CN117053928A - Calibration method for thermal infrared imager lens distortion correction and imaging non-uniformity - Google Patents
Calibration method for thermal infrared imager lens distortion correction and imaging non-uniformity Download PDFInfo
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Abstract
The invention discloses a calibration method for lens distortion correction and imaging non-uniformity of an infrared thermal imager, which relates to the technical field of infrared thermal imager calibration, and solves the problem of measurement result errors of the infrared thermal imager caused by lens distortion and imaging non-uniformity of a photosensitive element by starting from the two aspects of improving the lens distortion and the response characteristic.
Description
Technical field:
the invention relates to the technical field of thermal infrared imager calibration, in particular to a calibration method for thermal infrared imager lens distortion correction and imaging non-uniformity.
The background technology is as follows:
the infrared thermal imager is an instrument capable of detecting and measuring infrared radiation on the surface of an object, and converts the infrared radiation emitted by the surface of the object into a thermal image or video by utilizing the principle of infrared radiation so as to display the thermal distribution condition of the target object. The thermal infrared imager can sense radiation in far infrared to near infrared bands and can operate in low light or full black environments. The working principle of the thermal infrared imager is that based on the thermal radiation characteristic of an object, any substance with the temperature higher than absolute zero (-273.15 ℃) can radiate energy outwards, and according to the Planckian radiation law, the temperature of the surface of the substance can be calculated by measuring the intensity of infrared radiation within a certain wavelength range. Thus, by observing the thermal image or video, the information such as the temperature distribution, the hot spot position and the like of the target object can be known.
Thermal infrared imagers are widely used in many fields. For example, military may be used for night vision, target detection and tracking; the method can be industrially used for fault diagnosis, equipment monitoring and maintenance; the medicine can be used for body temperature detection, disease diagnosis and the like. In addition, thermal infrared imagers are also widely used in the fields of construction, fire protection, security protection, and the like. In the temperature measurement process, the temperature measurement precision of the thermal infrared imager is the most important one. There are many factors that affect the temperature measurement accuracy, and lens distortion is a common problem for the current thermal infrared imagers.
The lens distortion refers to an image deformation phenomenon caused by a lens or a lens in an optical system, and may have a certain influence on the temperature measurement accuracy of the thermal infrared imager, which may cause image distortion on an imaging plane, so that the size and shape of an actual target are not completely consistent with those displayed in the image, or the position of a measurement point may not be accurately determined when the temperature of the target is measured, which may affect the spatial resolution of the thermal infrared imager to the target, thereby affecting the accuracy of temperature measurement. Some types of lens aberrations may also cause different magnification of different areas in the image measured by the thermal infrared imager, which may affect the selection and size of the measurement area. If regions of large temperature difference cannot be clearly displayed on an image due to distortion, temperature measurement of these regions may be affected to some extent. In addition, because of the temperature measurement principle of the thermal infrared imager, infrared radiation of a target object is detected, and an infrared detector sensitive to the infrared radiation is utilized to convert the infrared radiation into an electric signal and then a series of circuits are used for displaying a temperature image of the target. Thus for some thermal infrared imagers with large dynamic ranges, their response to different temperature ranges may not be linear and their response characteristics may change as the time of use increases. In view of the above factors affecting the temperature measurement accuracy of the thermal infrared imager, it is necessary to develop a calibration method for imaging non-uniformity of the thermal infrared imager.
In order to solve the above-mentioned problems, patent CN113670445a discloses a "calibration method for imaging non-uniformity of thermal infrared imager", which corrects non-uniformity of thermal infrared imager by dividing calibration area and calibration temperature section, but does not fully consider the influence of lens distortion. The invention starts from the two aspects of improving the lens distortion and the response characteristic, solves the problem of measuring result errors of the thermal infrared imager caused by the lens distortion and the imaging non-uniformity of the photosensitive element, reduces the influence on the temperature measuring result caused by the lens distortion and the response characteristic change of the thermal infrared imager, and further improves the temperature measuring precision of the thermal infrared imager.
The invention comprises the following steps:
the invention aims to overcome the defect of temperature measurement precision of a thermal infrared imager caused by lens distortion and response characteristic change, and provides a calibration method for correcting the lens distortion and imaging non-uniformity of the thermal infrared imager.
