Title of Invention

NEAR INFRARED SPECTROPHOTOMETER

Abstract

ABSTRACT INFRARED SPECTROPHOTOMETER This invention relates to infrared spectrophotometer which comprise a rotating mechanical chopper to chop poly chromatic radiation from the source of light. The chopped beam is focussed on to the entrance slit of a monochromator Concave grating of a monochromator disperses and focuses the beam to the slit Monochromated beam from the exit slit gets incident on a reference sample or sample under test. The emerging beam is detected by a two-color detector and gets processed. The data is acquired, interpreted and presented in the desired form on the display.

Full Text

The present invention relates to an infrared spectrophotometer, the main embodiment of which lies in the fact that the spectrophotometer has been provided with sample compartment which can be replaced at the customer site for reflectance or transmittance mode for solid sample or liquid sample analysis respectively BACKGROUND OF THE INVENTION Spectroscopy has been defined the study of the absorption and emission of light and other radiation, as related to the wavelength of the radiation. The spectroscopic methods are helpful in almost all technical fields, especially for identifying constituents and processes in any source that emits light. With spectroscopic methods one can achieve various aims which might vary from identifying an element in a mixture to determine the structure of an unknown molecule. Besides these, the spectroscopic techniques have also found place in unraveling the scientific principles behind the structure of atom and various properties exhibited by atoms of different elements. EP Patent No. EP 02 04090 teaches a spectrophotometer comprising a monochromator, a sample compartment and photodetector, which is in form of an integrating sphere. U.S. Patent No. 4,804,266 discloses a system for rapid scan spectral analysis comprising a concave holographic diffraction grating continuously rotated at a constant angular velocity . to provide a rapid scanning monochromator. The angular velocity and angular acceleration of the grating are calculated to control the analog to digital converter sampling rate across the region of interest. SUMMARY OF THE INVENTION The present invention relates to a spectrophotometer where the main embodiment of lies in the fact that the spectrophotometer has been provided with sample compartment which can be replaced at the customer site for reflectance or transmittance mode for solid sample or liquid sample analysis respectively. The amount of reflected or transmitted energy from the sample is a measure of the concentration of the constituent molecules in the sample. The two color detector used in this instrument has completely eliminated the movement of detector mechanism to select Silicon or Lead sulphide detector depending on the wavelength selected. Concave grating based monochromator increases the light throughput and hence the signal to noise ratio. Radiation from the source, a quartz halogen lamp is focused into a parallel beam which is projected onto the material to be measured. A rotating wheel containing near infrared optical interference filters, spinning in the optical path, provides the means of selecting narrow bands of infrared light at wavelengths corresponding to reference and Absorption wavelengths. Typically the wheel rotates between 2 0 and 4 0 Hz effectively providing a continuous measurement many times per second. The pulses of light arriving on the product are both partially scattered and partially absorbed. A portion of the scattered light is collected by 'Collecting mirror' and focused onto detector. The detected signal are amplified and since both the reference and absorption beams have, taken the same optical path any change in the value of computed log ratio will relate to absorption by the component to be measured. STATEMENT OF THE INVENTION According to the present invention there is providea an inrrarea spectrophotometer comprising a sample compartment for holding two sample cells, an optical system radiating a parallel beam projected onto the material to be measured, characterized in that it comprises a monochromator having light gathering capacity in the wavelength range of 600-2000 nm with a concave holographic grating having 3 00 lines/mm, means for selecting narrow bands of infrared light such as herein described corresponding to monochromator reference and absorption wavelengths, two colour detector, a peltier cell built into the said two color detector for controlling the temperature of said detector to enable the detector to operate at a constant temperature, cooling means attached to the said detector for cooling the said detector, a pair of operational amplifiers for summation of the out put from said detector for data acquisition and data processing and means for controlling the rotation of said grating and said sample holders such as herein described. DETAILED DESCRIPTION OF THE INVENTION Radiation from the source, a quartz halogen lamp is focused into a parallel beam which is projected onto the material to be measured. A rotating wheel containing near infrared optical