Method and device for inverting target infrared radiation image based on measured data
阅读说明：本技术 基于实测数据反演目标红外辐射图像的方法和装置 (Method and device for inverting target infrared radiation image based on measured data ) 是由 李朝晖 李刚 张迪 于 2019-09-20 设计创作，主要内容包括：本发明提供一种基于实测数据反演目标红外辐射图像的方法和装置,该方法包括：采用红外成像系统获取目标物体的红外辐射图像；根据红外成像系统的黑体标定曲线以及红外辐射图像中每个像素点的测量电压值,得到红外辐射图像中每个像素点的温度；采用傅立叶变换函数、红外成像系统的调制传递函数和傅立叶反变换函数,对红外辐射图像中每个像素点的温度进行反演修正,得到每个像素点的修正后的温度；根据每个像素点的修正后的温度获取每个像素点的修正后的电压值；根据每个像素点的修正后的电压值,获取修正后的红外辐射图像。克服了传统的模型解算法得到的目标图像数据不真实,以及传统的实测法存在的测量误差等缺陷。(The invention provides a method and a device for inverting a target infrared radiation image based on measured data, wherein the method comprises the following steps: acquiring an infrared radiation image of a target object by adopting an infrared imaging system; obtaining the temperature of each pixel point in the infrared radiation image according to a black body calibration curve of the infrared imaging system and the measured voltage value of each pixel point in the infrared radiation image; performing inversion correction on the temperature of each pixel point in the infrared radiation image by adopting a Fourier transform function, a modulation transfer function and an inverse Fourier transform function of an infrared imaging system to obtain the corrected temperature of each pixel point; acquiring a corrected voltage value of each pixel point according to the corrected temperature of each pixel point; and acquiring a corrected infrared radiation image according to the corrected voltage value of each pixel point. The defects that target image data obtained by a traditional model solution algorithm is not real, and measuring errors exist in a traditional actual measurement method are overcome.)
1. A method for inverting a target infrared radiation image based on measured data is characterized by comprising the following steps:
acquiring an infrared radiation image of a target object by adopting an infrared imaging system;
obtaining the temperature of each pixel point in the infrared radiation image according to a black body calibration curve V (T) of the infrared imaging system and the measured voltage value of each pixel point in the infrared radiation image;
performing inversion correction on the temperature of each pixel point in the infrared radiation image by adopting a Fourier transform function, a modulation transfer function and an inverse Fourier transform function of an infrared imaging system to obtain the corrected temperature of each pixel point;
acquiring a corrected voltage value of each pixel point according to the corrected temperature of each pixel point;
and acquiring a corrected infrared radiation image according to the corrected voltage value of each pixel point.
2. The method of claim 1, further comprising:
measuring multiple groups of blackbody temperatures by using a thermal infrared imager in an infrared imaging system to obtain multiple groups of blackbody temperature data T and corresponding voltage values V (T);
and according to the plurality of groups of black body temperature data T and the corresponding level values V (T), approximating by a curve fitting algorithm to obtain a black body calibration curve V (T) of the thermal infrared imager.
3. The method according to claim 2, wherein the obtaining of the black body calibration curve v (T) of the thermal infrared imager by approximating the black body temperature data T and the corresponding level values v (T) through a curve fitting algorithm comprises:
the calibration method is that the working waveband lambda of the thermal infrared imager is1～λ2Inner, lambda1<λ2Adjusting the working state parameters of the thermal infrared imager to the corresponding working state parameters during actual measurement, and determining the relationship between the output level V (T) of the thermal infrared imager and the black body temperature T:
wherein V (T) is the output voltage of the thermal imager and has a unit of V; l (lambda, T) is the radiation brightness of the black body spectrum in W cm-2·sr-1·μm-1(ii) a R (lambda) is the spectral responsivity of the thermal infrared imager, and the unit is as follows: V.W-1·cm2·sr·μm·μm-1(ii) a λ wavelength, unit: mu m;
when the working waveband range of the thermal infrared imager is smaller than the preset waveband range, the relation between the output level V (T) of the thermal imager and the temperature T of the black body is an exponential relation and can be expressed as
In the formula, λ0＝(λ1+λ2) H is Planck constant in W.s2C is the speed of light in cm s-1Second radiation constant c2＝1.438786×104Unit μm · K, temperature T, unit K; λ ═ λ2-λ1，
When the operating band range of the thermal infrared imager is not less than the preset band range, the relationship between the thermal infrared imager output level v (T) and the black body temperature T can be expressed as:
a, B is two constants, and is determined by the least square method according to multiple groups of blackbody temperature data T and corresponding voltage values V (T).