In order to achieve the above purpose, the present invention provides a calibration method for correcting distortion and imaging non-uniformity of a thermal infrared imager lens, which comprises the following steps:
(1) Manufacturing a checkerboard calibration plate aiming at the thermal infrared imager and correcting lens distortion of the thermal infrared imager;
(2) Dividing a calibration temperature section according to the dynamic range of the thermal infrared imager, uniformly dividing the dynamic range of the thermal infrared imager into continuous calibration temperature sections according to a certain gradient, wherein the lowest temperature and the highest temperature in a single calibration temperature section are respectively T min And T is max ；
(3) Dividing temperature measurement grids, collecting blackbody furnace images in each temperature measurement grid, and then cutting and splicing the images of the blackbody furnace covering each grid to obtain complete temperature measurement data of the thermal infrared imager at a certain temperature;
(4) Dividing calibration areas according to the resolution ratio of the thermal infrared imager, collecting temperature data for each calibration area of the thermal infrared image in each temperature section, and converting the obtained temperature data into radiance data at the temperature according to a radiance conversion formula;
(5) And obtaining correction coefficients of all pixel points of the thermal infrared imager in the temperature measuring range, calculating the corrected radiance by using the correction coefficients, and reversely pushing back the temperature for analysis.
In a further technical scheme, the step (1) specifically adopts two rectangular sheets of different materials, namely a polished aluminum sheet and a black frosted plastic sheet, which are alternately arranged according to grid patterns in a checkerboard form, are adhered to a bottom plate of a calibration plate, and calibrate the lens distortion of the thermal infrared imager according to a Zhang Zhengyou calibration method.
In a further technical scheme, the step (3) specifically includes the following steps:
(3.1) dividing a thermal image of the thermal infrared imager into mutually connected acquisition areas according to the resolution of a detector of the thermal infrared imager;
(3.2) adjusting the horizontal and vertical positions of the thermal infrared imager without changing the position of the black body furnace, so that the black body furnace covers each grid in the thermal infrared imager preview picture, and collecting a plurality of images covering each grid;
and (3.3) intercepting the corresponding covering grid parts of the acquired multiple blackbody furnace images, and then splicing the intercepted parts according to the original grid sequence to finally obtain the complete response thermal image of the blackbody furnace of the thermal infrared imager at the temperature.
In a further technical scheme, the step (4) specifically includes the following steps:
(4.1) dividing a thermal image of the thermal infrared imager into continuous calibration areas according to the resolution of a detector of the thermal infrared imager and the square annular equidistant;
(4.2) setting the innermost region as an A region, the outermost region as an H region, and numbering all the regions in sequence from inside to outside A to H;
(4.3) selecting a temperature segment to be calibrated, and determining T min And T is max Selecting two temperature values T 1 And T is 2 So as to satisfy T min <T 1 <T 2 <T max ；
(4.4) setting the blackbody furnace temperature to T 1 Obtaining the temperature T of the blackbody furnace through the step (3) 1 After which the temperature data is divided into the following divisions according to the division of the calibration area A T m1 To the point of H T m1 A total of 8 temperature data matrices;
wherein the left subscript a represents the measured thermal image a region; the left subscript H represents the measured thermal image H region;
(4.5) since the response of the thermal infrared imager is linear with the radiance, it is necessary to convert the temperature into the radiance and then perform the subsequent process. Setting the blackbody furnace to T 1 Converting the temperature data matrix obtained by temperature processing into a radiance matrix through a radiance conversion formula A L m1 To the point of H L m1 The radiance conversion formula is:
wherein the constant a 1 ＝2hc 2 ＝1.191066×10 -12 W·cm 2 ；
a 2 ＝hc/k＝1.438833×10 4 μm·K；
The unit of the radiance L is W cm -2 ·sr -1 Represents 1 square centimeter of black body at a wavelength interval (width) Δλ=λ 2 -λ 1 Power radiated by the internal unit solid angle.
