RU2292038C2 - Method and device for measuring refractivity - Google Patents

Abstract

FIELD: measuring technique.

SUBSTANCE: device for measurement of refractivity has measuring prism. There is slit diaphragm onto side face of prism. Light-emitting diode is glued behind slot diaphragm. Multiple-element receiver is glued to another side face of prism. Third face of prism has to be plane of separation through which plane the optical contact is made with tested matter. Output of multiple-array receiver is connected to threshold unit. Output of threshold unit is connected with control input of switch. Signal input of switch is connected with output of pulse generator. Outputs of switch and pulse generator are connected to counting inputs of two counters correspondingly. Zero adjustment inputs of counter and their outputs are connected with corresponding inputs of microprocessors. Output of microprocessor is connected with display.

EFFECT: improved precision of measurement; higher manufacturability of structure; reduced cost.

4 cl, 4 dwg

Description

The invention relates to optical electronic instrumentation, and in particular to methods and means of measuring the refractive index of liquid and pasty substances using the method of limiting angle, and can be used to create measuring instruments of both optically transparent and optically opaque liquids, pastes, gels, fine powders, etc. substances.

A method of measuring the refractive index using the phenomenon of total internal reflection (the method of limiting angle) has been known for a long time [1]. When using this method, the critical or limiting angle is measured at which the effect of violation of the total internal reflection at the boundary of two media is observed, the refractive index of one of which is known. As a rule, a glass prism is used as a measuring element with a known refractive index, although it is possible to use other optical elements: hemispheres, plane-parallel plates, etc. [1].

Two options for measuring the critical angle are known:

- on the plane of contact of the two media direct a beam of light rays from the side of the medium with a large refractive index and register the reflected light,

- on the plane of contact of the two media direct a sliding beam of light from the side of the medium with a lower refractive index and register the refracted rays of light. This option is only suitable for transparent environments.

In both cases, the position of the border of light and shadow, depending on the critical angle, is determined by which the refractive index of the test substance is determined.

The critical angle is measured by any goniometric device, the readings of which are converted into the refractive index of the test substance.

A disadvantage of the known methods is that the measurement of the limiting angle is carried out at the border of light and shadow, which is not clear enough for high-precision measurements, and this leads to errors in determining the refractive index. In addition, the position of the border of light and shadow is measured at one point - the point of intersection of the border with either the scale or with one element of the photodetector line, which also does not contribute to obtaining high accuracy of measurements of the refractive index.

Widely known are devices based on the method of limiting angle, for example, flow-through refractometers of the PR series from K-PATENTS [2]. In these devices using the method of limiting angle, the optical element - a prism - is in contact with the test substance. The boundary between two media: the optical element and the test substance is illuminated by a non-parallel beam of light. Part of the light rays incident on the contact boundary at an angle less than critical goes into the test substance, and part of the light rays incident at large angles on the contact border undergoes total internal reflection and is projected onto a photodetector (FPU) - CCD line. The position of the border of light and shadow on a CCD line determines the refractive index of the test substance.

A disadvantage of the known device lies in the disadvantage of the applied method: the countdown is carried out at one point of intersection of the fuzzy border of light and shadow on one photodetector.

The proposed method for determining the refractive index and the device built on its basis is designed to eliminate the above disadvantages.

In the proposed method of measuring the refractive index, also using the method of limiting angle, the test substance is in contact with the working face of the prism with a known refractive index exceeding the refractive indices of the investigated substances. The plane of contact through the first side face of the prism is illuminated by a diverging beam of light from a monochromatic point or slit light source. The part of the light that has undergone total internal reflection on the plane of contact of the working face of the prism with the test substance is sent through the second side face of the prism to a multi-element matrix photodetector (FPU).

The difference of the proposed method is that the refractive index of the test substance is determined by the relative area of the shadow on the photosensitive surface of the matrix FPU obtained from the test substance, and the relative area of the shadow on the photosensitive surface of the FPU is found by repeatedly reading the signals from all the photosensitive elements of the FPU, the signal amplitude of which compare with a given threshold value, and calculate the ratio of the number of signals that have not reached the threshold value to the total number read from the PD signals. The indicated ratio is proportional to the relative area of the shadow on the photosensitive surface of the FPU. The relative shadow area refers to the ratio of the shadow area to the area of the entire photosensitive surface of the FPU.

