CN114719999B - Test system for thermal imaging performance under background clutter interference - Google Patents

Test system for thermal imaging performance under background clutter interference Download PDF

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CN114719999B
CN114719999B CN202210468200.4A CN202210468200A CN114719999B CN 114719999 B CN114719999 B CN 114719999B CN 202210468200 A CN202210468200 A CN 202210468200A CN 114719999 B CN114719999 B CN 114719999B
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radiation
target
background
image
temperature difference
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CN114719999A (en
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夏寅辉
刘建东
王霞
高山
魏冉峰
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Beijing Dongyu Hongda Technology Co ltd
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Beijing Dongyu Hongda Technology Co ltd
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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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Abstract

The application provides a thermal imaging performance's test system under background clutter interference, include: the system comprises a background clutter generating component, a target generating component, a collimation component, a thermal imager and a measurement and control component; the background clutter generating component modulates the first path of infrared light and the background clutter into a first image; reflecting the first image through a target generating component to obtain background radiation; transmitting the second path of infrared light through a target plate to generate target radiation, and emitting the target radiation together; obtaining parallel radiation by modulation of the collimation assembly; the thermal imager displays the parallel radiation as a target pattern under clutter; and the measurement and control component determines the value of the minimum distinguishable temperature difference according to the second temperature difference and the target pattern, and when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent. The method and the device can improve the performance and the application capacity of the thermal imager under complex background, and establish the performance test method and the standard of the thermal imager under background clutter interference.