(4.6) setting the blackbody furnace temperature to T 2 Repeating the steps (4.4) and (4.5) to obtain a radiance matrix A L m2 To the point of H L m2 。
In a further technical scheme, the step (5) specifically comprises the following steps:
(5.1) for the radiance matrices obtained in steps (4.5) and (4.6), in a single calibration region, e.g. region a, there is the following set of equations:
wherein, A L m1 (i, j) and A L m2 (i, j) is represented by T 1 And T is 2 The pixel point of the ith row and the jth column in the A area radiance matrix corresponds to the radiance value at the temperature;
L 1 (i, j) and L 2 (i, j) represents the corresponding radiance value at the blackbody furnace set temperature;
representing a temperature section T min ～T max Attenuation coefficients of the radiance values of all pixel points in the inner area A;
representing a temperature section T min ～T max Drift coefficients of the radiance values of all pixel points in the inner area A;
by the combined type (2) and (3), the attenuation coefficient matrix is calculatedAnd an offset coefficient matrixExpressed as:
(5.2) sequentially calculating all the calibration areas in the temperature section T according to the step (5.1) min ～T max Attenuation coefficient matrix inAnd drift coefficient matrix->
(5.3) for the point with coordinates (x, y) in the nth calibration area, the thermal image measurement temperature corresponds to the radiance L measure The real temperature of the point corresponds to the radiance L calibrated The relationship between them is:
the beneficial effects of the invention are as follows: the invention starts from the two aspects of improving lens distortion and response characteristics, solves the problem of measurement result errors of the thermal infrared imager caused by lens distortion and imaging non-uniformity of a photosensitive element, and particularly reduces the influence on a temperature measurement result caused by lens distortion and response characteristic change of the thermal infrared imager by manufacturing a specific calibration plate of the thermal infrared imager and calibrating the thermal infrared imager in a segmented and regional mode, thereby improving the temperature measurement precision of the thermal infrared imager.
Description of the drawings:
FIG. 1 is a physical image and an infrared image of a thermal infrared imager specific calibration plate in embodiment 1 of the present invention;
FIG. 2 is a graph showing the comparison of the effects of the calibration plate in example 1 of the present invention;
fig. 3 is a schematic diagram of a black body furnace covering a first square in a preview screen of a thermal infrared imager in embodiment 1 of the present invention;
FIG. 4 shows two-dimensional temperature data acquired by the thermal infrared imager at 40℃in the blackbody furnace after the interception and splicing treatment in example 1 of the present invention;
FIG. 5 is a schematic diagram illustrating the division of calibration areas of a thermal infrared imager according to embodiment 1 of the present invention;
FIG. 6 is a graph showing comparison of temperature data before and after calibration in a temperature range of 0 to 100℃under 60℃verification in example 1 of the present invention;
FIG. 7 is a graph showing temperature data comparison between the temperature before and after correction in example 1 of the present invention at the rest of the temperature ranges;
fig. 8 is a graph comparing temperature data corrected by the calibration method of the present invention and the calibration method of the patent CN113670445 a.
The specific embodiment is as follows:
the invention is further described below with reference to specific embodiments and illustrations in order to make the technical means, the creation features, the achievement of the purpose and the effect of the implementation of the invention easy to understand.
Example 1
The embodiment provides a calibration method for imaging non-uniformity of a thermal infrared imager, which specifically comprises the following steps:
s1, using FLIR A40M uncooled long-wave thermal infrared imager. 40 highly polished aluminum sheets and three black frosted plastic panels were prepared. And (3) sticking the three black frosted plastic plates into a whole to enhance the deformation resistance, and then alternately arranging 40 polished aluminum sheets according to a grid pattern in a checkerboard form and sticking the polished aluminum sheets on the stuck black frosted plastic plates.
And S2, under the condition that the imaging is good at a position 50cm away from the thermal infrared imager, locking a temperature threshold (measuring range), ensuring stable image quality in the whole shooting process, and shooting 40 calibration plate thermal images with different poses. The real object diagram and the shooting effect of the calibration plate are shown in figure 1.
S3, screening the acquired images, and eliminating thermal images with low imaging quality. And (5) introducing the qualified thermal image into Matlab for calibration treatment to obtain an internal reference matrix and a distortion coefficient of the thermal infrared imager. And comparing the original image with the corrected image to obtain a comparison effect diagram, as shown in fig. 2.
