A high-precision solution refractive index measurement system and method

Abstract

The application belongs to the field of optical measurement, and relates to a high-precision solution refractive index measurement system and method, the system comprising a light source, an optical flat, a fiber collimator, a beam splitter, a correction glass, a mirror, a cuvette, a transparent solution sample holder, an adjustable diaphragm and a demodulation module. The application proposes using the correction glass as the optical path adjustment mode of the cage type Mach-Zehnder structure, solves the displacement limitation problem caused by the cage type structure, adjusts the optical path without displacement, and can replace the correction glass with different thickness according to different samples, thereby expanding the measurement range of the sample; the sample loading device used can meet the clamping of transparent solid samples and transparent solution samples, the measurement structure does not need to be modified for different measurement samples, and the measurement efficiency is improved; the solution refractive index measurement standard deviation can reach 1.5*10 ‑5 , about 0.001% of the average value of the measurement, the measurement system has a simple structure and high measurement precision.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical measurement, and relates to a solution refractive index measurement technology, in particular to a high-precision solution refractive index measurement system and method based on a Mach-Zehnder structure. BACKGROUND

[0002] The refractive index is one of the important evaluation criteria for describing the characteristics of a solution, and the refractive index of a solution often needs to be measured with high precision and at a high speed in the fields of biomedicine, chemical industry, petroleum industry and food industry. Common methods for measuring the refractive index of a solution include a capacitive sensing method, an ultrasonic detection method, a chemical chromatography method and an optical method. The optical method has become one of the main methods for measuring the refractive index due to its simple structure, high measurement precision and no pollution to the original solution. The optical method for measuring the refractive index mainly includes a geometric optical method and an interference method. The geometric optical method measures the refractive index and other parameters of a solution by using the exit angle of a light beam after the light beam passes through the solution, and an Abbe refractometer is a common device for measuring the refractive index by using this method. In the interference method, two light beams emitted from a single light source form a measurement light and a reference light through a light splitting system, and then the measurement light and the reference light are coupled and interfered after passing through a measurement path and a reference path respectively. The thickness and the refractive index of a sample to be measured are reflected by the change in the optical path difference before and after the sample is inserted. Common methods include a Newton ring method and a wedge method. According to the propagation mode of light in the optical path, the interference measurement system can be divided into a reflection type interference system and a transmission type interference system. Since the sample to be measured is a solution, the reflectivity is usually low, and therefore the transmission type interference system is used for measurement in the present application.

[0003] A Mach-Zehnder interferometer (MZI) is a transmission type interferometer, and two light beams obtained after a light beam emitted from a light source passes through a beam splitter are subjected to interference after passing through a reference path and a measurement path respectively. The stability of the system can be improved by using a cage structure to build the Mach-Zehnder interference system.

[0004] The existing cage type Mach-Zehnder structure does not have a design structure for adjusting the optical path and loading a solution in the optical path design, the measurement range is small, and the optical path difference cannot be adjusted according to the characteristics of different samples, and the refractive index of a solution cannot be measured. Therefore, an improved scheme is needed to realize high-precision solution refractive index measurement based on the Mach-Zehnder structure. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art, and provides a high-precision solution refractive index measurement method. On the basis of the cage type Mach-Zehnder structure, the initial optical path difference is adjusted by changing the thickness of the correction glass to expand the measurement range, and a solution loading structure is added to realize high-precision measurement of a transparent liquid sample.

[0006] The present application is implemented by using the following technical solutions:

[0007] The first aspect of the present application provides a high-precision solution refractive index measurement system, comprising a light source, an optical flat, a first fiber collimator, a second fiber collimator, a first beam splitter, a second beam splitter, a first correction glass, a second correction glass, a first mirror, a second mirror, a cuvette, a transparent solution sample holder, an adjustable diaphragm and a demodulation module.

[0008] The first fiber collimator is connected to the first beam splitter through a cage rod and a cage plate, and the light beam enters the measurement light path after being reflected by the first beam splitter, is incident on the first mirror after passing through the first correction glass, and enters the second beam splitter after being reflected and passing through the cuvette; the light beam that transmits through the first beam splitter enters the reference light path, is incident on the second mirror after passing through the second correction glass, and is incident on the second beam splitter after being reflected and passing through the adjustable diaphragm. The two light beams are combined and then enter the optical fiber through the second fiber collimator.