interference filters, spinning in the optical path, provides the means of selecting narrow bands of infrared light at wavelengths corresponding to reference and Absorption wavelengths. Typically the wheel rotates between 20 and 40 Hz effectively providing a continuous measurement many times per second. The pulses of light arriving on the product are both partially scattered and partially absorbed. A portion of the scattered light is collected by 'Collecting .mirror' and focused onto detector. The detected signal are amplified and since both the reference and absorption beams have taken the same optical path any change in the value of computed log ratio will relate to absorption by the component to be measured. The main principle behind the spectrophotometer is: that when incident on a sample gets reflected or transmitted depending on the nature of the sample. In the reflectance mode incident radiation penetrates the surface of the sample through some layers, excites the vibrational modes of the molecules of the sample and gets scattered in all directions. In transmission mode, the incident radiation gets transmitted through the sample with diminished energy. The amount of reflected or transmitted energy from the sample is a measure of the concentration of the constituent molecules in the sample. The present invention relates to a NIR Spectrophotometer with sample compartment which can be replaced at the customer site for reflectance or transmittance mode for solid sample or liquid sample analysis respectively. The two color detector used in this instrument has completely eliminated the movement of detector mechanism to select Si or PbS detector depending on the wavelengfid|\selected. Concave grating based monochromator increases the light throughout the signal to noise ratio. DESCRIPTION OF THE ACCOMPANYING DRAWINGS The present invention can be understood in a better way with the help of the accompanying drawings, wherein:- Fig. 1. relates to an instrument block diagram for the near infrared spectrophotometer. Fig. 2. is an optical layout of NIR spectrophotometer. Fig. 3. represents power supply block diagram for near infrared spectrophotometer. Fig. 4. covers a thermoelectric cooler block diagram, Fig. 5. represents stepper motor controller block diagram, and Fig. 6. shows the PC block diagram. The block diagram of NIR Spectrophotometer is shown in figure 1. Each block of figure 1 is explained below with cross reference to the detailed figures indicated v/ithin the brackets after the block heading. BLOCK 1 LIGHT SOURCE A high wattage tungsten halogen lamp having a quartz envelop emitting in the range 340-3000 nm is used as a light source. The source is mounted in a well ventilated source chamber so that no conductive, convective or radiative heat transfer takes place from the source housing to the rest of the instrument. BLOCK 2 SOURCE OPTICS (Refer Figure 2) A reflective optical system is employed to collect the radiant Energy beam from the light energy source (1) and focus the same onto the entrance slit of the monochromator (4). This optical system consists of the following optical components: A plane mirror (2A) to direct the radiant energy beam from the light source towards another concave mirror (2B). A concave mirror (2B) collects the above directed radiant energy beam and focuses onto the entrance slit (4A) of monochromator. BLOCK 3 LIGHT CHOPPER (Refer figure 2) A chopper wheel (3A), with multiple slots , driven by a constant rpm DC motor (3B) is positioned at the entrance slit (4A) of the monochromator(4) to pulse the input radiant energy beam received by the dispersive element (4C) . The chopper wheel while rotating passes through the slot of an optical switch (3C) placed at appropriate position, and interrupts the light emitted by the light emitting diode of the optical switch resulting in a pulsating waveform at the detector output of the optical switch (3C). This pulse train is utilised to synchronise the data acquisition sequence of Analog to Digital converter (12A) with the pulsed radiant energy beam received by the dispersive element (4C) from the light source(1). The chopper motor (3B) and the optical switch {3C) are powered by chopper motor power supply (21). 6L0CK 4 MONOCHROMATOR (Refer figure 2) Since the monochromator is the heart of the equipment, features are provided to ensure spectral purity, resolution and light gathering capacity in the specified wavelength range of 600-2500 nra. An optical filter (4B) is placed at the entrance slit (4A) of the monochromator to cutoff the radiant energy below 600 nm coming from the light source. The wavelength discrimination is achieved by employing a 40X45 mm concave holographic grating (4C) having 300 lines/mm with sufficient light gathering capacity. Use of the concave grating eliminates the need for two additional mirrors required (for collimating the input polychromatic radiant energy beam and decollimating the dispersed spectrum onto the entrance slit of the monochromator) when the plane grating is used, thereby increasing the light through put from the monochromator. The grating is mounted on a stable platform having thrust and ball