein each prism assembly is constructed in a manner that the hypotenuse faces of all prisms in the assembly are parallel and coplanar.
24. The apparatus as claimed in claims 12 and 23, wherein the 45° corner of each prism in each prism assembly is in contact with the similar corner of the prism adjacent to it.

25. The apparatus as claimed in claims 12, 23-24, wherein the prisms in a prism assembly are so arranged that their corresponding ground faces are coplanar.
26. The apparatus as claimed in claims 12, 23-25, wherein one of the ground faces of all prisms in a prism assembly are glued to a parallel rectangular glass plate by suitable strain free glue while the other ground faces of all prisms in the prism assembly are glued to another parallel rectangular glass plate by suitable strain free glue such that prealigned prisms are held together rigidly sandwiched between two glass plates.

27. The apparatus as claimed in claims 12, 23-26, wherein for each screen, each prism assembly formed by sandwiching between two glass plates is housed inside the first column such that the hypotenuse faces of the prism array are parallel to the column.
28. The apparatus as claimed in claims 12, 23-27, wherein for each screen, each prism assembly formed by sandwiching between two glass plates is housed inside the second column such that the hypotenuse faces of the prism array are parallel to the column.
29. The apparatus as claimed in claims 12, 23-28, wherein all the prism assemblies in all the respective columns are rigidly held in place yet can be easily taken out for replacement by tightening or loosening a set of screws.
30.The apparatus as claimed in claims 12, 27-29, wherein the first and second columns of each screen are so mounted that the hypotenuse faces of the prism assembly in the first column are parallel to and face the corresponding surfaces of the prism assembly in the second column.
31. The apparatus as claimed in claims 12, 27-30, wherein all the mechanical columns together with prism assemblies housed inside, can be easily and rigidly fixed to a frame, inter-column distances being maintained by a set of field replaceable spacer bars, in such a manner that the columns and space-bars can

be taken out of the sliding carriers for replacement by larger or smaller columns to enable on site upscaling or downscaling the screen size by changing the screen width varying from 100mm to 1500 mm and the screen height varying from 100 mm to 1500 mm in steps of an integral multiple of the hypotenuse length of a prism and/or changing of the inter-screen distances in the range of 100 mm to 1000 mm.
32. The apparatus as claimed in claim 12, wherein all the internal walls of all mechanical parts housing optical and electro-optical components or devices are painted dull black minimizing the optical noise due to internal reflections of stray light from the inner walls of mounts and houses.
33. The apparatus as claimed in claim 12, wherein the photodetector module is mounted in a recess in the first column in each laser screen assembly blocking any light other than expanded collimated laser beam making the apparatus performance independent of ambient light.
34. The apparatus as claimed in claim 12, wherein, in each laser screen assembly, a continuous curtain of laser light of rectangular geometry is created by the back and forth reflection of the same laser beam enabled by the transmission and reflection properties of a 45°-90°-45° prism when it receives collimated laser beam incident normally on one half of the hypotenuse face.

35. The apparatus as claimed in claims 12, and 34, wherein, in each laser screen assembly, a continuous curtain of laser light of rectangular geometry is created by the displacement of the laser beam and its emergence from the other half of the hypotenuse face using transmission and reflection properties of a 45°-90°-45° prism when it receives collimated laser beam incident normally on one half of the hypotenuse face.
36. The apparatus as claimed in claim 12, wherein an electronic power supply module supplies required stabilized power supplies to laser diodes, photodetectors, electronic circuitry, display devices and other opto-electronic, electronic devices, components and circuits incorporated in the apparatus.
37. The apparatus as claimed in claims 12 and 36, wherein the power supply module is powered either by mains supply or 24 V, 300 mA battery.
38. The apparatus as claimed in claim 12, wherein a dual channel signal processing unit is provided such that first channel processes the signal received from the

photodetector module of the first screen and the second channel processes the signal received from the photodetector module of the second screen.
39. The apparatus as claimed in claims 12 and 38, wherein in each channel the
received signal from each photodetector module is converted from current to
voltage.
40. The apparatus as claimed in claims 12, 38 and 39, wherein the signal processing unit inverts and amplifies the input signals.
41. The apparatus as claimed in claims 12, 38-40, wherein in each channel a variable dc signal is generated and subtracted from the inverted amplified signal to offset drifts affecting long term stability.
42. The apparatus as claimed in claims 12, 38-41, wherein in each channel a reference signal is generated after subtraction.
43. The apparatus as claimed in claims 12, 38-42, wherein the variable voltage in each channel is provided to fine tune the apparatus for higher precision and also to offset low frequency drifts in laser power outputs and aging effects of the laser diodes and the photodetectors.
44. The apparatus as claimed in claims 12, 38-43, wherein a potentiometer is provided in each channel for manual adjustment of the voltages generated for subtraction.