[0009] The light source is connected to the first fiber collimator through an optical fiber, the first fiber collimator is connected to the cage plate through a threaded adapter, and the first beam splitter is fixed on the optical flat through the cage rod and the connecting rod support.

[0010] The first beam splitter, the cage plate clamping the first correction glass and the first mirror are connected through the cage rod to ensure that the relative height of the system is basically consistent, and a quartz glass sheet with a diameter of 1 inch and different thicknesses can be placed at the correction glass to adjust the initial optical path. The cage rod is connected to the second beam splitter to install the transparent solution sample holder, the solution to be measured is loaded into the cuvette, and the clamping pressure arm on the transparent solution sample holder is adjusted to press the cuvette for measurement.

[0011] The first beam splitter, the cage plate clamping the second correction glass and the second mirror are connected through the cage rod, the second mirror, the adjustable diaphragm and the second beam splitter are also connected through the cage rod to ensure that the height of the system is basically consistent and the light beam is perpendicular to each element as much as possible.

[0012] The second beam splitter is fixed on the optical flat through the connecting rod and the connecting rod support, the light beam exit is connected to the cage plate with the threaded adapter through the cage rod, the second fiber collimator is connected to the threaded adapter for fixation, the second fiber collimator is connected to the demodulation module through an optical fiber, and then connected to a computer for data transmission.

[0013] Further, the transparent solution sample holder has two clamping pressure arms, the two clamping pressure arms can slide on the transparent solution sample holder and can be fixed on the transparent solution sample holder by bolts, and the cuvette is clamped between the two clamping pressure arms.

[0014] The second aspect of the present application provides a measurement method based on the above measurement system, comprising the following steps:

[0015] 1) A quartz glass of appropriate thickness is mounted as the first and second correction glasses, so that the initial optical path difference (the difference between the optical path of the measurement light path and the optical path of the reference light path when no sample is placed) is positive and as small as possible, and the light intensity of the measurement light path and the reference light path is adjusted to a suitable value;

[0016] 2) Adjust the adjustable diaphragm to the maximum aperture, mount the cuvette in the transparent solution sample holder, collect the interference spectrum data when only the cuvette is placed, and calculate the initial optical path difference OPD1;

[0017] 3) Place the sample to be measured in the measurement light path, and add the sample solution to be measured in the cuvette. Keep the adjustable diaphragm at the maximum aperture, collect the interference spectrum data when the sample is placed, and calculate the optical path difference OPD2 when the sample is placed;

[0018] 4) Remove the cuvette and use visual measurement to obtain the thickness H between the two walls;

[0019] 5) By simultaneously measuring the optical path difference, calculate the required optical parameters, and then obtain the refractive index n of the transparent solution sample.

[0020] Further, step 1) comprises:

[0021] (1.1) Turn on all devices, and set the integration time of the demodulation module to an appropriate size through the computer;

[0022] (1.2) Adjust the angle adjustment frame of the first and second mirrors so that the two beams of light can interfere at the second beam splitter;

[0023] (1.3) Block the reference light path or the measurement light path, and observe the light intensity information collected by the demodulator so that the light intensity of the two paths is basically the same;

[0024] (1.4) Adjust the thickness of the first and second correction glasses so that the calculated initial optical path difference is positive and as small as possible while ensuring that it is separated from the fundamental frequency, so as to obtain a larger measurement range.

[0025] After setting the appropriate angle in step (1.2), the angle cannot be adjusted again in subsequent measurements.

[0026] The judgment of the initial optical path difference (the difference between the optical path of the measurement light path and the optical path of the reference light path) in step (1.4) can be achieved by gradually increasing the optical path in the measurement light path to observe the change of the calculated optical path difference. If the optical path difference decreases first and then increases, it means that the optical path of the measurement light path is smaller than the optical path of the reference light path, i.e. the initial optical path difference is negative. If the optical path difference continues to increase, it means that the optical path of the measurement light path is larger than the optical path of the reference light path, i.e. the initial optical path difference is positive, which meets the experimental requirements. The thickness and position of the first correction glass and the second correction glass cannot be adjusted again in subsequent measurements.