And S4, aligning the lens of the thermal infrared imager to the blackbody furnace mouth, wherein the front end of the lens is 50cm away from the blackbody furnace mouth. The tripod and blackbody furnace positions are unchanged in the whole experimental process. According to the performance parameters of the thermal infrared imager and the blackbody furnace, the calibration temperature section is divided into 0-100 ℃, 100-200 ℃, 200-300 ℃ and 300-400 ℃.
S5, firstly calibrating a temperature range of 0-100 ℃ and selecting 40 ℃ as T 1 T at 80 DEG C 2 . Setting the temperature of the blackbody furnace to 40 ℃, and continuously waiting for 30min after the temperature reading of the blackbody furnace reaches a target value to stabilize the temperature.
S6, the resolution of the FLIR A40M thermal infrared imager is 320 multiplied by 240, and the preview picture is divided into 16 rectangular blocks with the size of 80 multiplied by 60 by using a camera software self-contained drawing tool. The thermal infrared imager position is adjusted so that the blackbody furnace completely covers the first square area at the preview interface, as shown in fig. 3, and then the entire thermal image is acquired.
And S7, continuously adjusting the position of the thermal infrared imager, enabling the black body furnace to cover the other 15 rectangular areas in sequence, collecting the thermal images, and finally collecting the thermal images of each square covered by the 16-spoke black body furnace.
S8, reading the acquired 16 thermal images by Matlab software, intercepting matrix areas of the thermal images covered by the blackbody furnace, and then splicing according to the sequence of the original matrix areas to obtain a complete thermal image of the thermal infrared imager of the blackbody furnace, wherein the thermal image is set at 40 ℃, as shown in FIG. 4.
S9, processing by Matlab software to obtain a two-dimensional temperature data matrix T of a thermal image measured by an infrared thermal imager at 40 DEG C m1 And the two-dimensional temperature data matrix T is matched at equal intervals m1 Partitioning to obtain A T m1 To the point of H T m1 A total of 8 square annular two-dimensional temperature data matrices are shown in fig. 5.
S10, as the response of the thermal infrared imager and the radiance are in linear relation, 8 square annular two-dimensional temperature data matrixes obtained in the step S9 are obtained A T m1 To the point of H T m1 The data are converted into 8 square annular two-dimensional radiance data by a radiance conversion formula (1)Matrix array A L m1 To the point of H L m1 。
Taking an H area as an example, the H area two-dimensional temperature data matrix is H T m1 The H-region radiance matrix is obtained after calculation of the radiance conversion formula H L m1 。 H T m1 And (3) with H L m1 The following is shown:
it is noted that all the annular matrix areas except the area A are square annular areas with the transverse width of 20 and the longitudinal width of 15, the corresponding areas except the annular inner elements are corresponding measured temperature values, and the rest elements are all set to 0.
S11, setting the temperature of the blackbody furnace to 80 ℃, and continuously waiting for 30 minutes to stabilize the temperature after the temperature reading of the blackbody furnace reaches the target value.
S12, obtaining 8 square annular two-dimensional radiance data matrixes at 80 ℃ according to the method of the steps S4-S9 A L m2 To the point of H L m2 . For the H region in the temperature range of 0-100 ℃, there is the following set of equations:
H L m1 (i,j)＝ H K 0～100 (i,j)L 1 (i,j)+ H B 0～100 (i,j) (8)
H L m2 (i,j)＝ H K 0-100 (i,j)L 2 (i,j)+ H B 0-100 (i,j) (9)
where (i, j) is the coordinates of each pixel point in the H region. The combination of the formula (8) and the formula (9) can be calculated to obtain 0 to 10Attenuation coefficient matrix of H region in 0 ℃ temperature section H K 0-100 And drift coefficient matrix H B 0-100 。
Attenuation coefficient matrix H K 0～100 The following is shown:
drift coefficient matrix H B 0-100 The following is shown:
s13, repeating the step S12, and calculating attenuation coefficient matrixes corresponding to all areas in the temperature range of 0-100 DEG C n K 0-100 And drift coefficient matrix n B 0-100 . Thus, the calibration for the temperature section of 0-100 ℃ is completed.
