CN113494963A - Radiation calibration method suitable for rapid quantitative processing of thermal imager - Google Patents
Radiation calibration method suitable for rapid quantitative processing of thermal imager Download PDFInfo
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- 238000002474 experimental method Methods 0.000 claims abstract description 9
- 238000012360 testing method Methods 0.000 claims abstract description 8
- 230000005457 Black-body radiation Effects 0.000 claims description 20
- 238000003384 imaging method Methods 0.000 claims description 15
- 230000036760 body temperature Effects 0.000 claims description 14
- 230000003595 spectral effect Effects 0.000 claims description 9
- 210000001747 pupil Anatomy 0.000 claims description 6
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- 238000011088 calibration curve Methods 0.000 description 7
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
- G01J5/02—Constructional details
- G01J5/08—Optical arrangements
- G01J5/0887—Integrating cavities mimicking black bodies, wherein the heat propagation between the black body and the measuring element does not occur within a solid; Use of bodies placed inside the fluid stream for measurement of the temperature of gases; Use of the reemission from a surface, e.g. reflective surface; Emissivity enhancement by multiple reflections
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
- G01J2005/0077—Imaging
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Abstract
The invention provides a radiation calibration method suitable for rapid quantitative processing of a thermal imager, and relates to the field of infrared testing, wherein the method comprises the following steps of 1: converting the temperature of the black body into radiation brightness of the black body according to a Planck formula; step 2: reading the relation between the number of charges received by the detector and the exposure time under different blackbody temperatures; and step 3: and acquiring the charge flux densities F corresponding to different blackbody temperatures through a calibration experiment, and performing linear fitting on the blackbody temperatures and the charge flux densities F. The radiometric calibration parameter is not dependent on the exposure time of the thermal infrared imager any more, and the rapid quantitative measurement of the thermal infrared imager and the expansion of the temperature range of the infrared quantitative measurement are facilitated.
Description
Technical Field
The invention relates to the field of infrared testing, in particular to a radiation calibration method suitable for rapid quantitative processing of a thermal imager.
Background
Radiometric calibration of the thermal infrared imager is the basis for realizing quantitative measurement of the thermal infrared imager, and laboratory radiometric calibration is to calculate calibration coefficients Gain and Offset by using algorithms according to the radiance value L measured by different thermal infrared imagers and the imaging digital count DN. As shown in fig. 1, the currently popular laboratory radiometric calibration includes the following steps: (1) setting the thermal infrared imager to carry out non-uniform correction on the thermal infrared imager within a certain exposure time, and taking a black body radiation source as a standard; (2) calculating a blackbody radiation brightness value L according to the blackbody radiation temperature and the blackbody radiation emissivity; (3) imaging black body radiation through a thermal infrared imager, and recording a gray value (namely a digital count DN) of the black body radiation imaged on the thermal infrared imager; (4) changing the black body radiation temperature, recording the one-to-one correspondence between a plurality of groups of black body radiation brightness values L and gray scale values (DN), and obtaining calibration coefficients Gain and Offset through linear fitting of the black body radiation brightness values L and the gray scale values (DN); (5) obtaining a functional relation between the black body radiation brightness value L and a gray value (DN): l ═ Gain × DN + Offset.
In the traditional laboratory radiometric calibration, different exposure times of the thermal infrared imagers correspond to a set of calibration parameters, so that the calibration results of several exposure times need to be obtained, and the radiometric calibration needs to be carried out for several exposure times. Because the exposure time of the thermal infrared imager needs to be changed according to the radiation intensity of a target in the testing process of the thermal infrared imager, calibration before testing is difficult to ensure that the exposure time of the thermal infrared imager subjected to radiation calibration is matched with the exposure time used for testing, and the method is very disadvantageous for acquiring quantitative data of the thermal infrared imager in real time and rapidly and quantitatively measuring the thermal infrared imager.
Disclosure of Invention
In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a radiometric calibration method suitable for rapid quantitative processing of thermal imagers, wherein radiometric calibration parameters no longer depend on exposure time of the thermal infrared imagers, which is beneficial for rapid quantitative measurement of the thermal infrared imagers and for expanding temperature ranges of the infrared quantitative measurement.
The invention provides a radiation calibration method suitable for rapid quantitative processing of a thermal imager, which comprises the following steps:
step 1: converting the temperature of the black body into radiation brightness of the black body according to a Planck formula;
step 2: reading the relation between the number of charges received by the detector and the exposure time under different blackbody temperatures;
and step 3: and acquiring the charge flux densities F corresponding to different blackbody temperatures through a calibration experiment, and performing linear fitting on the blackbody temperatures and the charge flux densities F.