[0027] Further, step 2) comprises:

[0028] (2.1) Adjust the adjustable diaphragm to the maximum aperture, so that the light of the reference light path and the measurement light path can all enter the demodulation module;

[0029] (2.2) Place the cuvette on the transparent solution sample holder for subsequent loading of the solution to be measured;

[0030] (2.3) Set the demodulation module to acquisition mode by computer and run the acquisition program;

[0031] (2.4) Pause the acquisition program and save the interference spectrum data at this time. Calculate the saved spectrum data by the calculation program, record the interference spectrum data when only the cuvette is placed, and calculate the initial optical path difference OPD1.

[0032] In step (2.2), after the cuvette and the transparent solution sample holder are placed, they cannot be adjusted again in subsequent measurements.

[0033] Further, step 3) comprises:

[0034] (3.1) When measuring a transparent liquid sample, use a pipette to add the solution to be measured in the cuvette;

[0035] (3.2) Adjust the adjustable diaphragm to the maximum aperture, so that the light of the reference light path and the measurement light path can all enter the demodulation module;

[0036] (3.3) Start the acquisition program by computer, collect the interference spectrum information after adding the sample, and calculate the optical path difference OPD2 at this time.

[0037] In step (3.1), the height of the solution in the cuvette must exceed the position of the light beam, so that all the light beams can pass through the solution to be measured.

[0038] Further, the thickness H between the two walls obtained in step 4) can be used continuously in subsequent measurements of other solution samples without repeated measurement.

[0039] Further, the refractive index n of the transparent solution sample in step 5) satisfies the formula:

[0040]

[0041] Wherein OPD2 is the optical path difference after adding the sample obtained in step (3.3), OPD1 is the initial optical path difference without placing the sample obtained in step (2.4), and H is the thickness between the two walls of the cuvette obtained in step 4.

[0042] The present application has the advantages and beneficial effects that:

[0043] 1. The present application proposes to use a correction glass as the optical path adjustment mode of the cage type Mach-Zehnder structure, solves the displacement limitation problem caused by the cage type structure, adjusts the optical path without displacement, and can replace different thickness correction glasses according to different samples to expand the measurement range of the sample.

[0044] 2. The sample loading device used in the present application can meet the measurement of different solution samples, and only needs to replace the measured solution to use the proposed method for measurement, thereby improving the measurement efficiency.

[0045] 3. The present application can also measure transparent solid samples, without modifying the measurement structure for different measurement samples, thereby improving the multifunctionality and applicability of the system.

[0046] 4. The present application uses an interference measurement method, and the standard deviation of the solution refractive index measurement can reach 1.5x10 -5 , which is about 1.1% of the average value of the measurement, and has higher measurement accuracy and better system stability than the reflection spectrum method.

[0047] 5. The present application uses a non-contact measurement method, which will not cause damage to the measurement sample. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a top view of the high-precision solution refractive index measurement system of the present application;

[0049] Figure 2 is a perspective structural view of the high-precision solution refractive index measurement system of the present application;

[0050] Figure 3 is a schematic diagram in the data solving process of the present application;

[0051] Figure 4 is a flowchart of the thickness and refractive index measurement method of the transparent sample in the measurement process of the present application;

[0052] Figure 1In the diagram, 1 is an optical plate, 2 is a first fiber collimator, 3 is a first beam splitter, 4 is a first correction glass, 5 is a first reflecting mirror, 6 is a cuvette, 7 is a transparent solution sample holder, 7-1 is a clamping arm, 8 is a second beam splitter, 9 is a second fiber collimator, 10 is an adjustable aperture, 11 is a second reflecting mirror, and 12 is a second correction glass. Detailed Implementation

[0053] The present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0054] like Figure 1 As shown, the experimental system proposed in this invention is a high-precision solution refractive index measurement system. The system includes a light source, an optical plate 1, a first fiber collimator 2, a second fiber collimator 9, a first beam splitter 3, a second beam splitter 9, a first correction glass 4, a second correction glass 12, a first reflector 5, a second reflector 11, a cuvette 6, a transparent solution sample holder 7, an adjustable aperture 10, and a demodulation module.