S14, measuring the temperature by using the calibrated thermal infrared imager, wherein in the calibrated temperature section, for a pixel point with coordinates (x, y) in an nth region, the radiation brightness corresponding to the temperature measured at the point in a thermal image is L measure Calculating the radiance value L corresponding to the real temperature of the point by using the formula (10) calibrated 。
The equation (10) shows that the temperature data are converted into the radiance data, then the attenuation coefficient matrix and the drift coefficient matrix corresponding to the temperature section are used for correcting the original radiance data, and finally the temperature data are converted back, so that a more accurate temperature measurement result can be obtained.
S15, setting the temperature of the blackbody furnace to 60 ℃, and comparing and verifying temperature data (before correction) obtained by directly measuring the blackbody furnace with temperature data (after correction) processed by the calibration method of the invention by using a thermal infrared imager.
The temperature data before and after correction are compared as shown in fig. 6. The average temperature data for each calibration area before and after correction is compared as shown in table 1.
TABLE 1
From the data in table 1, it can be calculated: the mean square error of the average temperature of the calibration area obtained by measurement of the thermal infrared imager before correction is sigma 1 3.6872, and the mean square error of the average temperature of the calibration area is sigma 2 The method of the invention has larger improvement on the temperature measurement precision of the thermal infrared imager at 0-100 ℃.
S16, changing the temperature of the blackbody furnace, calibrating the thermal infrared imager at the temperature of 100-200 ℃ and 200-300 ℃ and 300-400 ℃ according to the method of the steps S5-S13, respectively calculating an attenuation coefficient matrix and a drift coefficient matrix corresponding to each temperature section, and finally obtaining temperature data before correction and after correction and comparing.
A comparison of measured temperatures before and after correction with blackbody furnace temperature is shown in fig. 7. The measured temperatures before and after correction are shown in table 2.
TABLE 2
From the data in table 2, it can be calculated: the mean square error of the average temperature measured by the thermal infrared imager before correction is sigma 1 4.5278, and the mean square error of the average temperature calculated after correction is σ 2 From the above, 1.6129, the calibration method of the present invention has different ranges for the temperature measurement accuracy of the thermal infrared imager at 100-200deg.C, 200-300deg.C, 300-400deg.CAnd (5) improving the degree.
S17, in order to intuitively compare the effect of the calibration method of the invention on improving the temperature measurement precision of the thermal infrared imager, a set of comparison experiments are established in the calibration method disclosed in the patent CN113670445A, and the calculation is respectively carried out in each temperature section of 100-200 ℃, 200-300 ℃ and 300-400 ℃. For convenience of explanation, the calibration method of the present invention will be referred to as "the present method" and the calibration method of patent CN113670445a will be referred to as "the comparative method" hereinafter.
Temperature data calculated using the present method and the comparative method are shown in fig. 8. The temperature deviation of this method from the comparative method is shown in table 3.
TABLE 3 Table 3
From the data in table 3, it can be calculated: the mean square error of the calculation result obtained by using the method is sigma 1 = 1.6129, the mean square error of the calculated result obtained using the comparison method is σ 2 = 2.3896, the accuracy of the calculation result of the method is better than that of the comparison method.
The foregoing has shown and described the basic principles and main features of the present invention and the advantages of the present invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, and that the above embodiments and descriptions are merely illustrative of the principles of the present invention, and various changes and modifications may be made without departing from the spirit and scope of the invention, which is defined in the appended claims. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (5)
1. The method for calibrating the lens distortion correction and imaging non-uniformity of the thermal infrared imager is characterized by comprising the following steps of:
(1) Manufacturing a checkerboard calibration plate aiming at the thermal infrared imager and correcting lens distortion of the thermal infrared imager;
(2) Root of Chinese characterDividing a calibration temperature section according to the dynamic range of the thermal infrared imager, uniformly dividing the dynamic range of the thermal infrared imager into continuous calibration temperature sections according to a certain gradient, wherein the lowest temperature and the highest temperature in a single calibration temperature section are respectively T min And T is max ；
(3) Dividing temperature measurement grids, collecting blackbody furnace images in each temperature measurement grid, and then cutting and splicing the images of the blackbody furnace covering each grid to obtain complete temperature measurement data of the thermal infrared imager at a certain temperature;
(4) Dividing calibration areas according to the resolution ratio of the thermal infrared imager, collecting temperature data for each calibration area of the thermal infrared image in each temperature section, and converting the obtained temperature data into radiance data at the temperature according to a radiance conversion formula;
(5) And obtaining correction coefficients of all pixel points of the thermal infrared imager in the temperature measuring range, calculating the corrected radiance by using the correction coefficients, and reversely pushing back the temperature for analysis.
