Optical interferometric gas sensor structure and self-calibration method

Abstract

The present application belongs to the technical field of gas concentration measurement, and specifically proposes a kind of optical interference gas sensor structure and self-calibration method.Sensor is constituted by measurement gas chamber, photosensitive element and the like, and measurement gas chamber adopts four-chamber structure.Its working principle is that light emitted by same light source passes through different gas chambers, and because of different gas concentrations, optical path difference and phase shift are generated, and then interference fringes are formed, and gas concentration detection is realized by detecting fringe movement amount.Two interference fringes generated by four gas chambers are compared to realize zero point self-calibration, and the present application has the advantages of high non-linear compensation accuracy, can avoid reading error caused by mechanical vibration and temperature change, and provides a reliable scheme for accurate measurement of gas concentration.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of gas concentration measurement, and specifically relates to a gas detection method, system and device based on the optical interference principle. BACKGROUND

[0002] Gas concentration monitoring plays a crucial role in safety production. By timely and accurate monitoring of gas components and concentrations in industrial environments, enterprises can effectively prevent potential safety risks. In industrial production processes, certain gases are harmful to the human body and may even cause explosions or fires. Gas concentration monitoring equipment can monitor the concentration of these gases in real time, and once the safety threshold is exceeded, the system will issue an alarm to remind employees to take timely measures to prevent accidents, improve production safety, and help protect the personal safety of employees, as well as production equipment and the environment. In addition, gas concentration data can also provide strong support for production decision-making and process optimization, thereby improving product quality and production efficiency.

[0003] The measurement accuracy of gas concentration sensors is crucial for safety monitoring, but in actual applications, the accuracy and stability of zero-point calibration are often affected due to factors such as sensor contamination, aging or physical damage. In addition, environmental factors such as high temperature, high humidity, dust, etc. may also interfere with sensor measurement results. These problems may cause errors in the sensor after zero-point calibration, affecting its accuracy and reliability in actual applications. The current zero-point calibration method for gas sensors mainly uses standard gas samples to calibrate manually, which is tedious and labor-intensive, requiring a lot of manpower and resources, and is difficult to meet the requirements of real-time and accurate measurement. For optical interference gas sensors, existing technologies still have deficiencies in non-linear error suppression and zero-point drift suppression, making it difficult to achieve real-time and accurate measurement of gas concentration. New designs are needed in terms of sensor optical path structure, signal acquisition, calibration method, etc. to fully utilize the advantages of relative measurement of optical interference optical path difference, realize automatic calibration of the sensor in actual use, suppress zero-point drift, and improve measurement accuracy and stability. SUMMARY

[0004] The application relates to a light interference gas sensor structure, which comprises a measuring gas chamber, a photosensitive element, a linear light source, a plane mirror, a triangular prism and a compensation prism; the measuring gas chamber adopts a four-chamber structure and comprises three reference gas chambers and one sampling gas chamber; reference gas is filled in the upper two rectangular reference gas chambers, reference gas is filled in the lower rectangular reference gas chamber, and the gas to be measured is filled in the lower rectangular sampling gas chamber; the upper two rectangular reference gas chambers are arranged in vertical and parallel mode with the lower rectangular reference gas chamber and the lower rectangular sampling gas chamber; the two compensation prisms are located between the upper gas chamber and the plane mirror; the photosensitive element adopts a linear CCD to collect the interference fringe signals in the vertical direction.

[0005] Further, the light emitted by the linear light source is reflected by the plane mirror coated with a total reflection film, is divided into two parallel beams, and respectively passes through the upper and lower rectangular reference gas chambers, forms two groups of light, is horizontally incident into the triangular prism after passing through the rectangular reference gas chambers, is reflected by the triangular prism twice, respectively passes through the upper rectangular reference gas chamber and the lower rectangular sampling gas chamber, and is incident into the plane mirror coated with a total reflection film on the back surface and is reflected, and the two groups of light respectively generate interference fringes at the photosensitive element, the interference fringes are divided into two parts, and are recorded as a first interference fringe and a second interference fringe.