4. The method of claim 3, wherein the performing an inverse correction on the temperature of each pixel point in the infrared radiation image by using the fourier transform function, the modulation transfer function of the infrared imaging system, and the inverse fourier transform function to obtain the corrected temperature of each pixel point comprises:
the temperature T of each pixel point (x, y) in the infrared radiation image is measuredx,ySubstituting into the Planck formula (10) to obtain a target spectrum radiation degree f' (x, y);
performing Fourier transform on the target spectrum radiation degree F '(x, y) according to the formula (7) to obtain a transformed spectrum domain function F' (u, v);
dividing the transformed spectral domain function F' (u, v) by the corresponding MTF according to equation (8) to obtain modified spectral data F (u, v);
performing inverse fourier transform on the corrected spectrum data F (u, v) according to equation (9), and correcting the temperature F (x, y) of each pixel point; wherein the content of the first and second substances,
F(u,v)＝MTF(u,v)-1×F'(u,v) (8)
in the formula, MTF (u, v) is a modulation transfer function corresponding to the infrared imaging system, x is an abscissa of a pixel point in the infrared radiation image, and y is an ordinate of the pixel point in the infrared radiation image; m, N isRespectively corresponding to the maximum abscissa and the maximum ordinate of the infrared radiation image; m and N are positive integers, a first radiation constant c1＝3.741832×104Unit W.cm-2·μm4U and v are the spatial frequencies of the detector in the infrared imaging system in the horizontal direction and the vertical direction respectively.
5. The method of claim 4, wherein the modulation transfer function is expressed as MTF (u, v) -MTFOptics·MTFdetector·MTFSignal；
Wherein the MTFSignalRepresenting the modulation transfer function, MTF, of a signal processing module in an infrared imaging systemDetectorRepresenting a modulation transfer function of a detector module in the infrared imaging system; MTFOpticsRepresenting the modulation transfer function of an optical system block in an infrared imaging system,
where σ is the standard deviation of the gaussian distribution of the diffuse speckle energy due to aberrations, in units: mm;
where α, β are the horizontal and vertical widths of the detector modules, respectively, in units: mm; deltah,ΔvHorizontal offset and vertical offset sampling interval between two adjacent frames, respectively, unit: the thickness of the film is mm,
MTFSignal＝[1+(u/f0)2]-1/2
in the formula f0At a frequency of 3dB in the spatial frequency domain.
6. The method of claim 2, wherein measuring multiple sets of black body temperatures using a thermal infrared imager in an infrared imaging system comprises:
before and after an infrared imaging system is adopted to obtain infrared radiation images of a target object, a thermal infrared imager in the infrared imaging system is adopted to measure the temperature of a plurality of groups of black bodies;
the size of the aperture of the black body is larger than or equal to the solid angle of a field of view of the thermal infrared imager for receiving radiation energy.
7. An apparatus for inverting an infrared radiation image of a target based on measured data, comprising:
the infrared radiation image acquisition module is used for acquiring an infrared radiation image of a target object by adopting an infrared imaging system;
the temperature acquisition module is used for acquiring the temperature of each pixel point in the infrared radiation image according to a black body calibration curve V (T) of the infrared imaging system and the measured voltage value of each pixel point in the infrared radiation image;
the correction module is used for carrying out inversion correction on the temperature of each pixel point in the infrared radiation image by adopting a Fourier transform function, a modulation transfer function and an inverse Fourier transform function of the infrared imaging system to obtain the corrected temperature of each pixel point;
the voltage value acquisition module is used for acquiring the corrected voltage value of each pixel point according to the corrected temperature of each pixel point;
and the corrected image acquisition module is used for acquiring a corrected infrared radiation image according to the corrected voltage value of each pixel point.
8. The apparatus of claim 7, further comprising:
the black body calibration curve acquisition module is used for measuring a plurality of groups of black body temperatures by adopting an infrared thermal imager in an infrared imaging system to obtain a plurality of groups of black body temperature data T and corresponding voltage values V (T);
and according to the plurality of groups of black body temperature data T and the corresponding level values V (T), approximating by a curve fitting algorithm to obtain a black body calibration curve V (T) of the thermal infrared imager.
