|Title of Invention||
"AN OPTO-ELECTRONIC REFRACTOMETER FOR MEASURING THE REFRACTIVE INDEX OF LIQUIDS."
|Abstract||This invention relates to an opto-electronic refractometer (1) for measuring the refractive index of liquids comprises, an isosceles prism (2) having a base (3) and having a first and second to equal faces (8,9) disposed opposite to each other, a source of light (5) with a lens system (6) disposed in the proximity of said first face (8) as to provide a convergent beam of light (7) fall at right angle on said first face, the base (3) of said prism (2) being interfaced so that the light beam is partially reflected and partially transmitted; characterized in that, an array of pinholes (10) disposed in the planeparallel to said second face (9), a converging lens (11) for receiving said light from the array(10), an measuring means (12) such as optical power meter for measuring the light from said lens.|
|Full Text||FIELD OF INVENTION
The present invention relates to an opto-electronic refractometer sensor having a high sensitivity and capable of determining the refractive index (r. i) of liquids with a very high degree of sensitivity and accuracy.
Reference may be made to application no. 968/Del/99 of 13.07.1999 in the name of same Applicant. Almost all the conventional refractometers measure angles to determine the r.i. Abbe. Pulfrich. Hilger and Chance refractometers are some of the examples of such type of refractometers. The automatic versions of these refractometers available in the market (like those manufactured by Dr. Wolfgang Kernchen Optil-Electronik-Automation. Germany) make use of an opto-electronic scanner to detect the bright/dark borderline. Angle measurements involved in all these types of refractometers cause the size of the instrument to enlarge with the increase in the precision with which r.i. is measured.
Designs available for more compact fibber optical sensors (K. Spenner. M.D. Singh. II. Schulte and H. I Boehnel. SPIE MS 108. pp.424-426) employ uncaldded glass fibber as sensing probe. Consequently, these devices are very fragile necessitating very careful handling. Also, the process for cleaning the sensor probe after each use is very cumbersome as it involves cleaning of the fibber with acid as well in an ultrasonic bath containing alcohol.
OBJECTS OF THE INVENTION
The main object of the present invention is to propose an opto-electronic refractometer sensor fabricated which has a very high sensitivity over a large range of refractive indices.
Another object of this invention is to propose a refractometer sensor which is sturdy, cost effective and compact in size.
Still another object of this invention is to propose a refractometer sensor which has refractometer sensitivity approaching theoretical limits.
Yet another object of this invention is to propose a refractometer sensor which is opto-electronic in nature.
A further object of this invention JS to propose a refractometer sensor which can measure refractive index of the ambient on-line, continuously.
A still further object of this invention is to propose a refractometer sensor, which has no moving parts.
STATEMENT OF INVENTION
According to this invention there is provided an opto-electronic refractometer for measuring the refi active index of liquids comprising of :
(a) an isosceles prism having a base and a first and second equal faces disposed opposite to each other;
(b) a source of light disposed in the proximity of said first face so as to provide a convergent beam of light falling at right angle on said first face;
(c) the base of said prism being interfaced so that the light beam is partially reflected and partially transmitted; characterized in that;
(d) an array of pinholes disposed in a plane parallel to said second face;
(e) a screen having a pinhole is provided in front of the array;
(f) a converging lens for receiving said light from the array and
(g) a measuring means such as optical power meter for measuring the intensity of light from said lens wherein the base of said prism is interfaced with the ambient such that the light beam at this interface is partially transmitted and partially reflected wherein the reflected light carries with it the information on the refractive index (ua) of the ambient.
(a) an array of pinholes disposed in the proximity of said second face.
(b) a converging lens for receiving said light from the array.
(c) an measuring means for measuring the light from said lens.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Further object and advantages of this invention will be more apparent from the ensuring description when read in conjunction with the accompanying drawing and wherein Fig. 1 shows the refractometer.