2. The calibration method according to claim 1, wherein the step (1) is characterized in that two kinds of rectangular sheets of different materials, namely polished aluminum sheets and black frosted plastic sheets, are alternately arranged according to a grid pattern in a checkerboard form, are adhered to a bottom plate of a calibration plate, and calibrate lens distortion of the thermal infrared imager according to a Zhang Zhengyou calibration method.
3. The calibration method according to claim 1 or 2, wherein the step (3) specifically comprises the steps of:
(3.1) dividing a thermal image of the thermal infrared imager into mutually connected acquisition areas according to the resolution of a detector of the thermal infrared imager;
(3.2) adjusting the horizontal and vertical positions of the thermal infrared imager without changing the position of the black body furnace, so that the black body furnace covers each grid in the thermal infrared imager preview picture, and collecting a plurality of images covering each grid;
and (3.3) intercepting the corresponding covering grid parts of the acquired multiple blackbody furnace images, and then splicing the intercepted parts according to the original grid sequence to finally obtain the complete response thermal image of the blackbody furnace of the thermal infrared imager at the temperature.
4. A calibration method according to any one of claims 1-3, characterized in that: the step (4) specifically comprises the following steps:
(4.1) dividing a thermal image of the thermal infrared imager into continuous calibration areas according to the resolution of a detector of the thermal infrared imager and the square annular equidistant;
(4.2) setting the innermost region as an A region, the outermost region as an H region, and numbering all the regions in sequence from inside to outside A to H;
(4.3) selecting a temperature segment to be calibrated, and determining T min And T is max Selecting two temperature values T 1 And T is 2 So as to satisfy T min <T 1 <T 2 <T max ；
(4.4) setting the blackbody furnace temperature to T 1 Obtaining the temperature T of the blackbody furnace through the step (3) 1 After which the temperature data is divided into the following divisions according to the division of the calibration area A T m1 To the point of H T m1 A total of 8 temperature data matrices;
wherein the left subscript a represents the measured thermal image a region; the left subscript H represents the measured thermal image H region;
(4.5) setting the blackbody furnace to T 1 Converting the temperature data matrix obtained by temperature processing into a radiance matrix through a radiance conversion formula A L m1 To the point of H L m1 The radiance conversion formula is:
wherein the constant a 1 ＝2hc 2 ＝1.191066×10 -12 W·cm 2 ；
a 2 ＝hc/k＝1.438833×10 4 μm·K；
The unit of the radiance L is W cm -2 ·sr -1 ；
(4.6) setting the blackbody furnace temperature to T 2 Repeating the steps (4.4) and (4.5) to obtain a radiance matrix A L m2 To the point of H L m2 。
5. The calibration method according to any one of claims 1-4, characterized in that: the step (5) specifically comprises the following steps:
(5.1) for the radiance matrices obtained in steps (4.5) and (4.6), in a single calibration region, e.g. region a, there is the following set of equations:
wherein, A L m1 (i, j) and A L m2 (i, j) is represented by T 1 And T is 2 The pixel point of the ith row and the jth column in the A area radiance matrix corresponds to the radiance value at the temperature;
L 1 (i, j) and L 2 (i, j) represents the corresponding radiance value at the blackbody furnace set temperature;
representing a temperature section T min ～T max Attenuation coefficients of the radiance values of all pixel points in the inner area A;
representing a temperature section T min ～T max Drift coefficients of the radiance values of all pixel points in the inner area A;
by combining (2) and (3), the attenuation system is calculatedNumber matrixAnd an offset coefficient matrixExpressed as:
(5.2) sequentially calculating all the calibration areas in the temperature section T according to the step (5.1) min ～T max Attenuation coefficient matrix inAnd drift coefficient matrix->
(5.3) for the point with coordinates (x, y) in the nth calibration area, the thermal image measurement temperature corresponds to the radiance L measure The real temperature of the point corresponds to the radiance L calibrated The relationship between them is:
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