Technical Field
The invention belongs to the field of infrared physics, and particularly relates to a method and a device for inverting a target infrared radiation image based on measured data.
Background
The infrared radiation of the target enters the infrared imaging system after being transmitted by the atmosphere, and the infrared imaging system converts the infrared radiation signal of the target into an electric signal so as to be used for post-processing the infrared radiation information of the target by a signal processor of the infrared imaging system. On the premise of not considering atmospheric effect and other effect disturbance, due to the influence of the self effect of the infrared imaging system, the infrared radiation image data measured by the infrared imager is not the real radiation value of the object surface but the value after the disturbance of the self effect of the infrared imaging system. Therefore, to acquire the real radiation data of the object surface, the measurement data must be corrected, and the key point of the correction is to establish a correction model to obtain a correction function. However, at present, the research content of inverting the real infrared radiation of the target by establishing a correction reduction model through the actual measurement data of the target object is not common at home and abroad, and similar research results are rarely reported.
Through scientific and technological research, research in the field of infrared radiation imaging at home and abroad at present mainly focuses on two aspects, one is that in the aspect of the traditional infrared scene generation technology, an infrared image is generated according to a theoretical calculation result based on an infrared radiation calculation model. Due to the limitation of the technical level, the currently applied infrared radiation calculation model is quite rough, and a large error exists between the calculation result and the actual measurement data, so that an infrared image generated by the radiation calculation model is natural and unreal, and a large error is generated for the subsequent engineering technology research and development of infrared radiation detection products; on the other hand, most of the methods focus on researching an extraction algorithm of target features by using theoretical methods such as wavelet analysis, hidden Markov random fields, neural networks and the like, researching features such as motion, texture, spectrum and the like of a target, generating an infrared image based on the features, and solving the problem that a large error exists between a calculation result and actual measurement data.
Disclosure of Invention
The purpose of the invention is as follows:
the invention provides a method and a device for inverting a target infrared radiation image based on measured data, which can eliminate the self-effect disturbance of an infrared imager as much as possible, scientifically, quantitatively and objectively provide a target origin infrared image and achieve the purpose of determining and extracting the essential characteristics of a target irrelevant to a specific thermal imaging system.
The technical scheme of the invention is as follows:
the invention provides a method for inverting a target infrared radiation image based on measured data, which comprises the following steps:
acquiring an infrared radiation image of a target object by adopting an infrared imaging system;
obtaining the temperature of each pixel point in the infrared radiation image according to a black body calibration curve V (T) of the infrared imaging system and the measured voltage value of each pixel point in the infrared radiation image;
performing inversion correction on the temperature of each pixel point in the infrared radiation image by adopting a Fourier transform function, a modulation transfer function and an inverse Fourier transform function of an infrared imaging system to obtain the corrected temperature of each pixel point;
acquiring a corrected voltage value of each pixel point according to the corrected temperature of each pixel point;
and acquiring a corrected infrared radiation image according to the corrected voltage value of each pixel point.
Optionally, the method for inverting the target infrared radiation image based on the measured data further includes:
measuring multiple groups of blackbody temperatures by using a thermal infrared imager in an infrared imaging system to obtain multiple groups of blackbody temperature data T and corresponding voltage values V (T);
and according to the plurality of groups of black body temperature data T and the corresponding level values V (T), approximating by a curve fitting algorithm to obtain a black body calibration curve V (T) of the thermal infrared imager.
The obtaining of the black body calibration curve V (T) of the thermal infrared imager by approximating the black body temperature data T and the corresponding level values V (T) through a curve fitting algorithm comprises the following steps:
the calibration method is that the working waveband lambda of the thermal infrared imager is1～λ2Inner, lambda1<λ2Adjusting the working state parameters of the thermal infrared imager to the corresponding working state parameters during actual measurement, and determining the relationship between the output level V (T) of the thermal infrared imager and the black body temperature T:
in the formula, V(T) is the output voltage of the thermal imager, and the unit is V; l (lambda, T) is the radiation brightness of the black body spectrum in W cm-2·sr-1·μm-1(ii) a R (lambda) is the spectral responsivity of the thermal infrared imager, and the unit is as follows: V.W-1·cm2·sr·μm·μm-1(ii) a λ wavelength, unit: mu m;
when the working waveband range of the thermal infrared imager is smaller than the preset waveband range, the relation between the output level V (T) of the thermal imager and the temperature T of the black body is an exponential relation and can be expressed as
In the formula, λ0＝(λ1+λ2) H is Planck constant in W.s2C is the speed of light in cm s-1Second radiation constant c2＝1.438786×104Unit μm · K, temperature T, unit K; λ ═ λ2-λ1，
When the operating band range of the thermal infrared imager is not less than the preset band range, the relationship between the thermal infrared imager output level v (T) and the black body temperature T can be expressed as:
a, B is two constants, and is determined by the least square method according to multiple groups of blackbody temperature data T and corresponding voltage values V (T).