In an embodiment of the present invention, the step 1 specifically includes the following steps:
step 1.1: obtaining spectral radiation exitance W according to Planck's formulaλ：
Wherein, WλIs the spectral radiant emittance (W.cm)-2.μm-1) (ii) a λ is wavelength (μm); c1Is the first radiation constant of 3.7415 × 104(W.cm-2.μm4)；C2Is the second radiation constant 1.4388 × 104(μm.k); t is the absolute temperature (K);
step 1.2: obtaining the radiant exitance W of the thermal infrared imager in the imaging wave band Delta lambda△λ：
Wherein, W△λRadiation emittance (W.cm) of delta lambda band-2)；ελIs the spectral emissivity; lambda [ alpha ]1The lower limit (mum) of the imaging waveband of the thermal infrared imager is set; lambda [ alpha ]2The upper limit (mum) of the imaging wave band of the thermal infrared imager;
step 1.3: will epsilonλConsider a constant and rewrite equation (2) to:
wherein epsilon is a constant;
step 1.4: obtaining the blackbody radiation brightness Lbb：
Wherein L isbbIs the blackbody radiation brightness of the delta lambda wave band.
In an embodiment of the present invention, the formula of the step 3 for linearly fitting the black body temperature and the charge flux density F is as follows:
wherein, the black body temperature and the charge flux density F are subjected to calibration experiment to obtain multiple groups of corresponding data, G and FoffFor calibration parameters, they were obtained by fitting the test data T and F.
In an embodiment of the invention, in step 3, when the calibration experiment obtains the charge flux densities F corresponding to different black body temperatures, the black body needs to cover the thermal infrared imager imaging system, so that the entrance pupil brightness of the thermal infrared imager is the black body radiation brightness.
As described above, the radiation calibration method suitable for the rapid quantitative processing of the thermal imager of the present invention has the following beneficial effects: in order to ensure the universality of a calibration curve after the exposure time is controlled quickly, introducing parameters of charge flux density, and taking the charge flux density as data collected by a thermal infrared imager and reducing the radiation brightness of a black body; different calibration curves are not required to be called any more in different exposure times, parameters such as different gain coefficients and exposure times are synthesized into the same calibration curve, the waiting time for changing the calibration curve after the parameters of the thermal infrared imager are modified is greatly reduced, and the method has important significance for rapid acquisition and storage of detection of the thermal infrared imager.
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FIG. 1 shows a flow chart of laboratory radiometric calibration as disclosed in the prior art of the present invention.
FIG. 2 is a graph showing a digital count-exposure time fit curve for various blackbody temperatures disclosed in an embodiment of the present invention.
FIG. 3 is a graph showing the fitted charge-exposure time curves for different blackbody temperatures disclosed in an embodiment of the present invention.
Fig. 4 shows a fitted graph of charge flux density versus temperature as disclosed in an embodiment of the present invention.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict.
It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention, and the components related to the present invention are only shown in the drawings rather than drawn according to the number, shape and size of the components in actual implementation, and the type, quantity and proportion of the components in actual implementation may be changed freely, and the layout of the components may be more complicated.
As shown in fig. 2, for a given pixel, the digital count (DN) is fitted to the Exposure Time (ET) as the black body temperature increases, ideally the accumulated digital count increases linearly with increasing exposure time, the detected electron flux (slope of the curve) increases with increasing black body temperature, but the intercept of each curve is always the same, so only the offset level of the thermographic imaging system is read. Therefore, based on the principle, the invention provides a radiometric calibration method suitable for rapid quantitative processing of a thermal imager, which is closely combined with the physical performance of the thermal infrared imager, facilitates correction of influences appearing in different dynamic ranges, does not depend on a function corresponding to the change of the digital level of the thermal infrared imager and the temperature of a black body or a radiation change function in a wave band range, and takes the charge flux density as the reading sensitivity of different exposure times; the method specifically comprises the following steps:
step 1: because the digital level output by the thermal infrared imager is in direct proportion to the black body radiation brightness, for comparison, the black body temperature is converted into the black body radiation brightness according to the Planck formula:
wherein, WλIs the spectral radiant emittance (W.cm)-2.μm-1) (ii) a λ is wavelength (μm); c1Is the first radiation constant of 3.7415 × 104(W.cm-2.μm4)；C2Is the second radiation constant 1.4388 × 104(μm.k); t is the absolute temperature (K);
therefore, the spectral radiation emittance of the black body in the imaging waveband delta lambda of the thermal infrared imager is the integral of the imaging waveband delta lambda:
wherein, W△λRadiation emittance (W.cm) of delta lambda band-2)；ελIs the spectral emissivity; lambda [ alpha ]1The lower limit (mum) of the imaging waveband of the thermal infrared imager is set; lambda [ alpha ]2The upper limit (mum) of the imaging wave band of the thermal infrared imager;
for black body,. epsilonλConsidered a constant, equation (2) is therefore rewritten as:
wherein epsilon is a constant;
obtaining the blackbody radiation brightness Lbb：
Wherein L isbbIs the blackbody radiation brightness of the delta lambda wave band.