In other words, the essential differences are that, firstly, the measurement is carried out not on one point at the border of light and shadow, but on the entire area of the shadow, including the border of light and shadow; secondly, the relative shadow area is calculated by the ratio of the unlit photosensitive elements of the FPU matrix to their total number. Thanks to this approach, the error in the measurement of the refractive index, caused by the fuzziness of the border of light and shadow, is significantly reduced due to the averaging of a large number of individual measurements over the entire set of photosensitive elements of FPU.

The functional dependence of the refractive index on the relative shadow area on the FPU for each specific device (refractometer) is determined by a well-known calibration method for reference liquids with known refractive indices, for example, calibrated aqueous solutions of acids, salts, bases, sucrose, etc. ([1], tables 6-8). The found functional dependence is recorded in the permanent memory of the microprocessor.

The invention is illustrated by drawings (see figure 1, 2, 3, 4). Figure 1 schematically shows the course of light rays in the measuring element, which for example is made in the form of a prism, figure 2 and 3 - the distribution of shadow and light on the photosensitive surface of the FPU for substances with different refractive indices, figure 4 - functional diagram devices (refractometer).

The diverging light flux from the light source 1 through the slit diaphragm 2 (see Fig. 1) is directed to the plane of separation of two media 3: prism 4 and the test substance 5, the refractive index of which should be less than the refractive index of the prism. To expand the range of the investigated substances, the refractive index of prism 4 should be as large as possible. As a prism material, sapphire (n _D = 1.76808) and STK-9 glass according to GOST 13659-78 (n _D = 1.7460) having a high refractive index and good mechanical strength can be recommended.

Ray “c” falls on the separation plane at a critical angle. Rays incident at smaller angles to the separation plane (such as beam "a") will go into the test substance 5, and rays incident at large angles to the separation plane (such as beam "b") will undergo total internal reflection and fall on the photosensitive surface 6 FPU 7. In this case, part of the photosensitive surface 6 will be unlit (from the beginning of the photosensitive surface 6 to the beam "c"), and part - illuminated (from the beam "c" to the end of the photosensitive surface 6). The magnitude of the unlit area of the photosensitive surface of the FPU is dependent on the refractive index of the test substance. Therefore, by measuring the unlit portion of the photosensitive surface of the FPU, we can find the refractive index of the test substance. To exclude the dependence of the measurement result on the area of the photosensitive surface of the FPU, measure the relative area of the shadow: the area of the shadow, referred to the entire photosensitive surface of the FPU.

Figure 2 shows an example of the distribution of light and shadow on the photosensitive surface of the FPU for the test substance with a relatively low refractive index, and in figure 3 with a higher refractive index.

The shadow area is calculated by the number of unlit photosensitive elements of the FPU. By sampling the elements of the FPU and comparing the signal from them with a given threshold, determine the number of unlit elements of the FPU. Next, find the ratio of unlit elements to the total number of elements of the FPU, which is proportional to the relative area of the shadow. The selection of elements can be carried out both deterministically, for example, by line-by-line sampling of all FPU elements, or randomly (Monte Carlo method [4]). A prerequisite is an equal probability of sampling for any element of the FPU.

Devices implementing the limit angle method have also been known for a long time. For example, automatic refractometers [4], [5], [6], which contain a light source, a measuring prism, photodetectors (FPUs) and FPU signal processing devices that normalize the output signal.

It is also known to use a multi-element matrix photodetector in a differential refractometer for measuring the refractive index by the magnitude of the shift in the position of the image of the slit relative to the initial position [7].

A refractometer using the limit angle method [8] contains a illuminator with an optical circuit and a refracting prism that implements the limit angle method, an output optical circuit with a mirror, an FPU in the form of a line of photosensitive elements, a microprocessor, and a display. The optical scheme of the illuminator generates light rays that are directed to the input side face of the prism made of glass with a high refractive index. The working face of the prism is in contact with the test substance, which has a lower refractive index. The output optical circuit is coupled to another side face of the prism and directs light rays to the FPU using a mirror. The output of the FPU is connected to a microprocessor designed to calculate the refractive index of the test substance. The microprocessor output is connected to a display on which the results of the measurements are displayed.