Optionally, the performing inversion correction on the temperature of each pixel point in the infrared radiation image by using a fourier transform function, a modulation transfer function of the infrared imaging system, and an inverse fourier transform function to obtain a corrected temperature of each pixel point includes:
the temperature T of each pixel point (x, y) in the infrared radiation image is measuredx,ySubstituting into the Planck formula (10) to obtain a target spectrum radiation degree f' (x, y);
performing Fourier transform on the target spectrum radiation degree F '(x, y) according to the formula (7) to obtain a transformed spectrum domain function F' (u, v);
dividing the transformed spectral domain function F' (u, v) by the corresponding MTF according to equation (8) to obtain modified spectral data F (u, v);
performing inverse fourier transform on the corrected spectrum data F (u, v) according to equation (9), and correcting the temperature F (x, y) of each pixel point; wherein the content of the first and second substances,
F(u,v)＝MTF(u,v)-1×F'(u,v) (8)
in the formula, MTF (u, v) is a modulation transfer function corresponding to the infrared imaging system, x is an abscissa of a pixel point in the infrared radiation image, and y is an ordinate of the pixel point in the infrared radiation image; m and N respectively correspond to the maximum abscissa and the maximum ordinate of the infrared radiation image; m and N are positive integers, a first radiation constant c1＝3.741832×104Unit W.cm-2·μm4U and v are the spatial frequencies of the detector in the infrared imaging system in the horizontal direction and the vertical direction respectively.
Optionally, MTF (u, v) ═ MTFOptics·MTFdetector·MTFSignalIs a modulation transfer function;
wherein the MTFSignalRepresenting the modulation transfer function, MTF, of a signal processing module in an infrared imaging systemDetectorRepresenting a modulation transfer function of a detector module in the infrared imaging system; MTFOpticsRepresenting the modulation transfer function of an optical system block in an infrared imaging system,
where σ is the standard deviation of the gaussian distribution of the diffuse speckle energy due to aberrations, in units: mm;
where α, β are the horizontal and vertical widths of the detector modules, respectively, in units: mm; deltah,ΔvHorizontal offset and vertical offset sampling interval between two adjacent frames, respectively, unit: the thickness of the film is mm,
MTFSignal＝[1+(u/f0)2]-1/2
in the formula f0At a frequency of 3dB in the spatial frequency domain.
Optionally, the measuring the temperatures of the multiple groups of black bodies by using the thermal infrared imager in the infrared imaging system includes:
before and after an infrared imaging system is adopted to obtain infrared radiation images of a target object, a thermal infrared imager in the infrared imaging system is adopted to measure the temperature of a plurality of groups of black bodies;
the size of the aperture of the black body is larger than or equal to the solid angle of a field of view of the thermal infrared imager for receiving radiation energy.
The second aspect of the present invention provides an apparatus for inverting an infrared radiation image of a target based on measured data, which is used for executing the method in the first aspect, and has the same and similar technical means and technical effects.
The invention provides a device for inverting a target infrared radiation image based on measured data, which comprises:
the infrared radiation image acquisition module is used for acquiring an infrared radiation image of a target object by adopting an infrared imaging system;
the temperature acquisition module is used for acquiring the temperature of each pixel point in the infrared radiation image according to a black body calibration curve V (T) of the infrared imaging system and the measured voltage value of each pixel point in the infrared radiation image;
the correction module is used for carrying out inversion correction on the temperature of each pixel point in the infrared radiation image by adopting a Fourier transform function, a modulation transfer function and an inverse Fourier transform function of the infrared imaging system to obtain the corrected temperature of each pixel point;
the voltage value acquisition module is used for acquiring the corrected voltage value of each pixel point according to the corrected temperature of each pixel point;
and the corrected image acquisition module is used for acquiring a corrected infrared radiation image according to the corrected voltage value of each pixel point.