Description

Test system for thermal imaging performance under background clutter interference
Technical Field
The embodiment of the application relates to the field of thermal imaging, in particular to a test system for thermal imaging performance under background clutter interference.
Background
In a practical scene, background clutter acts as an interference, and has a great influence on imaging performance and target detection capability of a thermal imager, and is an important factor for limiting target acquisition performance. The minimum resolvable temperature difference (minimum resolvable temperature difference, MRTD) is a core parameter for comprehensively evaluating the temperature resolution and the spatial resolution of the thermal imager, can characterize the thermal sensitivity and the high-frequency limit resolution of the thermal imager, and is the basis for designing the high-level thermal imager. In the process of target detection, background clutter can hold down the attention of an observer and influence the time of searching a target by the observer; background clutter also interferes with the viewer's judgment of the target, affecting the probability of the viewer's target recognition. The influence of the background clutter on the acquisition performance of the imaging system target is mainly reflected by correcting the existing performance model through the clutter scale. A correction scheme of various performance models is provided on the basis of clutter scale outside the country. With the improvement of the performance and application capability requirements of the thermal imaging instrument in a complex background, the performance test method and standard of the thermal imaging instrument in the background clutter interference need to be established.
Disclosure of Invention
The embodiment of the application aims to improve the performance and application capacity of the thermal imager under a complex background and establish a performance test method and standard of the thermal imager under background clutter interference.
In order to achieve the above objective, the embodiment of the present application provides a system for testing thermal imaging performance under background clutter interference. The system comprises: the background clutter generation assembly is used for modulating the first path of infrared light and the background clutter to obtain a first image; projecting the first image; the intensity of the first path of infrared light is adjustable; the target generation assembly is used for receiving the first image and reflecting the first image through the target plate to obtain background radiation; obtaining a second path of infrared light, transmitting the second path of infrared light through a target plate to generate target radiation, and emitting the background radiation and the target radiation together; the intensity of the second path of infrared light is adjustable, and the intensity difference between the first path of infrared light and the second path of infrared light is a first temperature difference; the collimation component is used for modulating the background radiation and the target radiation through double-mirror reflection to obtain parallel radiation; the thermal imager is used for receiving the parallel radiation and displaying the parallel radiation as a target pattern under clutter; the contrast of brightness between the image of the background radiation and the target radiation in the target pattern varies with the adjustment of the first temperature difference; the measurement and control assembly is used for detecting the difference value between the temperature of the background radiation and the temperature of the target radiation to obtain a second temperature difference, wherein the second temperature difference changes along with the adjustment of the first temperature difference; determining a value of a minimum resolvable temperature difference according to the second temperature difference and the target pattern, wherein the minimum resolvable temperature difference is a temperature difference when the contrast of brightness between an image of target radiation and an image of background radiation is gradually increased from zero to an image of target radiation which can be resolved by human eyes; and when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent.
As one embodiment, the background clutter generating component sequentially comprises: the first blackbody radiation source is used for emitting the first infrared light and adjusting the intensity of the first infrared light; the imaging light path is used for obtaining an infrared image from the first path of infrared light through the collecting lens; and the DMD light modulator is used for obtaining the background clutter and modulating the infrared image and the background clutter to obtain the first image.
As one embodiment, the DMD light modulator includes: the video circuit is used for obtaining the background clutter and converting the background clutter into digital signals to be output; the infrared digital micro mirror array is used for obtaining the digital signal, converting the digital signal into a clutter image under the action of the driving voltage, and modulating the clutter image and the infrared image to obtain the first image.
As one embodiment, the target generating assembly sequentially comprises, along a path of light propagation: the second blackbody radiation source is used for emitting the second path of infrared light and adjusting the intensity of the second path of infrared light; the target plate comprises a group of hollowed targets and a reflecting film layer, wherein the hollowed targets are used for transmitting the second path of infrared light to generate target radiation, and the reflecting film layer is used for reflecting the first image to obtain background radiation.
As one embodiment, the number of the hollowed-out targets is multiple, and the aspect ratio of each hollowed-out target in the multiple hollowed-out targets is 7:1.
As one embodiment, the collimating component sequentially includes, along a path of light propagation: the plane reflector is used for obtaining the background radiation and the target radiation and reflecting the background radiation and the target radiation out together; an off-axis parabolic reflector for obtaining the background radiation and the target radiation, and reflecting the background radiation and the target radiation into a collimator; the collimator is used for modulating the background radiation and the target radiation into parallel radiation and emitting the parallel radiation; the plane reflector is positioned in the incidence notch at the middle section of the collimator, and the off-axis parabolic reflector is positioned at the bottom section of the collimator and has a fixed relative position.
As one embodiment, the measurement and control assembly includes: a scanning radiometer for detecting the temperature of the target radiation and the temperature of the background radiation in the target pattern under the clutter background respectively; the controller comprises a control module and a test module; the control module is used for generating the background clutter to be input into the video circuit and generating a driving voltage to be input into the infrared digital micromirror array; the test module is used for obtaining the temperature of the target radiation and the temperature of the background radiation, and obtaining the value of the minimum distinguishable temperature difference according to the adjustment of the temperature of the target radiation and the temperature of the background radiation, so as to obtain the test result of the thermal imaging performance of the thermal imager.
The background clutter includes, as one embodiment, light, medium and heavy background clutter.
As an embodiment, the scanning of the scanning radiometer is performed at least at four spatial frequencies, the spatial frequencies being those which meet the operating requirements of the thermal imager.
As one embodiment, the test module includes: a detection unit for detecting the variance of pixel gray values of an image of target radiation and an image of background radiation in the target pattern, and determining a statistical variance; obtaining the temperature of the background radiation and the temperature of the target radiation, and calculating the difference between the two to obtain the second temperature difference; a level determining unit, configured to determine a value of a signal-to-noise ratio according to the second temperature difference and the statistical variance, and determine a level of clutter of the target pattern according to the value of the signal-to-noise ratio; an analysis unit, configured to analyze a contrast of brightness between an image of target radiation and an image of background radiation in the target pattern according to the level of clutter of the target pattern and the second temperature difference; determining a value of a minimum resolvable temperature difference according to a temperature difference from zero to an image which can be resolved by human eyes according to the contrast ratio of the brightness; and when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent.