[0006] Further, the compensation prism is provided with a mechanical adjusting device, the optical path difference of the light can be changed to realize mechanical zero adjustment, and the interference fringes can be ensured to exist all the time.

[0007] Further, the self-calibration method of the light interference gas sensor structure comprises the following steps.

[0008] 1. The photosensitive element respectively detects the first interference fringe and the second interference fringe.

[0009] 2. The distribution period and the average phase of the first interference fringe and the second interference fringe are respectively obtained through a signal analysis method.

[0010] 3. The average phase of the second fringe is calibrated by taking the average phase of the first fringe as a zero point reference.

[0011] 4. The concentration value of the measured gas is further obtained according to the corresponding relationship between the fringe phase and the gas concentration.

[0012] Further, the linear light source is a single-wavelength visible light source, and the linear light source and the photosensitive element are vertically installed, and the light emitted by the linear light source generates interference fringes at the photosensitive element after passing through the optical path system.

[0013] Further, the front and back of the measuring gas chamber is sealed and isolated from the outside by high-transmittance plane prisms, and the other directions are sealed and isolated from the outside by metal, and the gas chambers are sealed and isolated from each other by metal, and there is a communication area between the three reference gas chambers to ensure that the gases in the three reference gas chambers are completely consistent.

[0014] Further, the gray value peaks of the received light interference fringes are detected by using a peak detection algorithm to obtain effective positions for positioning the light interference fringes, and the effective positions of the processed light interference fringes generated by the upper and lower gas chambers are compared to realize the determination of the gas concentration.

[0015] Further, the self-calibration method pre-processes the light interference signal, specifically including:

[0016] 1. Gray scale processing is performed on the signal, and gamma transformation is used to map the wide gray scale range in the input image to the narrow gray scale range of the output image, compress the dark pixel value, and expand the high gray scale value.

[0017] 2. Noise removal is performed on the collected image, and the median value of the surrounding pixel values is used to replace the value of each pixel to remove salt and pepper noise.

[0018] 3. The light interference signal after image processing is extracted to obtain the peak position of each interference fringe as the comparison position of the light interference fringes of the upper and lower gas chambers.

[0019] Further, the self-calibration method, the gas concentration detection process is as follows:

[0020] 1. The standard reference gas is filled into the reference gas chamber from the air inlet;

[0021] 2. Open the gas inlet of the sampling gas chamber and place it in the environment to be measured. After the sampling gas chamber is filled with the gas to be measured, the linear light source emits light, which is reflected and refracted by the plane mirror coated with a total reflection film, and finally two parallel light beams are emitted from the plane mirror;

[0022] 3. The two parallel light beams, after passing through the gas chamber structure, are incident into the three prisms and are reflected, and then re-enter the gas chamber structure. The light emitted by the linear light source passes through the upper standard gas chamber and the lower sampling gas chamber after being split by the three prisms, and is incident into the plane mirror through the compensating prism. After being reflected on the front and back surfaces of the plane mirror coated with a total reflection film, interference fringes are generated and incident into the photosensitive element;

[0023] 4. Combined with the reference interference fringes on the upper half of the photosensitive element, the accurate concentration of the gas in the sampling gas chamber can be accurately obtained by measuring the light intensity on the lower half of the photosensitive element.

[0024] Further, the self-calibration method, the program algorithm logic of the automatic tracking detection of the stripe movement amount is as follows:

[0025] 1. Perform gray scale transformation on the original signal of the collected stripe, adjust the gray scale range of the signal, and filter the signal to remove the noise mixed in the collection signal process;

[0026] 2. After the signal is processed by the digital image, the signal is converted from the time domain to the frequency domain through the fast Fourier transform, and the average distribution period of the stripe signal is calculated And the average phase ψ;

[0027] 3. Based on the stripe period and the average phase, the stripe signal in the window is divided into n intervals;

[0028] 4. Extreme value search is performed in each interval to obtain the extreme value of each interval;