Optionally, the device for inverting the target infrared radiation image based on the measured data further includes:
the black body calibration curve acquisition module is used for measuring a plurality of groups of black body temperatures by adopting an infrared thermal imager in an infrared imaging system to obtain a plurality of groups of black body temperature data T and corresponding voltage values V (T);
and according to the plurality of groups of black body temperature data T and the corresponding level values V (T), approximating by a curve fitting algorithm to obtain a black body calibration curve V (T) of the thermal infrared imager.
The invention has the advantages that:
the method comprises the specific steps of firstly mapping between infrared image measurement data and corresponding radiation values, and then correcting the corresponding radiation data to obtain real radiation data. The data obtained by the infrared imaging device measurement is not the real radiation data of the object surface, but the level value after the imaging system is acted. The actual radiation value of the target object obtained from the measurement data is the inverse process of the infrared radiation measurement, so the steps of correcting the measurement data and the imaging process are reversed. The output level data obtained by infrared measurement is mapped to obtain a radiation value which is disturbed by an imaging system and is not converted into the level data, and the step is data calibration; and performing signal processing effect inversion, detector effect inversion and optical system effect inversion on the data obtained by calibration to obtain radiation data after imaging system effect correction.
According to the invention, the Modulation Transfer Function (MTF) of the infrared imaging system is constructed, and the output of the level signal of the thermal imaging system is converted into the target apparent radiation data by means of a blackbody test calibration method, so that the defects that the target image data obtained by the traditional model solution algorithm is not real, the measurement error exists in the traditional real measurement method and the like are overcome.
The technology and the calculation method thereof are not limited to specific and specific data, but determine the target original source infrared characteristic data according to the measured sequence image data by utilizing an inversion and reconstruction method, thereby providing a method for researching how to extract the essential characteristics of weak target signals, researching an infrared detection technology, establishing an infrared target characteristic model and solving the analysis of target detection images; the research result can provide basic research data for airborne/missile-borne infrared imaging guidance, reconnaissance and detection, and provides an effective technical approach for the design, optimization and performance evaluation of an airborne photoelectric system.
Drawings
FIG. 1 is a schematic diagram of infrared image data inversion correction;
FIG. 2 is a calibration curve of a thermal imager;
FIG. 3 is a calibration curve of a thermal imager;
FIG. 4 is a schematic representation of an infrared radiation image.
Detailed Description
The present invention is described in further detail below.
The invention provides a method for inverting a target infrared radiation image based on actually measured data, namely, target imaging data obtained through actually measuring by an infrared thermal imager in a test is combined with a correction reduction model to invert real infrared radiation data on the surface of a target object influenced by the effect of an imaging system so as to obtain the infrared radiation image of the target. The method for inverting the target origin infrared radiation characteristic by using the digital image obtained by experimental measurement eliminates the self-effect disturbance of the infrared imager as much as possible, scientifically, quantitatively and objectively provides the target origin infrared image, and achieves the purpose of determining and extracting the essential characteristics of the target irrelevant to a specific thermal imaging system.
Fig. 1 is an infrared image data inversion correction schematic diagram, and with reference to fig. 1, a method for inverting a target infrared radiation image based on actual measurement data includes the following steps:
calibrating the thermal infrared imager, namely converting the voltage value of a target object radiation signal obtained by actual measurement into equivalent black body temperature, measuring multiple groups of black body temperatures under a laboratory condition, and approximating the black body temperature values corresponding to the level values among the multiple groups of black body temperature data by a curve fitting algorithm according to the multiple groups of black body temperature data T and the corresponding level values V to finally obtain a black body calibration curve V (T);
correcting imaging effect, namely calibrating the measured image according to a black body calibration curve V (T) to obtain V (T) corresponding to each point of the measured imagex,y) Wherein T isx,yObtaining the corresponding black body calibration temperature for the coordinate value x, y of each point of the measured image according to the black body calibration curve V (T), and enabling Tx,yF' (x, y), fourier transforming the scaled data according to equation (7), and then dividing the transformed spectral domain function by the corresponding MTF according to equation (8); and finally, carrying out inverse Fourier transform on the corrected frequency spectrum data according to the formula (9), and finishing the imaging effect correction of the calibrated data.