[0055] The light source is connected to the first fiber collimator 2 via an optical fiber. The first fiber collimator 2 is connected to the cage plate after being converted by a threaded adapter, and then connected to the first beam splitter 3 via a cage rod. The first beam splitter 3 is fixed on the optical plate 1 by a connecting rod and a connecting rod bracket. The first beam splitter 3, the cage plate that holds the first correction glass 4, and the first reflector 5 are connected by a cage rod. A transparent solution sample holder 7 is installed on the second beam splitter 8 via a cage rod. The solution to be tested is loaded into the cuvette 6, and the clamping arm 7-1 on the transparent solution sample holder 7 is adjusted to increase the clamping ratio. The colorimeter 6 is used for measurement; the first beam splitter 3, the cage plate that mounts the second correction glass 12, and the second reflector 11 are connected by a cage rod; the second reflector 11, the adjustable aperture 10, and the second beam splitter 8 are connected by a cage rod; the second beam splitter 8 is fixed on the optical plate 1 by a connecting rod and a connecting rod bracket; the beam exit point is connected to the cage plate with a threaded adapter installed by a cage rod; the second fiber collimator 9 is connected to the threaded adapter; the second fiber collimator 9 is connected to the demodulation module by an optical fiber, and then connected to a computer for data transmission;

[0056] The transparent solution sample holder 7 and the cuvette 6 constitute a sample loading device. The transparent solution sample holder 7 has two clamping arms 7-1, which can slide on the transparent solution sample holder 7 and can be fixed to the transparent solution sample holder 7 by bolts. The cuvette 6 is mounted between the two clamping arms 7-1.

[0057] The first fiber collimator 2 and the first beam splitter 3 are connected by a cage rod and a cage plate. After being reflected by the first beam splitter 3, the light beam enters the measurement optical path, passes through the correction glass 3, and then enters the first reflecting mirror 5. After reflection, it passes through the cuvette 6 and enters the second beam splitter 8. The light beam transmitted through the first beam splitter 3 enters the reference optical path, passes through the second correction glass 12, and then enters the second reflecting mirror 11. After reflection, it passes through the adjustable aperture 10 and enters the second beam splitter 8. The two beams are combined and then enter the optical fiber through the second fiber collimator 9. The optical fiber is connected to the demodulation module and transmits the measurement data to the computer.

[0058] like Figure 3 The diagram shown illustrates the principle of the data processing in this invention, where n is the sample refractive index, and H is the thickness between the two walls of the cuvette when measuring the solution sample. The initial optical path difference OPD1 is the optical path difference when only cuvette 6 is placed, i.e., the optical path difference between the measuring optical path and the reference optical path when only cuvette 6 is placed, which can be derived from E. t The result is obtained by solving the interference spectrum data of E1, that is:

[0059] OPD1 = l2 - l1

[0060] The optical path difference OPD2 during sample placement can be obtained through E t The interference spectrum data of E2 is obtained and can be expressed as follows when measuring solution samples:

[0061] OPD2=OPD1+nH-n=l2-l1+(n-1)H

[0062] After measuring the thickness H between the two walls of the cuvette, the refractive index n of the transparent solution sample can be calculated using the following formula:

[0063]

[0064] like Figure 4 As shown, the present invention provides a high-precision solution refractive index measurement method based on the Mach-Zehnder structure, comprising the following steps:

[0065] 1) Mount appropriately thick quartz glass as the first calibration glass 4 and the second calibration glass 12, ensuring that the initial optical path difference (the difference between the optical path of the measurement optical path and the optical path of the reference optical path when no sample is placed) is positive and as small as possible, and adjust the optical intensity of the measurement optical path and the reference optical path to appropriate values, including:

[0066] (1.1) Turn on all devices and set the demodulation module integration time to a suitable value via computer;

[0067] (1.2) Adjust the angle adjustment frame of the first reflector 5 and the second reflector 11 so that the two beams of light can interfere at the second beam splitter 8;

[0068] (1.3) Blocking the reference light path or the measurement light path, observing the light intensity information collected by the demodulator, so that the light intensity of the two paths is basically the same;

[0069] (1.4) By adjusting the thickness of the first correction glass 4 and the second correction glass 12, the initial optical path difference obtained by calculation is positive, and the value is as small as possible to ensure that it is separated from the fundamental frequency, so as to obtain a larger measurement range.