[0029] 5. Compare the extreme values of adjacent intervals, judge whether they are effective extreme values by comparing the distance of the horizontal coordinates, and obtain a new set of effective extreme values;

[0030] 6. Compare the horizontal coordinates of the effective extreme values with the previous stripe extreme value coordinate positions, and select the horizontal coordinates of the two extreme values closest in Euclidean distance;

[0031] 7. According to the first five values of the stripe extreme value coordinates, a polynomial fitting curve is adopted, and the change trend is compared, and the value with the smallest curve interpolation is selected as the effective stripe position of the measurement. The present application has the following beneficial effects:

[0032] (1) The zero point self-calibration of the sensor can be realized by comparing the two interference stripes generated by the four air chambers;

[0033] (2) The non-linear compensation accuracy is higher, and the product performance is improved;

[0034] (3) Avoid the sensor reading error caused by mechanical vibration and temperature change;

[0035] (4) The image processing algorithm is used in part of the received optical interference signal, gamma transformation and median filtering are used to improve the positioning accuracy of the optical interference signal;

[0036] (5) For the case that the interference stripe movement amount exceeds one bright-dark period, the software program is designed specifically to realize the automatic tracking monitoring of the stripe movement amount. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 is a structural schematic diagram of the optical interference gas concentration sensor system in the present application;

[0038] Figure 2is a perspective view of the gas chamber in the present application;

[0039] Figure 3 is a description of the use of different gas chambers in the present application;

[0040] Figure 4 is a flow chart of the detection of gas concentration in the present application;

[0041] Figure 5(a) is a schematic diagram of interference fringes generated by the upper and lower two-layer gas chambers in the present application;

[0042] Figure 5(b) is a schematic diagram of interference fringes when detecting concentration in the present application;

[0043] Figure 6 is a schematic diagram of interference fringes before and after image processing in the present application.

[0044] Among them, the line light source (1), the plane mirror (2), the measurement gas chamber (3), the three-prism (4), the compensation prism (5), the photosensitive element (6). DETAILED DESCRIPTION

[0045] The embodiments of the present application will be further described in detail below with reference to the accompanying drawings and examples. The following examples are used to illustrate the present application, but cannot be used to limit the scope of the present application.

[0046] The sensor system proposed in the present application is used for gas concentration measurement, has high measurement accuracy and good stability, a beam of light emitted by a single-wavelength line light source is reflected and refracted by a plane mirror and then divided into two beams, and the two beams pass through a standard reference gas chamber filled with air and a sampling gas chamber filled with the gas to be measured respectively, and then are reflected and refracted by the plane mirror again and meet each other, at which time the two beams generate interference fringes, and with the change of the concentration of the gas to be measured, the position of the interference fringes will change accordingly;

[0047] In the optical path design, the line light source is a single-wavelength visible light source, the photosensitive element adopts a linear CCD device, and the line light source and the photosensitive element are both vertically installed, the light emitted by the line light source generates interference fringes at the photosensitive element after passing through the optical path system; the measurement gas chamber adopts a four-chamber structure of upper and lower two layers, the upper two reference gas chambers are located in the upper half of the gas chamber structure and are arranged horizontally and side by side, and the lower one reference gas chamber and one sampling gas chamber are located in the lower half of the gas chamber structure and are arranged horizontally and side by side. Among them, the left side is the reference gas chamber, and the right side is the sampling gas chamber. The front and back of the gas chamber are sealed and isolated from the outside by high-transmittance plane prisms, and the other directions are sealed and isolated from the outside by metal;

[0048] The metal seal is used to isolate the gas chambers, and a communication area is arranged between the three reference gas chambers to ensure that the gases in the three reference gas chambers are consistent; the upper two reference gas chambers are used to generate standard interference fringes as the reference of 0 concentration, and the lower reference gas chamber and the sampling gas chamber are used to generate interference fringes under the current gas concentration to measure the methane concentration;