bearing, that can be rotated at any required speed. The system is rotated by a stepper motor driven micrometer for accuracy of wavelength and speed of rotation. BLOCK 5 SAMPLE IRRADIATION OPTICS A planoconvex lens (5) is place after the exit slit (4D) of the hionochromator (4) to render an intense parallel beam of monochromatic radiation illuminates the sample of wide area. BLOCK 6 SAMPLE COMPARTMENT (Refer figure 2) A large sample compartment that ensures a constant temperature of the sample while recording spectra is provided. A stepper motor driven sample holder (6A) mechanism accommodates two sample cells one for reference and one for one sample. BLOCK 7 REFLECTION COLLECTION OPTICS (Refer figure 2) The light reflected from the sample will have two components, diffuse and spicular. Only the diffuse component, produced by light that has penetrated just beneath the surface, contains any information about the composition of the sample because it has been exposed to the possible absorbers present in the sample. The spicular component, produced by the glare of the glass over the sample and by the surface glare of the sample, contains no information about the composition of the material. Therefore it is highly desirable to measure only the diffuse reflectance of a sample. Two concave mirrors (7A, 15A) are place on either side of the 0 sample at 45 angle to the surface of the sample to collect the diffuse radiation. This configuration minimises the amount of spicular radiation collected, A short focal length spherical lens (7B, 15B) is placed after each of the concave mirror to focus the diffuse' radiation. collected by the concave mirror, onto the two color detector (8, 16) . BLOCK 8 DETECTOR The sensing of the diffuse radiation is performed by a two color Si/PbS detector. The system covers the entire NIR range without loss of sensitivity. Si detector acts as a bandpass filter from 1100 nm onwards where PbS acts as detector. The detector is 0 configured to operate at a constant temperature of 10 ' C for efficient performance and noise reduction. The detector is cooled by a Peltier cell built into the detector itself. BLOCK 9 THERMOELECTRIC COOLER (PELTIER CELL) FOR THE DETECTOR (Refer figure 4) The resistance of the PbS is resistive to temperature. In order to get stable output from this detector, it is required to keep the temperature of this detector constant. It's resistivity is inversely proportional to the temperature. The two color detector (8,16) module consists of one Peltier cell (9) and one thermistor (9B). Peltier cell (9) is a semiconductor junction. Depending on the direction of the current flow it generates the heat or absorbs the heat. The resistance of thermistor (9B) decreases with the increase in temperature. These two elements_are used to keep the temperature of the detector constant an 10 C; A constant current source (9A} ot luu (A is empioyea co oxas the thermistor (9B). As the resistance of the thermistor changes due to temperature the voltage across the thermistor changes due to temperature the voltage across the thermistor also changes. This voltage is buffered by an operational amplifier configured as non-inverting buffer (9C). A fixed voltage is derived from a precision reference zener diode with suitable amplification by an operational amplifier (9D). The difference in thermistor voltage and the fixed voltage is given by an operational amplifier configured as differential amplifier (9E) . The output of difference amplifier is amplified by an operational amplifier. The output of this amplifier is given to the non inverting terminal of an operational amplifier. The output of this amplifier is given to the non inverting terminal of an operational amplifier used as driver amplifier (9F). Feed back (91) to the driver amplifier is given to the inverting terminal of the operational amplifier. This driver amplifier (9F) drives a complementary matched transistor pair (9G) depending on the direction of current flow required to keep the temperature of the detector at the set value (9D) . If the temperature of the detector is higher than the actual temperature, the voltage across the thermistor will be lower than the fixed set voltage (9D) and the output of the differential amplifier (9F) is negative. This voltage is amplifed and given to driver amplifier (9F) which drives the appropriate transistor (9G) to pump more current into the Peltier cell to cool the detector. When the temperature of the detector is lower than the actual voltage, another transistor is driven to reverse the direction of current flow in the Peltier cell to heat the detector. The high current transistors are powered by TE cooler power supply (16) . The other part of the circuit is powered by dual power supply (13). BLOCK 10 INTERFACE CIRCUIT FOR Si DETECTOR An operational amplifier, having ultra low bias current (fA), is configure as 'current to voltage converter' to convert the photocurrent produced by the Si detector into a voltage signal. This circuit operates on dual power supply (20). BLOCK 11 INTERFACE CIRCUIT FOR PbS DETECTOR The PbS detector's resistance decreases when light falls on it which is proportional to the