[0070] 2) Adjust the adjustable diaphragm 10 to the maximum aperture, place the cuvette 6 on the transparent solution sample holder 7, collect the interference spectrum data when only the cuvette 6 is placed, and calculate the initial optical path difference OPD1; specifically including:

[0071] (2.1) Adjust the adjustable diaphragm 10 to the maximum aperture, so that the light of the reference light path and the measurement light path can all enter the demodulation module;

[0072] (2.2) Place the cuvette 6 on the transparent solution sample holder 7, so as to load the solution to be measured subsequently;

[0073] (2.3) Set the demodulation module to collection mode through the computer, and run the collection program;

[0074] (2.4) Pause the collection program and save the interference spectrum data at this time, calculate the saved spectrum data through the calculation program, and record the interference spectrum data when only the cuvette 6 is placed, and calculate the initial optical path difference OPD1.

[0075] 3) Place the sample to be measured in the measurement light path, add the sample solution to be measured in the cuvette 6. Adjust the adjustable diaphragm 10 to the maximum aperture, collect the interference spectrum data when the sample is placed, and calculate the optical path difference OPD2 when the sample is placed; specifically including:

[0076] (3.1) When measuring a transparent liquid sample, use a pipette to add the solution to be measured in the cuvette 6;

[0077] (3.2) Adjust the adjustable diaphragm 10 to the maximum aperture, so that the light beam can all enter the demodulation module;

[0078] (3.3) Start the collection program through the computer, collect the interference spectrum information after adding the sample, and calculate the optical path difference OPD2 at this time.

[0079] 4) Remove the cuvette 6, and use visual measurement to obtain the thickness h between the two walls. When visually measuring, use a confocal microscope to obtain a three-dimensional image of the cuvette, project it in two dimensions, and measure the distance between the two walls as the thickness H between the two walls of the cuvette.

[0080] 5) Calculate the optical parameters required by the measured OPD, and get the sample refractive index n. The refractive index n of the transparent solution sample satisfies the formula:

[0081]

[0082] Where OPD3 is the OPD after adding the sample in step (3.3), OPD1 is the initial OPD without placing the sample in step (2.4), and H is the thickness between the two walls of the cuvette 6 obtained in step 4).

[0083] Table 1 and Table 2 are the calculation results and standard deviation data of measuring the refractive index of 5% and 1t% saline solution, respectively. The standard deviation of the refractive index of the two solutions can reach 1.5x1t -5 , which is about t.tt1% of the average value, which is better than the commercial measuring instrument, and the measuring system is simple in structure and high in efficiency.

[0084] Table 1

[0085]

[0086] Table 2

[0087]

[0088] The above only describes the preferred embodiments of the present application, but not the limitation thereof. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can modify the technical solutions described in the foregoing embodiments, or make equivalent replacements to some or all of the technical features, and these modifications or replacements do not make the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application. The non-essential improvements and adjustments or replacements made by those skilled in the art according to the content of the present application are within the scope of the present application.

Claims

1. A high precision solution refractive index measurement system characterized by: The device comprises a light source, an optical flat (1), a first fiber collimator (2), a second fiber collimator (9), a first beam splitter (3), a second beam splitter (8), a first correction glass (4), a second correction glass (12), a first mirror (5), a second mirror (11), a cuvette (6), a transparent solution sample holder (7), an adjustable diaphragm (10) and a demodulation module. The light source is connected to the first fiber collimator (2) through an optical fiber, the first fiber collimator (2) is connected to a cage plate through a threaded adapter, and the cage plate is connected to the first beam splitter (3) through a cage rod. The first beam splitter (3), the cage plate clamping the first correction glass (4) and the first mirror (5) are connected through a cage rod, the transparent solution sample holder (7) is installed on the second beam splitter (8) through a cage rod, the solution to be measured is loaded into the cuvette (6), and the clamping pressure arm (7-1) on the transparent solution sample holder (7) is adjusted to press the cuvette (6) for measurement. The first beam splitter (3), the cage plate clamping the second correction glass (12) and the second mirror (11) are connected through a cage rod, and the second mirror (11), the adjustable diaphragm (10) and the second beam splitter (8) are connected through a cage rod. The second beam splitter (8) is fixed on the optical flat (1) through a connecting rod and a connecting rod support, and the light beam exit is connected to the cage plate provided with a threaded adapter through a cage rod, the second fiber collimator (9) is connected to the threaded adapter, the second fiber collimator (9) is connected to the demodulation module through an optical fiber, and then connected to a computer for data transmission. After the light beam is reflected by the first beam splitter (3), it enters the measurement light path, is incident on the first mirror (5) after passing through the first correction glass (4), is reflected after passing through the cuvette (6) and enters the second beam splitter (8); the light beam transmitted by the first beam splitter (3) enters the reference light path, is incident on the second mirror (11) after passing through the second correction glass (12), is reflected, enters the second beam splitter (8) after passing through the adjustable diaphragm (10), and is combined after the two light beams are combined, enters the optical fiber through the second fiber collimator (9), and the initial optical path difference is adjusted by changing the thickness of the correction glass.