[0049] The plane mirror is located on the left side of the gas chamber structure, and the back surfaces of the two reflecting surfaces are coated with total reflection films for reflecting the light emitted by the linear light source; the three prisms are located on the right side of the gas chamber structure for reflecting the light after passing through the gas chamber; after twice reflection of the light by the three prisms, the light is parallel to the incident light, so that the reflected light can be incident into the gas chamber again;

[0050] The light emitted by the linear light source after reflection by the three prisms is incident into the upper reference gas chamber and the lower sampling gas chamber, and then is incident into the left plane mirror through the high-transmittance plane lenses on the two sides of the gas chamber; after reflection by the front and back surfaces of the plane mirror, the light is converged into a beam and generates interference fringes on the photosensitive elements, respectively; the gas concentration is measured by comparing the two interference fringes.

[0051] In the process of comparing the two interference fringes, the peak value detection algorithm is used to detect the gray peak value of the received light interference fringes to obtain the effective position for positioning the light interference fringes, and the effective positions of the light interference fringes generated by the upper and lower gas chambers after processing are compared to measure the gas concentration.

[0052] The application will be further described below in combination with the drawings and examples:

[0053] As Figure 1As shown, the present application proposes a high-precision optical interference methane concentration sensing system, which comprises a line light source 1, a plane mirror 2 coated with a total reflection film, a gas chamber structure 3 composed of four gas chambers, a three-prism 4, a compensation prism 5, and a photosensitive element 6. The plane mirror 2 coated with the total reflection film is arranged on the left side of the gas chamber structure 3, and the horizontal angle between the plane mirror 2 and the gas chamber 3 is 45°, so that the light rays entering the plane mirror can be horizontally emitted after being reflected by the plane mirror 2 coated with the total reflection film. The line light source 1 is arranged below the plane mirror 2 and also forms a 45° angle with the plane mirror 2, so that the light rays emitted by the line light source 1 can be parallelly incident into the gas chamber structure after being refracted by the plane mirror 2. The photosensitive element 6 is also arranged below the plane mirror 2, and the light rays returning from the gas chamber can be incident into the photosensitive element 6 after being reflected by the front and back surfaces of the plane mirror 2 and being converged. The line light source 1 is vertically arranged, so that the emitted light can pass through the upper reference gas chamber and the lower sampling gas chamber, respectively. The photosensitive element 6 is also vertically arranged, and the two light beams emitted from the gas chamber 3 and reflected by the front and back surfaces of the plane mirror 2 and converged can be incident into the photosensitive element 6 and generate interference fringes. The three-prism 4 is installed on the other side of the gas chamber 3 and is used for reflecting the light rays emitted by the line light source 1 after passing through the gas chamber 3. The two reflecting surfaces of the three-prism 4 are coated with total reflection films and are used for reflecting the light rays. The light rays emitted from the line light source 1 are reflected by the front and back surfaces of the plane mirror 2, divided into two light beams, and pass through the upper left reference gas chamber, the upper right reference gas chamber, the lower left sampling gas chamber, and the lower right reference gas chamber, respectively, to form four light beams. The four light beams are incident into the three-prism 4, reflected by the two surfaces of the three-prism 4, and returned to the upper right reference gas chamber and the lower right reference gas chamber. The two reflected light beams in the upper reference gas chamber are incident into the compensation prism 5 before being incident into the plane mirror 2. The purpose of the compensation prism 5 is to generate initial interference fringes when the methane concentration is 0, and a mechanical rotating structure is arranged at the position of the compensation prism 5 and can be used for adjusting the angle of the compensation prism 5 to realize mechanical zero adjustment of the sensor. The two light beams of the compensation prism 5 are incident into the plane mirror 2, reflected by the two surfaces coated with the total reflection film, converged into one light beam, and incident into the upper half area of the photosensitive element 6. The two light beams in the lower reference gas chamber are incident into the compensation prism 5 before being incident into the plane mirror 2, pass through the compensation prism, are reflected by the two surfaces coated with the total reflection film after being incident into the plane mirror 2, converged into one light beam, and incident into the lower half area of the photosensitive element 6.