45. The apparatus as claimed in claims 12, 38-44, wherein a Liquid Crystal Device (LCD) displays the signal level in each channel after subtraction.
46. The apparatus as claimed in claims 12, 38-45, wherein it is possible, using the two potentiometers, to manually and easily adjust the data, corresponding to the first and second laser screens as displayed on the two LCDs to within 30% of a fixed known value to offset the drift produced by electronic circuitry improving the sensitivity of measurement, the fixed value being same for both the laser screens.

47. The apparatus as claimed in claims 12 and 38-46, wherein two manual potentiometers can be adjusted easily to bring the two LCD displays very close to each other improving apparatus precision; the permissible limit of the difference being 10%.
48. The apparatus as claimed in claims 12 and 38, wherein an opto-isolator is provided in each channel in the signal processing unit to eliminate line interference and other cross channel noise.

49. The apparatus as claimed in claims 12 and 38, wherein, in the first channel, a
flipflop after receiving signal from a monoshort starts a clock whenever a projectile
interrupts the first laser screen.
50. The apparatus as claimed in claims 12, 38 and 49, wherein, in the second channel, the flipflop after receiving a signal from a monoshort stops the running clock whenever the projectile interrupts the second laser curtain.
51. The apparatus as claimed in claims 12 and 38, wherein reset circuit permits automatic as well as manual resetting through a reset button in the signal processing unit to initialize and reset the apparatus.
52.The apparatus as claimed in claim 12, wherein apparatus incorporates standard laser diode, optical components, standard high speed opto-electronic and electronic devices and is adequately shielded from EMI/EMC.
53. The apparatus as claimed in claim 12, wherein a computer interface hardware is provided enabling remote apparatus by any standard IBM PC or equivalent.
54. The apparatus as claimed in claim 12 and 53, wherein a firmware provides user friendly software for data entry needed for apparatus operation, controls the apparatus operation, computes the desired velocity from the time interval between the starting and stopping of the clock, stores data and formats these to give soft and hard copies.
55. The apparatus as claimed in claims 12, 53 and 54, wherein the firmware is installed in an IBM PC and creates a data base of results obtained by the apparatus, the amount of data stored depending upon the storage capacity of the computer hard disk.

56. The apparatus as claimed in claims 12 and 33, wherein a cover made of thin metal or any hard plastic and painted/anodised dull black on all surfaces is provided to ensure that the energy of any stray light falling on the active area of each photodetector is negligibly small compared to the energy of the focused laser beam falling on the same area of the same photodetector.
57. The apparatus as claimed in claim 12, wherein all the mechanical parts and the chassis housing the electronics are dully painted or anodized eliminating atmospheric corrosion.
58. The apparatus as claimed in claim 12, wherein an array of LEDs permits on- line health monitoring of the apparatus.

59. Laser Based Apparatus for Projectile Velocity Measurement substantially as herein described with reference to the examples and drawings accompanying this specification.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=8Q2EbX/MwtfARWfXi6IXbQ==&loc=+mN2fYxnTC4l0fUd8W4CAA==

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Patent Number

277154

Indian Patent Application Number

790/DEL/2005

PG Journal Number

48/2016

Publication Date

18-Nov-2016

Grant Date

11-Nov-2016

Date of Filing

31-Mar-2005

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN, RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	CHANDER MOHAN	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
2	RAM PRAKASH BAJPAI	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
3	VIRENDRA SINGH SETHI	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
4	MANPREET SINGH	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
5	RAJESH KUMAR VERMA	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
6	RAMESH CHAND KALONIA	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
7	GAUTAM KUMAR	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
8	AMOD KUMAR	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
9	BUDHI BALLABH BAHUGUNA	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
10	SHASHI SHARMA	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
11	ASHOK KUMAR SOBTI	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.
12	MUKESH KUMAR GUPTA	CENTRAL SCIENTIFIC INSTRUMENTS ORGANISATION (CSIO) SECTOR-30, CHANDIGARH-160030, INDIA.

PCT International Classification Number

G01C 3/08

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA