A method of measuring the concentration of a gas

Abstract

This invention specifically relates to a gas concentration measurement method, aiming to address the technical problems of existing gas concentration measurement methods, such as systematic inversion bias, sensitivity to environmental conditions, insufficient robustness to changes in system response, difficulty in balancing computational complexity and real-time performance, and insufficient inversion stability under low signal-to-noise ratio conditions. This invention detects a target area to obtain a reference spectral image, a spectral image of the gas to be measured, and a water vapor spectral image. Through dark-field and flat-field correction, it obtains the reference equivalent radiance, the equivalent radiance of the gas to be measured, and the equivalent radiance of water vapor. Normalization is performed using the reference equivalent radiance to obtain the logarithmic absorbance of the gas to be measured and the logarithmic absorbance of water vapor. Water vapor absorption interference is subtracted using the logarithmic absorbance of water vapor, finally yielding the concentration of the gas to be measured. This reduces the influence of water vapor absorption on the detection results and improves inversion stability.

Description

Technical Field

[0001] This invention specifically relates to a method for measuring gas concentration. Background Technology

[0002] In recent years, with the increasing demands for environmental monitoring and industrial safety, gas leak detection technology has received growing attention. Currently, the mainstream methods involve acquiring gas spectral images using point sensors, single-band infrared imaging, and fiber optic sensing, and then performing spectral inversion on these images to calculate the gas concentration. Common gas spectral measurement methods include differential absorption imaging, differential absorption spectroscopy (DOAS), multi-band ratio methods, and inversion methods based on radiative transfer models.

[0003] Differential absorption imaging typically estimates the concentration of a target gas by constructing a radiance ratio or absorbance between the gas absorption band and a reference band. However, this method relies on the core assumption that "the absorption contribution of non-target gases is negligible or approximately equal." This assumption generally does not hold true in the short-wave infrared band, especially in the 1.6–2.1 μm range, where water vapor exhibits broad-spectrum continuous absorption characteristics, and its absorption band significantly overlaps with those of other gases. This can lead to systematic biases during the inversion process.

[0004] Differential absorption spectroscopy (DOAS) can separate the absorption contributions of non-target gases to some extent by fitting absorption lines with high spectral resolution. However, this method requires high signal-to-noise ratio and spectral calibration accuracy, and its stability is poor in dynamic scenarios.

[0005] The multi-band ratio method constructs multiple band combinations to suppress background interference, but it is essentially an empirical method and lacks strict physical constraints. When environmental conditions (such as solar altitude angle, surface reflectivity, and atmospheric state) change, its inversion results have poor stability and limited generalization ability.

[0006] Inversion methods based on radiative transfer models (such as MODTRAN forward modeling + inversion optimization) can describe atmospheric propagation processes relatively accurately, but these methods have high computational complexity, usually relying on iterative solutions or lookup tables, making it difficult to meet real-time requirements. They are also quite sensitive to input parameters and have significant uncertainties.

[0007] In summary, existing methods for measuring gas concentration have the following problems:

[0008] (1) Water vapor cross-absorption was not effectively modeled, resulting in a systematic inversion bias;

[0009] (2) It is sensitive to environmental conditions such as changes in surface reflectivity and solar radiation;

[0010] (3) Lacks robustness to system gain inconsistencies and channel response differences;

[0011] (4) It is difficult to balance computational complexity and real-time performance;

[0012] (5) Insufficient inversion stability under low signal-to-noise ratio conditions.

[0013] Therefore, there is a need for a gas concentration measurement method that can achieve simultaneous detection of multi-component gases in a lightweight system and effectively suppress water vapor interference and the effects of multi-source error coupling. Summary of the Invention

[0014] The main objective of this invention is to address the technical problems of existing gas concentration measurement methods, such as systematic inversion bias, sensitivity to environmental conditions, insufficient robustness to changes in system response, difficulty in balancing computational complexity and real-time performance, and insufficient inversion stability under low signal-to-noise ratio conditions, and to provide a gas concentration measurement method.

[0015] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0016] A gas concentration measurement method is provided for measuring the concentration of a gas to be measured in each pixel within a target area by analyzing a spectral image; its key feature is that it includes the following steps:

[0017] Step 1: Detect the target area to obtain the reference spectral image, the gas spectrum image to be tested, and the water vapor spectrum image of the target area; perform dark field and flat field correction on the reference spectral image, the gas spectrum image to be tested, and the water vapor spectrum image respectively to obtain the reference equivalent radiance, the equivalent radiance of the gas to be tested, and the equivalent radiance of the water vapor.

[0018] Step 2: Using the reference equivalent radiance, normalize the equivalent radiance of the gas to be tested and the equivalent radiance of water vapor to obtain the logarithmic absorbance of the gas to be tested and the logarithmic absorbance of water vapor.

[0019] Step 3: Using the logarithmic absorbance of water vapor, the water vapor absorption interference is subtracted from the logarithmic absorbance of the gas to be tested to obtain the net absorbance of the gas to be tested.

[0020] Step 4: Use Beer-Lambert's law to invert the net absorbance of the gas to be measured, and obtain the path integral concentration distribution of the gas in each pixel in the target area, thus completing the gas concentration measurement.

[0021] Further, in step 1, the reference equivalent radiance, the equivalent radiance of the gas to be measured, and the equivalent radiance of water vapor are all obtained by the following formula:

[0022]

[0023] in, For reference equivalent radiance, equivalent radiance of the gas being measured, or equivalent radiance of water vapor, According to the gas category, , For reference, The gas to be tested, It is water vapor; For pixel coordinates, each Corresponding to one spatial sampling point; For column coordinates, Row coordinates;

[0024] For the same exposure time, use the reference spectral image, the spectral image of the gas to be tested, or the spectral image of water vapor. Output of pixels at a point; for Dark current bias term at the point; , and These represent the gain constant, exposure time, and photoelectric conversion ratio of the optical system's detection channel when the corresponding spectral image is obtained. for The flat field coefficient at a point.

[0025] Furthermore, in step 2, the logarithmic absorbance of the gas to be measured and the logarithmic absorbance of water vapor are both obtained by the following formula:

[0026] ;

[0027] in, for The logarithmic absorbance of the gas to be measured or the logarithmic absorbance of water vapor at the point; According to the gas category, .

[0028] Further, in step 3, the net absorbance of the gas to be tested is obtained by the following formula:

[0029] ;

[0030] in, The net absorbance of the gas to be measured; The cross-absorption coupling coefficient can be obtained through experimental calibration, radiative transfer simulation, or fitting to a priori spectral database.

[0031] Further, in step 4, the path integral concentration distribution of the gas to be measured on each pixel within the target area is obtained by the following formula:

[0032] ;

[0033] in, The concentration of the gas to be measured. For the gas to be tested at temperature air pressure Effective absorption cross section under the given conditions, in meters. 2 / mol; for The effective optical path length corresponding to a pixel at a point, measured in meters (m).

[0034] Furthermore, in step 4, while obtaining the path integral concentration distribution of the gas to be measured on each pixel within the target area, the following steps are also included:

[0035] The logarithmic absorbance of water vapor was inverted using the Beer-Lambert law, and the path integral concentration distribution of water vapor in each pixel of the target region was obtained by the following formula:

[0036] ;

[0037] in, Water vapor concentration, For water vapor at temperature air pressure Effective absorption cross section under the given conditions, in meters. 2 / mol.

[0038] Furthermore, the gas to be tested is methane or carbon dioxide.

[0039] Compared with the prior art, the beneficial effects of the present invention are:

[0040] 1. The present invention provides a gas concentration measurement method that constructs a coupled inversion model by using a reference spectral image, a spectral image of the gas to be measured, and a water vapor spectral image. This eliminates the need to rely on a complete atmospheric radiative transfer model or complex iterative optimization. Water vapor absorption interference can be subtracted using only the linear coupling coefficient, reducing the ill-posedness and computational complexity of parameter estimation and improving inversion stability. This method is suitable for embedded or real-time processing systems.

[0041] 2. The gas concentration measurement method provided by this invention has a fast detection response speed and high overall system detection efficiency, making it suitable for rapid and continuous gas leak monitoring operations in complex and ever-changing industrial sites and field environments.

[0042] 3. The gas concentration measurement method provided by the present invention introduces a reference equivalent radiance to construct the logarithmic absorbance of the gas to be measured and the logarithmic absorbance of water vapor. This method can convert multiplicative errors such as changes in solar irradiance, differences in surface reflectivity, and system gain into additive terms, thereby significantly reducing the impact of environmental changes on the inversion results and improving the robustness of the system in complex scenarios.

[0043] 4. The gas concentration measurement method provided by the present invention has a clear mathematical expression for the construction of the logarithmic absorbance of the gas to be measured and the process of subtracting water vapor absorption interference, which facilitates the establishment of an error propagation model and enables quantitative analysis and optimization design of the inversion accuracy.

[0044] 5. The gas concentration measurement method provided by the present invention improves the detection capability under weak absorption conditions of low-concentration gas by compressing the dynamic range through logarithmic transformation. Attached Figure Description

[0045] Figure 1 This is a flowchart illustrating an embodiment of a gas concentration measurement method according to the present invention;

[0046] Figure 2 This is a schematic diagram of the structure of a multi-channel parallel gas concentration measuring device used in an embodiment of a gas concentration measurement method of the present invention;

[0047] Figure 3 This is a 3D structural schematic diagram of a multi-channel parallel gas concentration measuring device used in an embodiment of a gas concentration measurement method of the present invention;

[0048] Explanation of reference numerals in the attached figures:

[0049] 1-Front common aperture objective lens group, 11-First lens, 12-Second lens; 2-Microlens array; 3-Filter array; 4-Image plane. Detailed Implementation

[0050] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0051] This embodiment provides a gas concentration measurement method that uses methane and CO2 as the analyte gases. The method measures the concentration of the analyte gas in each pixel within a target area by analyzing a spectral image. Figure 1 As shown, it includes the following steps:

[0052] Step 1: Detect the target area and obtain the reference spectral image, methane gas spectral image, CO2 gas spectral image, and water vapor spectral image of the target area. Simultaneously, the computer performs dark-field and flat-field correction on the four spectral images to obtain the equivalent radiance of methane, CO2, water vapor, and the reference equivalent radiance, respectively. As shown in the following formula:

[0053] ;

[0054] in, According to the gas category, , For reference only; For pixel coordinates, each Corresponding to one spatial sampling point; For column coordinates, Row coordinates;

[0055] For methane gas spectral images, CO2 gas spectral images, water vapor spectral images, and reference spectral images at the same exposure time, Output of pixels at a point; for Dark current bias term at the point; , and These are the gain constant, exposure time, and photoelectric conversion ratio of the detection channel of the optical system when the corresponding spectral image is obtained, respectively.

[0056] for The flat field coefficient at the point is used to compensate for the uneven response of the methane gas spectrum image, CO2 gas spectrum image, water vapor spectrum image and reference spectrum image during the acquisition process and the transmittance fluctuation of the optical lens.

[0057] Step 2: Using the reference equivalent radiance, normalize the equivalent radiance of methane, CO2, and water vapor respectively, to obtain... Logarithmic absorbance of methane, logarithmic absorbance of CO2, and logarithmic absorbance of water vapor at the spot. As shown in the following formula:

[0058] ;

[0059] in, According to the gas category, ;

[0060] Step 3: Using the logarithmic absorbance of water vapor, the logarithmic absorbance of methane and CO2 is subtracted for water vapor absorption interference, yielding the results. Net absorbance of methane and net absorbance of CO2 at the spot As shown in the following formula:

[0061] ;

[0062] in, According to the gas category, ; The cross-absorption coupling coefficient can be obtained through experimental calibration, radiative transfer simulation, or fitting to a priori spectral database.

[0063] Step 4: Using Beer-Lambert's law, the net absorbance of methane, net absorbance of CO2, and logarithmic absorbance of water vapor are inverted to obtain the path integral concentration distributions of methane, CO2, and water vapor in each pixel of the target area, as shown in the following formulas:

[0064] ;

[0065] ;

[0066] in, The path integral concentration distributions of methane and CO2 gases are given. The path integral concentration distribution of water vapor. For methane and CO2 at temperature air pressure Effective absorption cross section under the conditions, For water vapor at temperature air pressure Effective absorption cross section under the given conditions, in meters. 2 / mol; for The effective optical path length corresponding to a pixel at a point, in meters;

[0067] Based on the preset measurement threshold, it is determined whether the path integral concentration distribution of methane gas, CO2 gas and water vapor on each pixel exceeds the measurement threshold. Pixels that exceed the measurement threshold are identified as gas leak points, thereby completing the gas concentration measurement.

[0068] In step 1, the reference spectral image, methane gas spectral image, CO2 gas spectral image, and water vapor spectral image can be obtained through multiple optical systems, i.e., conventional mid-wave infrared cameras or hyperspectral cameras, or simultaneously acquired through a multi-channel gas concentration detection system based on mid-infrared absorption spectroscopy disclosed in Chinese Patent CN111707634A; alternatively, they can be obtained through... Figure 2 , 3 The device shown is a multi-channel parallel gas concentration measurement device that simultaneously acquires data.

[0069] like Figure 2 , 3 As shown, the multi-channel parallel gas concentration measurement device includes a front common-aperture objective lens group 1, a microlens array 2, and a filter array 3 arranged coaxially along the direction of light propagation, as well as an image sensor and a computer; wherein, the front common-aperture objective lens group 1 is used to receive the light beam to be measured and focus it on the front surface of the microlens array 2; the microlens array 2 and the filter array 3 are used to perform two-dimensional spatial sampling of the light beam focused by the front common-aperture objective lens group 1 and form four sub-images on the image plane 4.

[0070] The front common-aperture objective lens group 1 includes a first lens 11 and a second lens 12 arranged coaxially along the direction of light propagation; the first lens 11 is a cemented doublet lens, and the second lens 12 is a parallel plate. The meridional plane and the sagittal plane of the front common-aperture objective lens group 1 are perpendicular to each other and intersect at the optical axis, dividing the image space on the exit side of the front common-aperture objective lens group 1 into four quadrants.