intensity of light. The PbS detector is biased by resistor network. The light on the detector is chopped by chopper. The resistance of the PbS detector changes with the chopping sequence ON and OFF. The voltage changes across the PbS detector is amplified by an operational amplifier configured as non-inverting amplifier. The DC voltage across the detector is filtered by capacitors. This circuit operates on dual power supply (20). "BLOCK 12 PC (Reter tigure b; PC performs various functions like data acquisition, data processing, control of stepper motors through digital input output and timer (DIOT) card (12B), user interface through keyboard (12C), data presentation on video monitor (12D) and printer (12E). A standard multi channel 12 bit Analog to Digital converter card (12A) is used to acquire from the two detector channels (Si and PbS) of the instrument. BLOCK 13 SIGNAL CONDITIONING OF Si DETECTOR CHANNEL An operational amplifier is configured as summing amplifier to add both the outputs from Si detector interface circuits (10 & 18). The output of the summing amplifier is further amplified by a Programmable Gain amplifier. The programmable gain amplifier is configured as a non-inverting amplifier using an operational amplifier. The required gain of the programmable gain amplifier is set by selecting the appropriate gain setting resistor network of this non inverting amplifier. The Analog to Digital converter card (12A) used for data acquisition requires that input signal be in the range of -5 to +5V. The output of the programmable gain amplifier is in the range of Oto +10V. In order to utilize the full input range of the Analog to Digital card, it is required to offset the output of the programmable gain amplifier by -5V. This offset voltage is derived from a precision reference zener diode with suitable amplification by an operational amplifier. This offset voltage and the output of the programmable amplifier are fed to a differential amplifier, configured using operational amplifier. The output of this differential amplifier is in the range of -5 to +5V which is acquired by PC (12) through one channel of Analog to Digital Converter (12A). This circuit operates on dual power supply (20). BLOCK 14 SIGNAL CONDITIONING ON PbS DETECTOR CHANNEL An operational amplifier is configured as summing amplifier to add both the outputs from PbS detector interface circuits (11 &19) . The output of the Summing amplifier is further amplified by a Programmable gain amplifier. The programmable gain amplifier is configured as a non inverting amplifier using an operational amplifier. The required gain of the programmable gain amplifier is set by selecting the appropriate gain setting resistor network of this non inverting amplifier. The output of this programmable gain amplifier is acquired by PC (12) through another channel of Anlaog to Digital Converter card (12A) . This circuit operates on dual power supply (20). BLOCK 15 Refer block 7 BLOCK 16 Refer block 8 BLOCK 17 Refer block 9 BLOCK 18 Refer block 10 BLOCK 19 INTERFACE CIRCUIT FOR PbS detector Refer block 11 BLOCK 20 ANALOG POWER SUPPLY (+V,-V) (Refer figure 3) This supply is used to power the analog circuits. The offset voltage of the operational amplifiers used in the analog circuit is sensitive to the 'net difference in the positive and negative supply rails' applied to the circuit. If two independent regulators for positive and negative supplies are used, the difference in their drift characterisitics causes a drift go the net difference in power supply rails' which changes the offset voltage of the operational amplifiers. To avoid this problem a dual tracking regulator configuration is used where the negative supply tracks the positive supply resulting in zero change in the net difference in the power supply rails. An AC voltage of 15-18 V derived from a secondary winding of transformer (26) is rectified using a center tapped diode bridge rectifier (20A), and filtered with capacitive filter (20B). An adjustable floating regulator (20C) is used to provide the positive power supply. A potential divider network (20D) using a matched pair of resistors is connected across the positive and negative rails of the power supply. The voltage from this potential divider network (20D) is compared with the power supply return potential by an operational amplifier configured as error amplifier (13E) to generate the error signs. This error signal is amplified by the error amplifier (20E) to drive a series pass transistor (20F) to provide regulation of the negative supply. A short circuit protection is provided by means of a transistor (20G). BLOCK 21 CHOPPER MOTOR POWER SUPPLY (Vcc) (Refer figure 3) This supply is used to power chopper motor (3B) and optical switch (3C). An AC voltage of 8-lOV derived from secondary winding of transformer (20) is rectified, using a diode bridge rectifier (21A), and filtered with capacitive filter (21B). A,three terminal tixed voltage regulator i^iu; is usea to proviae the regulated power supply. BLOCK 22 STEPPER MOTOR POWER SUPPLY (Vm) (Refer figure 3) This supply is used to energi^ft a number of stepper motors used in the instrument to control various