[0054] The optical interference gas sensor of this type can measure gases with different concentration ranges, and the index table of the basic numerical range of the concentration of the gas to be measured and the change of the light intensity of the interference fringes on the photoelectric detection element is established.

[0055] At the same time, according to the calculation of the optical path difference in this optical path, the relationship between the optical path difference and the gas concentration can be obtained, assuming that the gas concentration is x, the length of the gas chamber is L, and the refractive index of the compensation prism is n c, the thickness of the compensation prism is h, the inclination angles of the two compensation prisms are α and β respectively, and the relationship between the optical path difference and the gas concentration is as follows:

[0056]

[0057] On this basis, in the accurate calculation of the gas concentration, the linear interpolation method is used to accurately calculate the gas concentration, and the light intensity difference between the upper and lower parts of the light-sensitive element is compared, so that the measured gas concentration is accurately obtained by measuring the interference fringe light intensity.

[0058] In the comparison of the interference fringe positions, the light interference signal is preprocessed first:

[0059] Step one: the signal is subjected to grayscale processing, the gamma transformation is used to map the wide gray scale range in the input image to the narrow gray scale range in the output image, the dark pixel value is compressed, and the high gray level value is expanded. The relationship is as follows: s = cr 2.7

[0060] Step two: the collected image is subjected to noise removal, the median filter is used to replace the value of each pixel with the median value of the surrounding pixel values, and the salt and pepper noise is removed.

[0061] Step three: the light interference signal after image processing is extracted, the peak position of each interference fringe is obtained, and the comparison position of the upper and lower gas chamber interference fringes is obtained.

[0062] The gas concentration detection process is as shown in Figure 4 , and the specific process is as follows:

[0063] Step one: the standard reference gas is filled into the reference gas chamber from the air inlet;

[0064] Step two: open the gas inlet of the sampling gas chamber and place it in the measured environment, after the sampling gas chamber is filled with the measured gas, the linear light source 1 emits light, which is reflected and refracted at the same time after being reflected by the plane mirror coated with a full reflection film, and finally two parallel light rays are emitted out of the plane mirror.

[0065] Step three: the two parallel light rays described in step two pass through the gas chamber structure and are incident into the three-prism, and the light path is reflected again and re-incident into the gas chamber structure. The light emitted by the linear light source passes through the upper standard gas chamber and the lower sampling gas chamber after being split by the three-prism, and is incident into the plane mirror through the compensation prism. After being reflected on the front and back surfaces of the plane mirror coated with a full reflection film, interference fringes are generated and incident into the light-sensitive element.

[0066] Step four: combined with the reference interference fringes on the upper half of the light-sensitive element, the accurate concentration of the gas in the sampling gas chamber can be accurately obtained by measuring the light intensity of the lower half of the light-sensitive element.

[0067] The detection principle is as follows:

[0068] When the light rays from the linear light source and pass through the plane mirror coated with total reflection film on the back, refraction and reflection occur, forming two light rays. The two light rays pass through the gas chamber and are reflected by the triangular prism, then pass through the compensation prism, are reflected by the plane mirror coated with total reflection film on the back and are refracted, thus a light path difference is generated, and interference fringes are formed on the photosensitive element. When the concentration of the gas to be detected in the lower sampling gas chamber changes, the interference fringes formed in the lower half of the photosensitive element will move. According to the movement amount of the interference fringes and the initial position of the interference fringes formed in the upper half of the photosensitive element, the refractive index of the gas in the lower sampling gas chamber can be deduced, and then according to the relationship between the refractive index of the gas and the concentration of the gas, the concentration of the sampled gas can be obtained. This is the basic principle of measuring the concentration of the gas by using the optical interference method.