F(u,v)＝MTF(u,v)-1×F'(u,v) (8)
Wherein f (x, y) is data that has not undergone imaging effects; f' (x, y) is data after imaging effect, and is also data after actual measurement data is calibrated; f (u, v) is a Fourier spectrum corresponding to F (x, y); f '(u, v) is a fourier spectrum corresponding to F' (x, y); MTF (u, v) is a modulation transfer function corresponding to the imaging system, and x and y respectively correspond to the abscissa and ordinate positions of the infrared field of view; m and N respectively correspond to the maximum abscissa and the maximum ordinate of the infrared field.
Calibrating the signal output level of the thermal infrared imager and the corresponding apparent temperature by using a black body of a test room, wherein the black body refers to: an object that is capable of absorbing all incident radiation of any wavelength at any temperature, that is to say a black body that has zero reflectivity and zero transmittance, and an absorption equal to 1.
Further, the method also comprises a step of constructing a Modulation Transfer Function (MTF).
The MTF of an imaging system is only dependent on the state of the system, independent of the system input and output data, and can therefore be abstracted as a filter. The MTF expression can be obtained through a technical specification of a thermal imager product manufacturer and product experiment data which are specifically used, and can also be obtained through theoretical modeling. A theoretical modeling approach is provided, and an imaging system consists of an optical system, a detector, signal processing and other links.
After the thermal infrared imager is calibrated and before the imaging effect is corrected,
the MTF expression of the system should be composed of these three parts, respectively MTFSignalMTF, MTF representing the signal processing moduleDetectorMTF representing the detector module; MTFOpticsRepresenting the MTF of the optical system module.
Where σ is the standard deviation of the gaussian distribution of the diffuse speckle energy due to aberrations, in units: mm is
Where α, β are the horizontal and vertical widths of the detector modules, respectively, in units: mm; deltah,ΔvRespectively, the horizontal offset and the vertical offset between two adjacent frames are sampling intervals, and the unit is: the thickness of the film is mm,
MTFSignal＝[1+(u/f0)2]-1/2
in the formula f03dB frequency in the spatial frequency domain; in addition, u and v in the above equation are spatial frequencies in the horizontal and vertical directions, respectively, and therefore the total MTF of the imaging system is equal to the product of the above components:
MTF(u,v)＝MTFOptics·MTFdetector·MTFSignal。
the modulation transfer function means: the transfer function reflects the frequency characteristics of the system itself, regardless of the input-output magnitude of the system. Imaging system effect inversion correction model: the idea is to construct the infrared imaging system as a composite from a series of subsystems with certain frequency characteristics (spatial or temporal). If the frequency characteristics of the subsystems can be obtained according to the convolution property of the linear invariant system, the frequency characteristics of the infrared imaging system are determined. In the invention, the model construction adopts a Modulation Transfer Function (MTF) concept to describe the frequency characteristics of the subsystems, the transfer functions of the subsystems are solved one by one, and then the transfer functions are multiplied to obtain the Modulation Transfer Function (MTF) of the whole infrared imaging system.
Further, the calibration of the thermal infrared imager is specifically as follows:
the calibration method is that the working waveband lambda of the thermal infrared imager is1～λ2Adjusting the working state parameters of the thermal infrared imager to the corresponding working state parameters during actual measurement, and determining the relationship between the output level V (T) of the thermal infrared imager and the black body temperature T:
wherein v (t) is the output level of the thermal imager, unit: v; l (λ, T) is the blackbody spectral radiance, in units: w.cm-2·sr-1·μm-1(ii) a R (lambda) is the spectral responsivity of the thermal imager, and the unit is as follows: V.W-1·cm2·sr·μm·μm-1(ii) a λ wavelength, unit: mu m;
if λ1<λ2When lambda is2-λ1When Δ λ/λ is equal to1<<1 or Delta lambda/lambda2<<1, i.e. when the operating band Δ λ of the thermal imager is narrow, the relationship between the thermal imager output level v (T) and the black body temperature T is exponential, i.e. the relationship between the thermal imager output level v (T) and the black body temperature T can be expressed as
In the formula, λ0＝(λ1+λ2) (iii)/2, planck constant h ═ (6.626176 ± 0.000036) × 10-34Unit W.s2The light speed c is (2.99792458 ± 0.000000012) × 1010In units of cm s-1Second radiation constant c2＝1.438786×104Unit μm · K, temperature T, unit K;
if Δ λ does not satisfy Δ λ/λ1<<1 or Delta lambda/lambda2<<1, when the working waveband of the thermal imager is wide, the relation between the output level V (T) of the thermal imager and the black body temperature T can obtain the best fitting effect by utilizing the following exponential function:
by measuring multiple sets of black body temperatures T1、T2、T3… … level V (T) output by thermal imager1)、V(T2)、V(T3) … …, two constants A, B can be determined by using the least squares method, thereby obtaining an approximate analytic expression between the temperature and the voltage value of the black body.