[0071] The microlens array 2 is disposed on the rear focal plane of the front common aperture objective lens group 1, and includes four microlenses, which are located in the four quadrants respectively. In this embodiment, each microlens is a plano-convex lens with a convex front surface and a planar rear surface.

[0072] The filter array 3 includes four narrowband filters, each of which is in close contact with the rear surface of the corresponding microlens. The four narrowband filters are used for filtering, dividing the four quadrants from the first quadrant to the fourth quadrant into a methane detection channel, a CO2 detection channel, a water vapor detection channel, and a reference channel, respectively. Specifically, the wavelength range of the methane detection channel is 1.62~1.75μm, the wavelength range of the CO2 detection channel is 1.95~2.1μm, the wavelength range of the water vapor detection channel is 1.8~1.9μm, and the wavelength range of the reference channel is 1.55~1.6μm.

[0073] An image sensor is set on image plane 4. A computer is electrically connected to the output of the image sensor to perform calculations on four sub-images to obtain the concentrations of methane gas, CO2 gas, and water vapor in the beam under test.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. For those skilled in the art, modifications can be made to the specific technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. However, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions protected by the present invention.

Claims

1. A gas concentration measurement method, used to measure the concentration of a gas to be measured in each pixel within a target area by analyzing a spectral image; characterized in that, Includes the following steps: Step 1: Detect the target area to obtain the reference spectral image, the spectral image of the gas to be tested, and the spectral image of water vapor in the target area; Dark field and flat field corrections were performed on the reference spectral image, the spectral image of the gas to be measured, and the water vapor spectral image to obtain the reference equivalent radiance, the equivalent radiance of the gas to be measured, and the equivalent radiance of the water vapor. Step 2: Using the reference equivalent radiance, normalize the equivalent radiance of the gas to be tested and the equivalent radiance of water vapor to obtain the logarithmic absorbance of the gas to be tested and the logarithmic absorbance of water vapor. Step 3: Using the logarithmic absorbance of water vapor, the water vapor absorption interference is subtracted from the logarithmic absorbance of the gas to be tested to obtain the net absorbance of the gas to be tested. Step 4: Use Beer-Lambert's law to invert the net absorbance of the gas to be measured, and obtain the path integral concentration distribution of the gas in each pixel in the target area, thus completing the gas concentration measurement.

2. The gas concentration measurement method according to claim 1, characterized in that: In step 1, the reference equivalent radiance, the equivalent radiance of the gas to be measured, and the equivalent radiance of water vapor are all obtained by the following formula: ； in, For reference equivalent radiance, equivalent radiance of the gas being measured, or equivalent radiance of water vapor, According to the gas category, , For reference, The gas to be tested, It is water vapor; For pixel coordinates, each Corresponding to one spatial sampling point; For column coordinates, Row coordinates; For the same exposure time, use the reference spectral image, the spectral image of the gas to be tested, or the spectral image of water vapor. Output of pixels at a point; for Dark current bias term at the point; , and These represent the gain constant, exposure time, and photoelectric conversion ratio of the optical system's detection channel when the corresponding spectral image is obtained. for The flat field coefficient at a point.

3. The gas concentration measurement method according to claim 2, characterized in that: In step 2, the logarithmic absorbance of the gas to be measured and the logarithmic absorbance of water vapor are both obtained by the following formula: ； in, for The logarithmic absorbance of the gas to be measured or the logarithmic absorbance of water vapor at the point; According to the gas category, .

4. The gas concentration measurement method according to claim 3, characterized in that: In step 3, the net absorbance of the gas to be tested is obtained by the following formula: ； in, For the gas to be tested in Net absorbance at the point; This represents the cross-absorption coupling coefficient.

5. The gas concentration measurement method according to claim 4, characterized in that: In step 4, the path integral concentration distribution of the gas to be measured on each pixel within the target area is obtained by the following formula: ； in, For the gas to be tested in Path integral concentration distribution at a point The gas to be tested was subjected to temperature air pressure Effective absorption cross section under the given conditions, in meters. 2 / mol; for The effective optical path length corresponding to a pixel at a point, measured in meters (m).

6. The gas concentration measurement method according to claim 5, characterized in that, Step 4, in addition to obtaining the path integral concentration distribution of the gas to be measured on each pixel within the target area, also includes: The logarithmic absorbance of water vapor was inverted using the Beer-Lambert law, and the path integral concentration distribution of water vapor in each pixel of the target region was obtained by the following formula: ； in, For water vapor in Path integral concentration distribution at a point For water vapor at temperature air pressure Effective absorption cross section under the given conditions, in meters. 2 / mol.

7. The gas concentration measurement method according to claim 1, characterized in that: The gas to be tested is methane or carbon dioxide.

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