2. The high precision solution refractive index measurement system of claim 1, wherein: The transparent solution sample holder (7) has two clamping pressure arms (7-1), the two clamping pressure arms (7-1) can slide on the transparent solution sample holder (7) and can be fixed on the transparent solution sample holder (7) by bolts, and the cuvette (6) is clamped between the two clamping pressure arms (7-1).

3. A measurement method of a high-precision solution refractive index measurement system according to claim 1 or 2, characterized by: The device comprises the following steps: 1) clamping quartz glasses with appropriate thicknesses as the first correction glass (4) and the second correction glass (12) respectively, so that the initial optical path difference is positive and as small as possible, the initial optical path difference is the difference between the measurement light path and the reference light path when no sample is placed, and the light intensities of the measurement light path and the reference light path are adjusted to appropriate values; 2) Adjust the tunable aperture (10) to the maximum aperture, place the cuvette (6) in the transparent solution sample holder (7), collect the interference spectrum data with only the cuvette (6) in place, and solve for the initial optical path difference ; 3) Place the sample to be measured into the measurement light path, add the sample of the solution to be measured into the cuvette (6), keep the adjustable diaphragm (10) at the maximum aperture, collect the interference spectrum data when the sample is placed, and calculate the optical path difference when the sample is placed ; 4) Remove cuvette (6), measure thickness between two walls of cuvette (6) ; 5) By simultaneous measurement of the optical path difference, the optical parameters required are calculated, and the refractive index of the transparent solution sample is obtained .

4. The method of claim 3, wherein: Step 1) further comprises: (1.1) turning on all devices, and setting the integration time of the demodulation module to an appropriate size through the computer; (1.2) Adjust the angle adjustment frame of the first mirror (5) and the second mirror (11) so that the two beams of light can interfere at the second beam splitter (8); (1.3) Block the reference light path or the measurement light path, and observe the light intensity information collected by the demodulator to make the two light intensities basically consistent; (1.4) Adjust the thickness of the first correction glass (4) and the second correction glass (12) so that the initial optical path difference obtained by calculation is positive and the value is as small as possible under the condition of being separated from the fundamental frequency, so as to obtain a larger measurement range.

5. The method of claim 4, wherein: The method for judging whether the initial optical path difference is positive in step (1.4) is: by gradually increasing the optical path in the measurement light path, observe the change of the calculated optical path difference, if the optical path difference first decreases and then increases, it means that the optical path of the measurement light path is smaller than that of the reference light path, that is, the initial optical path difference is negative; if the optical path difference continues to increase, it means that the optical path of the measurement light path is larger than that of the reference light path, that is, the initial optical path difference is positive.

6. The method of claim 3, wherein: Step 2) includes: (2.1) Adjust the adjustable diaphragm (10) to the maximum aperture so that the light of the reference light path and the measurement light path can all enter the demodulation module; (2.2) Place the cuvette (6) on the transparent solution sample holder (7); (2.3) Set the demodulation module to the collection mode by computer and run the collection program; (2.4) pausing the collection procedure and saving the interferometric spectral data at this time, solving the saved spectral data by the solving procedure, and recording the interferometric spectral data with the cuvette (6) placed alone, and solving the recorded initial optical path difference .

7. The method of measuring of claim 3, wherein: Step 3) The height of the solution in the cuvette (6) must exceed the position of the light beam so that all the light beams can pass through the solution to be measured.

8. The measurement method according to claim 3, characterized by, Step 4) Measure the thickness between the two walls using visual measurements .

9. The method of claim 3, wherein: Step 5) Refractive index of the transparent solution sample The calculation formula is: 。

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