[0069] The program logic of the measurement is as follows:

[0070] First, the original signals of the interference fringes generated by the light source passing through the upper and lower gas chambers are collected by the photosensitive element, and the average distribution period and the average phase of the upper fringe signals are calculated;

[0071] According to the average distribution period and the average phase of the interference fringes generated by the upper gas chamber, the fringe signals in the window are divided into n intervals;

[0072] Extreme value search is performed in each interval to obtain the extreme value inside each interval as the effective position of the fringe. Since the light path difference of the light source passing through the upper and lower gas chambers is different, there is a significant phase difference between the upper and lower interference fringes. The effective positions of the fringes obtained by the extreme value search of the intervals of the upper and lower interference fringes can be used to obtain the movement amount of the interference fringes;

[0073] In the neighborhood of the positions of the upper and lower interference fringes, continuous fringe original data is selected, the fringe original data is subjected to grayscale processing, the noise in the fringe original signal is removed by using median filtering, and the average value of the maximum position of the fringe is obtained as the fringe position by using the barycentric method;

[0074] The value fringe coordinate axes of the upper and lower interference fringes are compared, the calibration data are combined, and nonlinear compensation is performed to finally obtain the output value of the gas concentration.

[0075] The program algorithm logic of the automatic tracking detection of the fringe movement amount is as follows:

[0076] (1) As described above, the original signals of the collected fringes are subjected to grayscale transformation, the grayscale range of the signals is adjusted, and the signals are filtered to remove the noise mixed in the collection process of the signals;

[0077] (2) After the digital image processing, the signal is converted from time domain to frequency domain by fast Fourier transform, and the average distribution period T and the average phase ψ of the fringe signal are calculated;

[0078] (3) Based on the fringe period and the average phase, the fringe signal in the window is divided into n intervals, i.e. [x0, x1], [x1, x2], …, [x n-1 , x n ];

[0079] (4) Extreme value search is performed in each interval to obtain the extreme values x M1 , x M2 , …, x Mn ;

[0080] (5) The extreme values of adjacent intervals are compared, whether they are effective extreme values is judged by comparing the distance of the horizontal axis coordinates, and a new set of effective extreme values is obtained;

[0081] (6) Since the movement of the fringe will not produce a sudden change, the horizontal coordinates of the effective extreme values are compared with the last fringe extreme value coordinate position , and the horizontal coordinates x A , x B of the two extreme values with the smallest Euclidean distance are selected;

[0082] (7) According to the last 5 values of the fringe extreme value coordinates , a polynomial fitting curve is adopted, the change trend is compared with x A , x B , and the value with the smallest curve interpolation is selected as the effective fringe position of the measurement.

[0083] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above description is only for specific embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A self-calibration method for an optical interferometric gas sensor structure, characterized in that, The method comprises the following steps: a. The photosensitive element (6) detects the first interference fringes and the second interference fringes respectively; b. The signal analysis method is used to obtain the distribution period and the average phase of the first interference fringes and the second interference fringes respectively; c. The average phase of the second fringes is calibrated by taking the average phase of the first fringes as the zero point reference; d. The concentration value of the measured gas is further obtained according to the corresponding relationship between the fringe phase and the gas concentration. The optical interference gas sensor structure comprises a measurement gas chamber (3), a photosensitive element (6), a line light source (1), a plane mirror (2), a triangular prism (4) and a compensation prism (5). The measurement gas chamber (3) adopts a four-chamber structure and comprises three reference gas chambers and one sampling gas chamber. The upper two rectangular reference gas chambers are filled with reference gas, the lower rectangular reference gas chamber is filled with reference gas, and the lower rectangular sampling gas chamber is filled with the gas to be measured. The upper two rectangular reference gas chambers are arranged vertically and side by side with the lower rectangular reference gas chamber and the lower rectangular sampling gas chamber. The compensation prism (5) is located between the upper gas chamber and the plane mirror (2). The photosensitive element (6) adopts a linear CCD to collect the interference fringe signals in the vertical direction. The light emitted by the line light source (1) is reflected by the plane mirror (2) coated with a full reflection film, is divided into two parallel beams, passes through the upper and lower rectangular reference gas chambers, forms two groups of light, is horizontally incident into the triangular prism (4) after passing through the rectangular reference gas chambers, is reflected by the triangular prism (4) twice, passes through the upper rectangular reference gas chamber and the lower rectangular sampling gas chamber respectively, and is reflected by the plane mirror (2) coated with a full reflection film. The two groups of light generate interference fringes at the photosensitive element (6) after reflection. The interference fringes are divided into two parts, which are referred to as the first interference fringes and the second interference fringes.