DESCRIPTION WITH REFERENCE TO ACCOMPANYING DRAWING:
The refractometer 1 comprises an isosceles prism 2 of glass with base angles and refractive index ug. The base 3 of the prism has an attached temperature controlled steel bath 4 to fill the ambient (of refractive index ua) thus forming a glass-ambient, ug-ua interface. Light from a laser or a LED source 5 is passed through a system of lenses 6 to produce a convergent light beam 7. This convergent beam of light 7 passes through one of the isosceles faces 8 of prism 2. The light rays reflected from the base 3 of prism 2 (interfacing the ambient) emerge out from the other isosceles from 9 of prism 2. This bundle of rays is then allowed to fall on an array of pinholes 10. The array of pinholes is disposed in a plane parallel to the face 9. The number of pinholes is kept so large to obtain sensitivities as high as those theoretically obtainable. Light emerging out from these pinholes is focused by a converging lens 11 on an optical per meter 12 for measurement.
The divergence of the incident beam 7 is adjusted in such way that the central ray 7a of this beam cone with its apex at P is incident at right angle on the isosceles face of the prism, and also that the minimum/maximum angle of incidence of the rays constituting the input beam cone at the ug-ua interface is
equal to or smaller then the critical angle at this interface for the lowest/highest value of refractive index of the ambient under investigation.
Only one pinhole is opened at a time keeping all others shut. The rays of reflected light passing through it correspond to incident rays lying on a very narrow range of angles of incidence. For a practical device, having very small sized pinholes, this very narrow range of angles corresponds almost to a single angle of incidence.
In this way, all the angles lying between minimum and maximum angles of incidences as described above are covered by opening one hole at a time, one after the other. The arrangement is such that when sensitivity due to one hole starts decreasing, this hole is shut and the next one is opened. Just when the sensitivity due to this hole starts decreasing, this hole is shut and the next one is opened. Measurement of r.i. is thus carries out with a high sensitivity over the entire range of value of r.i. of the ambient.
The sensitivity of the refractometer refers to the ratio of change in Ir/Ii to the change in |.ia for a given \xg. Wherein Ir and Ii are the intensities of reflected light passing through a pinhole when ambient has refractive indices (r. i) given by ua and 1 (r.i. for air), respectively. However, if reflected light passing through this pinhole had reached there after suffering total internal reflection then Ir=Ii.
In yet another embodiment of the invention a part of the input beam of light may be used as reference beam of light to facilitate the measurement of Ir/Ii.
In yet another embodiment of the invention micorprocessors may be used to read and analyze Ir/Ii.
The steps followed for the measurement of r.i. are given below:-
Step 1 : Sample is kept in the steel bath maintained at a constant temperature
Step 2 : Out of all the pinholes that pin hole is opened corresponding to which a small
change in r.i. of the ambient gives the largest change in reflected intensity.
Step 3: Reflected intensity Ir for the u,a under investigation is then measured.
Step 4 : Comparison of measurements made in step 3 with the reference value for ua and
calibration charts or graphs provides the value for r.i. under investigation.
Step 5 : If the r.i. of the ambient increases to the extent that the changes in the reflected
intensity cease to be large enough then this hole is shut and the next hole is opened keeping all other holes shut so that the changes in reflected intensity become nearly as large as (or slightly larger than) before. When changes in reflected intensity of light again begin to get smaller, this hole is shut and the next adjacent hole opened. This process is continued until the r.i. of the ambient is measured with a high degree of sensitivity over its entire range.
Step 6 : If the refractometer sensor is microprocessor based, step 2 to 5 may be
accomplished with its help as follows: A microprocessor controlled screen (13 in fig 1) having only one pinhole (of slightly larger diameter) is placed in front of array of pinholes (shown by 10 in Fig. 1) and moved across it so that the light reflected from the glass-ambient interface 3 passes through only one pinhole at a time Ir/Ii can then be determined for each pinhole and refractive index ua of the ambient determined with the help of earlier calibration of the device (as in step 4). ua is determined most accurately, or with highest sensitivity, corresponding to that pinhole for which Ir/Ii comes out to be the largest.