The disadvantages of the known device are that:

- a illuminator with an optical circuit, a prism, an output optical circuit, a mirror, and a FPU are structurally disconnected from each other, which causes an error due to thermal and mechanical deformations of the base and mounting of these elements;

- the measurement is carried out on a line of photosensitive elements, which eliminates the possibility of spatial averaging and, therefore, limits the accuracy of the measurement of the refractive index;

- the complexity of the measuring part and the design of the refractometer as a whole, the presence of a large number of nodes and the need for their mutual adjustment and adjustment reduce the manufacturability of the product and increase its cost.

The proposed device is designed to reduce these disadvantages, improve the accuracy of measuring the refractive index, as well as improve the manufacturability of the design and, therefore, reduce the cost of the product, which is a significant factor in mass production.

The proposed device contains (see Figs. 1 and 4) a measuring prism 4, on the first side face of which a slit diaphragm 2 is applied, behind which a light source - LED 1 is glued to the prism, and FPU 7 is glued to the second side face of the prism 4. Third face prism 4 is a section 3 plane through which optical contact is made with the test substance 5. LED 1 with a slit diaphragm 2, measuring prism 4 and FPU 7 form a single monolithic measuring unit. FPU is made on the basis of a multi-element photodetector, for example, a CCD or CMOS television matrix. The output of the FPU 7 is connected to a threshold device 8, the output of which is connected to the first input of the key 9, to the second input of which the output of the pulse generator 10 is connected. The outputs of the key 9 and the pulse generator 10 are connected to the counting inputs of the pulse counters 11 and 12, respectively. The outputs of the counters 11 and 12 are connected to the information inputs of the microprocessor 13, the control output of which is connected to the zero-setting inputs of the counters 11 and 12. The output of the microprocessor 13 is connected to the display 14.

The device operates as follows. The diverging light rays formed by the slit diaphragm 2 from the light source 1 are directed to the separation plane of two media 3: prism 4 and the test substance 5. On the working edge of the prism 4, materializing the separation plane 3, some light rays undergo complete internal reflection through the second face of the prism 4 falls on FPU 7, another part of the rays goes into the test substance 5. On the photosensitive surface 6 of the FPU 7, an image is formed in the form of light and dark areas. Accordingly, at the output of the FPU 7, signals of different amplitudes appear: the bright section corresponds to a signal of large amplitude, the dark to small.

The threshold device 8 compares the amplitude of the signals of the FPU 7 with a threshold value. The threshold value is approximately 2/3 of the signal level from the light portion of the FPU. Signals whose amplitude is higher than the threshold value trigger the threshold device 8, while the threshold device closes the key 9 and prohibits the passage of pulses from the generator 10 to the input of the pulse counter 11. If the signals from the FPU 7 are lower than the threshold value, then the threshold device 8 does not work, the key remains open and passes the pulses of the generator 10 to the counter 11. Counter 11 counts the number of pulses passing through the key 9, counter 12 - the total number of pulses of the generator 10.

The microprocessor 13 takes values from the counters 11 and 12 and calculates their ratio. Further, the microprocessor 13 calculates the refractive index of the analyte 4 from the obtained ratio. It is obvious that the more individual measurements are made (the more pulses are counted), the more accurate, due to averaging, will be the measurement result. The microprocessor 13 sets the required number of measurements, sufficient to obtain the specified accuracy, after which it displays information on the display 14 and at the same time generates an impulse to set the counters 11 and 12 to zero to start the new measurement process.

The proposed method and the device built on its basis make it possible to obtain high accuracy in measuring the refractive index, automate the measurement process, and also improve the manufacturability of the design of the device for measuring the refractive index.

Improving the accuracy of the measurement is achieved by:

- use of spatial (area measurement) parameters during measurement, and not time (durations, periods, frequencies, etc.), which introduce elements of instability during measurements. So, in the above example, the instability of the frequency of the pulse generator 10 does not affect the accuracy of the measurement of the refractive index;

- creating a single monolithic (light source - prism-FPU) measuring unit, which significantly reduces the frustration of the optical system and virtually eliminates the temperature and mechanical errors of the measuring system of the device;

- a large number of elements and high spatial resolution of the FPU (the use of hundreds of lines with hundreds of elements in one line, located in increments of several microns) involved in the measurement;

- averaging the results of a large number of individual (from each element of the FPU) measurements.