Further, in the step 1 of calibrating the thermal infrared imager,
and calibrating the thermal imager by using a large-aperture black body in a time period before and after the test, wherein the aperture of the large-aperture black body is larger than or equal to the solid angle of the field of view for receiving the radiation energy of the infrared imager. In order to correlate the radiation temperature and voltage data, the thermal imager must be calibrated with a large aperture black body (i.e., a black body having an aperture size greater than or equal to the solid angle of the field of view of the infrared imager receiving the radiation energy) during the time period before and after the test.
The infrared radiation image data measured by the infrared imager is not the real radiation value of the surface of the object but the value after the disturbance of the self effect of the infrared imaging system. Therefore, to obtain the real radiation data of the object surface, the measurement data must be corrected, and the key point of the correction is to establish an imaging system effect inversion correction model to obtain a correction function, namely a Modulation Transfer Function (MTF); secondly, input and output of the thermal imaging system are calibrated.
Fig. 2 is a calibration curve of a thermal imager, fig. 3 is a calibration curve of a thermal imager, and fig. 4 is a schematic view of an infrared radiation image. Referring to fig. 1 to 4, the method of the present invention includes:
step 1, calibrating a thermal infrared imager
The method comprises the steps of measuring a plurality of groups of blackbody temperatures under laboratory conditions, and approximating blackbody temperature values corresponding to other level values by a curve fitting algorithm according to the data.
The target object radiation data obtained due to actual measurement is a voltage value (generally, a 14-bit data sample value) converted by the signal processor, not the radiation brightness of the object. If the radiant brightness of the target object is calculated according to theoretical analysis, the radiant energy of each point of the target object needs to be calculated according to the Planck formula, but the temperature value and the emissivity of the point need to be known in advance, and the actual temperature and the emissivity of each point on the target body are difficult to accurately obtain in actual measurement, so that the radiant energy of the target body cannot be directly calculated by using the Planck formula. The invention assumes that output level data D obtained by measurement of an infrared imager corresponds to a specific radiation value, and each radiation value corresponds to a specific equivalent blackbody temperature value T, namely, the output level data and the equivalent blackbody temperature have a one-to-one correspondence relationship. According to the relation, radiation measurement can be carried out on the black body in a laboratory, and the corresponding black body temperature T is obtained when the output level of the thermal imager is D, wherein T is the equivalent black body temperature corresponding to the measured level data D.
However, to completely measure the black body temperature corresponding to each output level value, the workload is very large, and it takes a long time. The method adopted by the invention is to measure a plurality of groups of blackbody temperatures under laboratory conditions, and with the data, blackbody temperature values corresponding to other level values can be approximated by a curve fitting algorithm.
In order to correlate the radiation temperature and voltage data, the thermal imager must be calibrated with a large aperture black body (i.e., a black body having an aperture size greater than or equal to the solid angle of the field of view of the infrared imager receiving the radiation energy) during the time period before and after the test. The calibration method is that the thermal imager works in the working waveband (lambda)1～λ2) And adjusting the working state parameters of the thermal imager to the corresponding working state parameters during measurement, and determining the relationship between the output level V (T) of the thermal imager and the black body temperature T, as shown in FIG. 2.
In order to correlate the radiation temperature and voltage data, the thermal imager must be calibrated with a large aperture black body (i.e., a black body having an aperture size greater than or equal to the solid angle of the field of view of the infrared imager receiving the radiation energy) during the time period before and after the test. The calibration method is that the thermal imager works in the working waveband (lambda)1～λ2) And adjusting the working state parameters of the thermal imager to the corresponding working state parameters during measurement, and determining the relationship between the output level V (T) of the thermal imager and the black body temperature T.
Wherein V (T) is the output level of the thermal imager (unit: V); l (lambda, T) is the radiation intensity (unit: W. cm) of the blackbody spectrum-2·sr-1·μm-1) (ii) a R (lambda) is the spectral responsivity (unit: V.W) of the thermal imager-1·cm2·sr·μm·μm-1) (ii) a Lambda wavelength (unit: μm).