Step 7 : Step 1 to 6 may be repeated by varying the temperature of the samples to
dtermine its r.i. at those temperatures.
Use of the refractometer for the determination of refractive indices in cane sugar and beverage industries.
In the cane sugar and beverage industries there is a requirement for measurement of r.i. with quite a high degree of accuracy (upto 5th decimal place of its value) over quite a wide range of its values. Some of the refractometers used there [eg. the Zeiss model "C. A Brow me & F. WQ. Zerban". Physical & Chemical Methods of Sugar Analysis, John Wiley and Sons Ltd.. New York, 2nd Edition, pp. 85-135 (1948)] employs 8 to 10 intercahngeable prisms to cover the entire range over which r.i. is required to be measured accurately.
It is noteworthy that in the present invention only one prism is used. The beam cone illuminating one of the isosceles faces of the prism contains rays falling at continuously changing angles of incidence. By opening one pinhole at a time, one after the other, this arrangement allows measurement of r.i. with a high degree of sensitivity, over the entire range of its values. A screen (13) having only one pinhole with a diameter larger than the diameter of the pinhole is placed in front of the array of pinholes and moved across it to open only one pinhole at a time. Keeping all others shut. This movement is microprocessor controlled in a refractometer which is completely automatic.
The main advantages of the present invention are that:
1. It is an Opto-electronic device capable of yielding directly the values of r.i.
2. It is a device, which has no moving parts involved in its construction except for some not so-sophisticated arrangement required tor closing and opening of the holes one by one.
3. Its microprocessors-based version can be made completely automatic.
4. It is a robust device and can be put to use under rugged conditions.
5. It is a cost-effective device.
6. It is a device which can operate with ambient samples of such small volumes as a few c.c.
7. The sensitivity of this device approaches its theoretically achieveable limits.
1. An opto-electronic refractometer (1) for measuring the refractive index of liquids comprising of :
(a) an isosceles prism (2) having a base (3) and a first and second equal faces (8, 9) disposed opposite to each other;
(b) a source of light (5) disposed in the proximity of said first face (8) so as to provide a convergent beam of light (7) falling at right angle on said first face;
(C) the base (3) of said prism (2) being interfaced so that the light beam is partially reflected and partially transmitted; characterized in that;
(d) an array of pinholes (10) disposed in a plane parallel to said second face (9);
(e) a screen (13) having a pinhole is provided in front of the array;
(f) a converging lens (11) for receiving said light from the array (10) and
(g) a measuring means (12) such as optical power meter for measuring the intensity of light from said lens wherein the base of said prism is interfaced with the ambient such that the light beam at this interface is partially transmitted and partially reflected wherein the reflected light carries with it the information on the refractive index (pa) of the ambient.
2. An opto-electronic refractometer as claimed in claim-1 wherein the apex angle of the input light beam cone is such that the minimum/maximum angle of incidence of the rays constituting the
input beam cone at the prism base ambient µg-µa interface is equal to or smaller than the critical angle at the interface for the lowest/highest value of refrac :h e index µa of the ambient.
3. An opto-electronic refractometer as claimed in claim 1 wherein a
hole is kept open at one instance while taking observation of
reflected light intensity corresponding to which the refractometer
has maximum sensitivity and the refractometer comprising of a
lens system (6).
4. An opto-electronic refractometer for measuring the refractive index
of liquids substantially as herein described and illustrated.
|Indian Patent Application Number||1100/DEL/2002|
|PG Journal Number||35/2010|
|Date of Filing||01-Nov-2002|
|Name of Patentee||LALIT MOHAN BALI|
|Applicant Address||A-5/1 NIRALA NAGAR, LUCKNOW-226020, INDIA.|
|PCT International Classification Number||G01N21/43|
|PCT International Application Number||N/A|
|PCT International Filing date|