Simplification of the design of refractometers that implement the proposed method is achieved due to the extremely simple optical design of the refractometer (see Fig. 1): a light source with a diaphragm - a prism-FPU.

High manufacturability is ensured by eliminating mutual adjustment and alignment of the elements of the measuring part of the refractometer, and the initial exhibition of parameters is provided by software embedded in the microprocessor using reference liquids with known refractive indices.

The proposed method and the device based on it involve the use of modern high technologies, such as small-sized high-resolution FPUs (CCD arrays, CMOS arrays, etc.), microprocessors (microcontrollers) with the appropriate software, small-sized semiconductor light sources (monochromatic LEDs) . All this allows you to create small-sized, including pocket, means of measuring the refractive index of various substances, and by the refractive index to find consumer parameters of substances (concentration of certain substances: sugar, salt, etc. in products; dry residue: Brix scale; percentage of content alcohol in alcoholic beverages, etc.), which can be calculated by a microprocessor according to known dependencies or listed in tables in its memory.

Literature

1. Ioffe B.V. Refractometric Methods of Chemistry, chapters 7–9, ed. "Chemistry", Leningrad branch, 1974

2. H. Salo. Refractometer, US patent No. 6067151, IPC G 01 N 21/41, May 23, 2000

3. N.P. Buslenko, Yu.A. Schreider. Statistical Test Method. M., State. published physical and mathematical literature, 1961

4. A.G. Klimanov, V.K. Shuvaev. Refractometer, application No. 93010878, IPC G 01 N 21/41, application filing date 01.03.93, publication date 02.27.95, Russia.

5. L.P. Brusilovsky and others. Automatic refractometer for monitoring the parameters of liquid media, application No. 21113710, IPC G 01 N 33/04, G 01 N 21/43, application filing date - 12/26/97, publication date - 06/20/98, Russia.

6. A.I. Penkovsky, N.A. Petranovsky. Refractometer, application No. 2049985, IPC G 01 N 21/43, application filing date - 08/28/92, publication date - 10/12/95, Russia.

7. A.I. Penkovsky et al. Method for measuring the strength of vodka and device for its implementation, RF patent No. 2241220, IPC G 01 N 33/14, priority December 13, 2001, publication date November 27, 2004, Bull. No. 33.

8.K.D. Sharma, K.R. Bleyley. Hand-held pocket automatic refractometer, US patent No. 6816248, IPC G 01 N 21 / 41.09 November 2004

Claims (3)

1. The method of measuring the refractive index, based on the phenomenon of total internal reflection on the plane of contact of the test substance with the optical element, which is illuminated by a diverging monochromatic beam of light from a point or slit light source, the part of the light that underwent total internal reflection is sent to a multi-element matrix photodetector ( FPU), on the photosensitive surface of which zones of light and shadow are formed, characterized in that the refractive index of the investigated substance TWA is determined by the relative area of the shadow on the photosensitive surface of the matrix FPU, and the relative area of the shadow is found by repeatedly reading the signals from all the photosensitive elements of the FPU, the signal amplitude of which is compared with a predetermined threshold value, the number of signals that have not reached the threshold value is calculated, and the total number read from FPU signals, and calculate the ratio of the number of signals that have not reached a given value to the total number of read signals, which is proportional to the relative area di shadows. 2. A device for measuring the refractive index containing a light source, an optical element made in the form of a prism made of glass with a high refractive index, the working face of which is in contact with the test substance, an array photodetector (FPU), a microprocessor and a display connected to the information output of the microprocessor characterized in that a threshold device, a pulse generator, a key and two pulse counters are introduced into it, wherein the output of the FPU is connected to a threshold device, the output of which is connected to the control input of the key, to the signal input of which the output of the pulse generator is connected, the outputs of the key and the pulse generator are connected to the counting inputs of the corresponding counters, the outputs of the counters are connected to the corresponding information inputs of the microprocessor, and the inputs for setting the counters zero are connected to the control output of the microprocessor. 3. The device according to claim 2, characterized in that the light source is made in the form of a slit diaphragm deposited on the input side face of the prism, and an LED mounted immediately behind the slit and glued to the side face of the prism, and the FPU is glued to the second side face of the prism.

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