2. The self-calibration method of claim 1, wherein, The line light source (1) is a single-wavelength visible light source, and the line light source (1) and the photosensitive element (6) are vertically installed. The light emitted by the line light source (1) generates interference fringes at the photosensitive element (6) after passing through the optical path system.

3. The self-calibration method of claim 1, wherein, The front and back surfaces of the measurement gas chamber (3) are sealed and isolated from the outside by high-transmittance plane prisms, and the other directions are sealed and isolated from the outside by metal. The three reference gas chambers are connected by a communication area to ensure that the gases in the three reference gas chambers are consistent.

4. The self-calibration method of claim 1, wherein, The gray value peak detection algorithm is used to detect the gray value peak of the received optical interference fringes to obtain the effective position for positioning the optical interference fringes. The effective positions of the optical interference fringes generated by the upper and lower gas chambers after processing are compared to measure the gas concentration.

5. The self-calibration method of claim 1, wherein, The optical interference signal is preprocessed, specifically including: a. Gray processing is performed on the signal. Gamma transformation is used to map the wide gray scale range in the input image to the narrow gray scale range in the output image, compress the dark pixel value, and expand the high gray scale value; b. The collected image is subjected to noise removal. The median filter is used to replace the value of each pixel with the median value of the surrounding pixel values to remove salt and pepper noise. c. Extract the light interference signal after image processing, get the peak position of each interference fringe as the contrast position of the upper and lower gas chamber light interference fringes.

6. The self-calibration method of claim 1, wherein, The gas concentration detection process is as follows: a. The standard reference gas is filled into the reference gas chamber from the air inlet; b. Open the air inlet of the sampling gas chamber and place it in the environment to be measured. After the sampling gas chamber is filled with the gas to be measured, the linear light source (1) emits light, which is reflected and refracted at the same time when it enters the plane mirror (2) coated with a total reflection film, and finally exits the plane mirror (2) as two parallel light rays; c. The two parallel light rays pass through the gas chamber structure (3) and enter the three-prism (4), where they are reflected and re-enter the gas chamber structure (3). The light emitted by the linear light source (1) passes through the upper standard gas chamber and the lower sampling gas chamber after being split by the three-prism (4), and then enters the plane mirror (2) through the compensating prism (5). After being reflected on the front and back surfaces of the plane mirror (2) coated with a total reflection film, interference fringes are generated and enter the photosensitive element (6); d. By combining the reference interference fringes on the upper half of the photosensitive element (6), the accurate concentration of the gas in the sampling gas chamber can be accurately obtained by measuring the light intensity on the lower half of the photosensitive element (6).

7. The self-calibration method of claim 1, wherein, The program algorithm logic for automatic tracking detection of the amount of stripe movement is as follows: a. Perform gray scale transformation on the original signal collected from the stripes, adjust the gray scale range of the signal, and filter the signal to remove noise mixed during the signal collection process; b. After digital image processing, the signal is converted from time domain to frequency domain by fast Fourier transform, and the average distribution period T and the average phase ψ of the stripe signal are calculated; c. Based on the stripe period and the average phase, the stripe signal in the window is divided into n intervals; d. Perform extreme value search in each interval to obtain the extreme value of each interval; e. Compare the extreme values of adjacent intervals to determine whether they are valid extreme values by comparing the distance of the horizontal coordinates, and obtain a new set of valid extreme values; f. Compare the horizontal coordinates of the valid extreme values with the previous stripe extreme value coordinate positions, and select the horizontal coordinates of the two extreme values with the smallest Euclidean distance; g. Based on the first five values of the stripe extreme value coordinates, use polynomial fitting curve and change trend comparison, and select the value with the smallest curve interpolation as the effective stripe position of the measurement.

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