When lambda is1-λ2Δ λ is equal to λ1Or λ2Relatively small, i.e. when Δ λ/λ1<1 or Delta lambda/lambda2<1, that is, when the operating band Δ λ of the thermal imager is narrow, the relationship between the thermal imager output level v (T) and the black body temperature T is considered to be an exponential relationship, that is, the relationship between the thermal imager output level v (T) and the black body temperature T can be expressed as
In the formula, λ0＝(λ1+λ2) (iii)/2, planck constant h ═ (6.626176 ± 0.000036) × 10-34(W·s2) The light speed c is (2.99792458 ± 0.000000012) × 1010(cm·s-1) Second radiation constant c2＝1.438786×104(. mu.m.K), temperature T (K).
When the working waveband of the thermal imager is wider, when delta lambda/lambda is1>1 and delta lambda/lambda2>1, the relation between the output level V (T) of the thermal imager and the black body temperature T can obtain the best fitting effect by utilizing the following exponential function:
by measuring different black body temperatures T1、T2、T3… … level V (T) output by thermal imager1)、V(T2)、V(T3) … … (typically 5 or more), the approximate analytical expression between the blackbody temperature and voltage values can be determined A, B using least squares.
The concrete case is as follows: table 1 is a table of the level value and the black body temperature for calibration measurement of a thermal imager of a certain model. Fig. 3 is a calibration curve corresponding to the calibration data.
Table 1 calibration data for thermal imager
Level value (DL)
Black body temperature (. degree. C.)
342
8.2
393
22.2
647
36.8
995
66.3
1846
118.9
The MTF of an imaging system is only dependent on the state of the system, independent of the system input and output data, and can therefore be abstracted as a filter. The MTF expression can be obtained through a technical specification of a thermal imager product manufacturer and product experiment data which are specifically used, and can also be obtained through theoretical modeling. A theoretical modeling approach is provided, and as shown in FIG. 1, an imaging system is composed of an optical system, a detector, signal processing and the like.
Therefore, the MTF expression of the system should be composed of these three parts, respectively MTFSignalMTF, MTF representing the signal processing moduleDetectorMTF representing the detector module; MTFOpticsRepresenting the MTF of the optical system module.
Where σ is the standard deviation (unit: mm) of the gaussian distribution of the diffuse speckle energy due to the aberration.
Wherein alpha and beta are respectively the horizontal width and the vertical width (unit: mm) of the detector; deltah,ΔvRespectively the level between two adjacent framesOffset and vertical offset by the sampling interval (unit: mm).
MTFSignal＝[1+(u/f0)2]-1/2
In the formula f03dB frequency in the spatial frequency domain; in the above formula, u and v represent spatial frequencies in the horizontal direction and the vertical direction, respectively. Thus, the overall MTF of the imaging system is equal to the product of the above sections:
MTF(u,v)＝MTFOptics·MTFdetector·MTFSignal
After the level data is calibrated and converted into the target apparent blackbody temperature through the step 1, the level data can be corrected. According to the physical principle of infrared imaging, the real radiation data of the target is converted into the actually measured infrared radiation data of the target through the formulas (4), (5) and (6).
F'(u,v)＝MTF(u,v)×F(u,v) (5)
Wherein f (x, y) is data that has not undergone imaging effects; f' (x, y) is data after imaging effect, and is also data after actual measurement data is calibrated; f (u, v) is a Fourier spectrum corresponding to F (x, y); f '(x, y) is a fourier spectrum corresponding to F' (x, y); MTF (u, v) is a modulation transfer function corresponding to the imaging system, and x and y respectively correspond to the abscissa and ordinate positions of the infrared field of view; m and N respectively correspond to the maximum abscissa and the maximum ordinate of the infrared field.
To correct the calibrated data, the inverse process corresponding to the physical process described by the equations (4), (5) and (6) must be performed. Firstly, carrying out Fourier transform on calibrated data, and then dividing a transformed spectrum domain function by a corresponding MTF; and finally, carrying out inverse Fourier transform on the corrected frequency spectrum data to finish the imaging effect correction of the calibrated data, wherein the whole process is as the formulas (7) to (9).
F(u,v)＝MTF(u,v)-1×F'(u,v) (8)
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