Step 2: reading the relationship between the number of charges received by the detector and the exposure time of the thermal infrared imager under different blackbody temperatures;
theoretically, the number of charges (C) received by the detector at the same blackbody temperature is linear with the Exposure Time (ET), as shown in FIG. 3, CoffRepresenting a calibrated bias parameter, namely the magnitude of dark current noise of the detected charge; the slope of each line represents the number of charges passing through the detector per unit time, i.e., the charge flux density F (counts/μ s) corresponding to different blackbody temperatures; performing linear fitting on the function of the charge flux density F and the black body temperature; firstly, the charge flux density F depends on the black body radiant energy received by the thermal infrared imager in unit time, namely is in direct proportion to the radiant power received by the entrance pupil of the thermal infrared imager and further in direct proportion to the radiant brightness received by the entrance pupil of the thermal infrared imager, and a black body is required to cover an imaging system of the thermal infrared imager in a calibration process, so that the entrance pupil brightness of the thermal infrared imager is the black body radiant brightness;
and step 3: acquiring charge flux densities F corresponding to different black body temperatures through a calibration experiment, and performing linear fitting on the black body temperatures and the charge flux densities F;
as shown in fig. 4, the abscissa identifies the black body temperature T for calibration, and the ordinate represents the charge flux density F; the curve is a final calibration curve F (T) to be obtained, does not contain exposure time parameters, and is suitable for inversion of entrance pupil radiance of the thermal infrared imager under various different exposure time conditions;
the linear fit formula used is:
wherein, the black body temperature T and the charge flux density F are subjected to calibration experiment to obtain multiple groups of corresponding data, G and Fo For calibration parameters, they were obtained by fitting the test data T and F.
In summary, the present invention is used for any exposure time supported by a given focal plane detector; (exposure time does not need to be predetermined while avoiding saturation); suitable for high contrast scenes (high dynamic range scenes); is suitable for any ambient temperature; the quantification is realized in the thermal infrared imager, and radiation calibration data processing is carried out according to the requirements of the thermal infrared imager on quick and real-time quantitative measurement, so that the traditional relative radiation calibration process is replaced, and the data processing time occupied by relative radiation calibration and the introduced uncertainty are reduced; meanwhile, charge flux density calibration is introduced according to the charge number of the pixel, and the traditional method of adopting different calibration curves for different exposure times is improved into a calibration curve which can be suitable for any exposure time, so that the quantitative processing process of data is greatly reduced, and the rapid real-time quantitative processing of the thermal infrared imager is realized. Therefore, the invention effectively overcomes various defects in the prior art and has high industrial utilization value.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.
Claims (4)
1. A radiation calibration method suitable for rapid quantitative processing of a thermal imager is characterized by comprising the following steps:
step 1: converting the temperature of the black body into radiation brightness of the black body according to a Planck formula;
step 2: reading the relation between the number of charges received by the detector and the exposure time under different blackbody temperatures;
and step 3: and acquiring the charge flux densities F corresponding to different blackbody temperatures through a calibration experiment, and performing linear fitting on the blackbody temperatures and the charge flux densities F.
2. The radiometric calibration method suitable for rapid quantitative processing of thermal imagers according to claim 1, characterized in that: the step 1 is specifically as follows:
step 1.1: obtaining spectral radiation exitance W according to Planck's formulaλ：
Wherein, WλIs the spectral radiant emittance (W.cm)-2.μm-1) (ii) a λ is wavelength (μm); c1Is the first radiation constant of 3.7415 × 104(W.cm-2.μm4)；C2Is the second radiation constant 1.4388 × 104(μm.k); t is the absolute temperature (K);
step 1.2: obtaining the radiant exitance W of the thermal infrared imager in the imaging wave band Delta lambda△λ：
Wherein, W△λRadiation emittance (W.cm) of delta lambda band-2)；ελIs the spectral emissivity; lambda [ alpha ]1The lower limit (mum) of the imaging waveband of the thermal infrared imager is set; lambda [ alpha ]2The upper limit (mum) of the imaging wave band of the thermal infrared imager;
step 1.3: will epsilonλConsider a constant and rewrite equation (2) to:
wherein epsilon is a constant;
step 1.4: obtaining the blackbody radiation brightness Lbb：
Wherein L isbbIs the blackbody radiation brightness of the delta lambda wave band.
3. The radiometric calibration method suitable for rapid quantitative processing of thermal imagers according to claim 2, characterized in that: the formula for linearly fitting the black body temperature and the charge flux density F in the step 3 is as follows:
wherein, the black body temperature and the charge flux density F are subjected to calibration experiment to obtain multiple groups of corresponding data, G and FoffFor calibration parameters, they were obtained by fitting the test data T and F.
4. The radiometric calibration method suitable for rapid quantitative processing of thermal imagers according to claim 3, characterized in that: and 3, when the charge flux densities F corresponding to different black body temperatures are obtained in a calibration experiment, covering the imaging system of the thermal infrared imager with the black body, so that the entrance pupil brightness of the thermal infrared imager is the black body radiation brightness.
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