mechanisms. Independent power supplies are used for each of the stepper motors used in the instrument. An AC voltage of 15-18 V derived from secondary winding of transformer (26) is rectified, using a diode bridge rectifier (22A, 22D), and filtered with capacitive filter (22B, 22E). Three terminal fixed voltage regulators (22C, 22F) are used to provide the regulated power supply. BLOCK 23 THERMOELECTRIC COOLER POWER SUPPLY (Refer figure 4) This power supply provides a constant voltage with high current capacity required to drive the Peltier cell (9). An AC voltage of 8 to lOV derived from secondary winding of transformer (26) is rectified, using a diode bridge rectifier (23A, 23F), and filtered with capacitive filter (23B, 23D,230,231) Three terminal fixed voltage regulators (23C, 23E,23H,23J) are used to provide the regulated power supply. BLOCK 24 LAMP POWER SUPPLY A Standard high wattage switching mode power, supply with less ripple and noise in the output, is used to power the light source (1) . BLOCK 25 MAINS INPUT (Refer figure 3) The instrument is powered from a 230(10 % Vac, 50Hz, 1 Phase . BLOCK 26 TRANSFORMER (Refer figure 3) The transformer primary is powered from mains supply (19) . Multiple secondary windings with appropriate voltage and current ratings are provided to supply to various power supplies (13, 14, 15, 16) . BLOCK 27 STEPPER MOTOR CONTROLLER (Refer figure 5) This instrument has number of stepper motor controllers to drive the various mechanisms like Grating drive Sample holder The PC (12) initialises these motors to their homing positions when the instrument is switched on with the help of a 'homimg sense signal' generated by the optical switches mounted on respective motors. The PC (12) sends the control signals through Digital Input Output and Timer (DIOT) card (12B) to respective transistor driver IC(27A) which drives respective motor (27B). The homing sense signal generated by the optical switch (27C) through DIOT card (12B) by the PC (12) to stop the control signals and thereby the motor (27B). The stepper motor controller circuit and the stepper motors are powered by stepper motor power supply (22). The homing circuit for grating consists of two optical switches. The detectors of these optical switches are connected in series so that it employes AND logic. The optical switch for the cuvetter homing circuit is positioned such that in the homing position reference Cuvetter comes in the light path. WE CLAIM:- 1- An infrared spectrophotometer comprising a sample compartment for holding two sample cells, an optical system radiating a parallel beam projected onto the material to be measured characterized in that it comprises a monochromator having light gathering capacity in the wavelength range of 600-2000 nm with a concave holographic grating having 300 lines/mm, means for selecting narrow bands of infrared light such as herein described corresponding to monochromator reference and absorption wavelengths, two colour detector, a peltier cell built into the said two color detector for controlling the temperature of said detector to enable the detector to operate at a constant temperature, cooling means attached to the said detector for cooling the said detector, a pair of operational amplifiers for summation of the out put from said detector for data acquisition and data processing and means for controlling the rotation of said grating and said sample holders such as herein described. 2. An infrared spectrophotometer as claimed in claim 1, wherein the said optical system comprises a reflective optical system to collect the radiant energy beam from light source and focus the same onto the entrance slit of the said monochromator. 3. An infrared spectrophotometer as claimed in claim 2, wherein the said reflective optical system comprises a plane mirror to direct radiant energy beam from the said light source towards a concave mirror which in turn focuses onto the entrance slit of the said monochromator. 4. An infrared spectrophotometer as claimed in claim 1, wherein the means for selecting narrow bands of infra red light comprises a chopper wheel with multiple slots, driven by a constant rpm motor rotating at 20 to 40 Hz positioned at the slit of the said monochromator. 5. An infrared spectrophotometer as claimed in claim 1, wherein an optical filter to cut off light waves having wave length below 600 nm is placed at the entrance slit of the said monochromator. 6. An infrared spectrophotometer as claimed in claim 1, wherein two color detector comprises a silicon photo detector and a lead sulphide detector. 7. An infrared spectrophotometer as hereinbefore described with reference to the accompanying drawings.

Documents:

1847-mas-1996 abstract duplicate.pdf

1847-mas-1996 abstract.pdf

1847-mas-1996 claims duplicate.pdf

1847-mas-1996 claims.pdf

1847-mas-1996 correspondence others.pdf

1847-mas-1996 correspondence po.pdf

1847-mas-1996 description (complete) duplicate.pdf

1847-mas-1996 description (complete).pdf

1847-mas-1996 drawings duplicate.pdf

1847-mas-1996 drawings.pdf

1847-mas-1996 form-1.pdf

1847-mas-1996 form-26.pdf

1847-mas-1996 form-4.pdf

1847-mas-1996 petition.pdf