According to the embodiment of the application, the clutter background scene is effectively simulated, the target scene under the interference background is realized in the laboratory environment, the MRTD test method and system of the thermal imager under the interference are established, the performance of the thermal imager can be tested more comprehensively, the thermal imager is evaluated more carefully, and the application level of the thermal imager is improved.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments disclosed in the present specification, the drawings required for the description of the embodiments will be briefly described below, and it is apparent that the drawings in the following description are only examples of the embodiments disclosed in the present specification, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a block diagram of a thermal imaging performance test system under background clutter interference according to an embodiment of the present application;
FIG. 2 is a schematic diagram of a test system according to an embodiment of the present application;
FIG. 3 is a pre-test preparation flow chart of the test system in an embodiment of the present application;
FIG. 4a is a flow chart of a test module of the test system according to an embodiment of the present application;
FIG. 4b is a flowchart illustrating a thermal imaging performance analysis under background clutter interference according to an embodiment of the present application;
FIG. 5a is a schematic diagram of a target pattern corresponding to a light clutter at a certain spatial frequency according to an embodiment of the present application;
FIG. 5b is a corresponding target pattern for medium clutter at a spatial frequency according to an embodiment of the present application;
FIG. 5c is a diagram of a target pattern for a corresponding heavy clutter at a spatial frequency in an embodiment of the present application.
Detailed Description
In the following description, reference is made to "some embodiments" which describe a subset of all possible embodiments, but it is to be understood that "some embodiments" can be the same subset or different subsets of all possible embodiments and can be combined with one another without conflict.
In the following description, the terms "first\second\third, etc." or component a, component B, component C, etc. are used merely to distinguish similar objects and do not represent a particular ordering for the objects, it being understood that particular orders or precedence may be interchanged where allowed, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.
In the following description, reference numerals indicating steps such as S110, S120, … …, etc. do not necessarily indicate that the steps are performed in this order, and the order of the steps may be interchanged or performed simultaneously as allowed.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing embodiments of the present application only and is not intended to be limiting of the present application.
The technical scheme of the present application is described in further detail below through the accompanying drawings and examples.
The Minimum Resolvable Temperature Difference (MRTD) is an important parameter for comprehensively evaluating the temperature resolution and the spatial resolution of a thermal imaging system, and is defined as follows: the minimum resolvable temperature difference is the temperature difference at which the human eye can resolve the image of the target radiation as the contrast of the brightness between the image of the target radiation and the image of the background radiation gradually increases from zero to the human eye.
In some embodiments, for an aspect ratio of 1 in a uniform blackbody background with a certain spatial frequency: 7, the observer performs infinitely long observation on the display screen of the thermal imager, and when the temperature difference between the target radiation and the background radiation gradually increases from zero to the time when the observer confirms that the target pattern of the four strips can be distinguished, the temperature difference between the target radiation and the background radiation becomes the minimum distinguishable temperature difference (MRTD) at the spatial frequency.
Spatial frequency refers to the number of grid cycles of sine modulation of brightness and darkness of images or stimulus patterns in each viewing angle, and the units are cycles/degree; in this application, it specifically refers to the period in which the bright-dark fringes formed by the four target bands in the unit viewing angle repeatedly appear.
The definition of thermal sensitivity (NETD) is "noise equivalent temperature difference", which refers to the ability of a thermal imager to resolve small temperature differences; data and units are generally given as < 50mk; the temperature measurement precision refers to the temperature precision of the system, and takes a large value at +/-2 ℃ or +/-2 ℃.
It should be understood that as long as NETD and MRTD values can be made smaller, the thermal imaging system has higher sensitivity to temperature differences, even if two or more objects are close to each other in actual imaging, the temperature differences between the temperatures are small, the thermal imaging can clearly distinguish the two or more objects from each other on a display image, and the real scene is fed back, which is the "high imaging effect" of infrared thermal imaging, and the thermal imaging performance of the thermal imager is excellent. Conversely, the higher the temperature difference value between the temperatures of two or more objects, the worse the imaging effect, and even the situation that the naked eyes only see 'black and big groups' in the image can occur, the thermal imaging performance of the thermal imager is poor.
The embodiment of the application provides a test system for thermal imaging performance under background clutter interference. The system can effectively evaluate the performance of the thermal imager containing the background interference condition under the laboratory condition and provide the test standard, and the method has good evaluation effect.
Fig. 1 is a functional block diagram of a test system for thermal imaging performance under background clutter interference according to an embodiment of the present application. As shown in fig. 1, the system includes a background clutter generating component 11, a target generating component 12, a collimation component 13, a thermal imager 14, and a measurement and control component 15.
The background clutter generating component 11 modulates the first path of infrared light and the background clutter to obtain a first image, and projects the first image to the target generating component 12. Wherein, the intensity of the first infrared light is adjustable.
The target generating component 12 receives the first image, and reflects the first image through the target plate to obtain background radiation; obtaining a second path of infrared light, transmitting the second path of infrared light through the target plate to generate target radiation, and emitting the background radiation and the target radiation to the collimation assembly 13; the intensity of the second infrared light is adjustable, and the difference value of the intensities of the first infrared light and the second infrared light is recorded as a first temperature difference.
The collimator assembly 13 modulates the background radiation and the target radiation by double-mirror reflection to obtain parallel radiation, and the parallel radiation is emitted to the thermal imager 14.
The thermal imager 14 receives parallel radiation and displays the parallel radiation as a target pattern under clutter; the contrast of the brightness between the background radiation in the target pattern and the image of the target radiation varies with the adjustment of the first temperature difference.
The measurement and control component 15 detects the difference between the temperature of the background radiation and the temperature of the target radiation to obtain a second temperature difference, wherein the second temperature difference changes along with the adjustment of the first temperature difference; determining a value of a minimum resolvable temperature difference according to the second temperature difference and the target pattern, wherein the minimum resolvable temperature difference is a temperature difference when the contrast of brightness between the image of the target radiation and the image of the background radiation gradually increases from zero to an image of which the human eyes can resolve the target radiation; when the value of the minimum distinguishable temperature difference is equal to or lower than the set threshold value, the thermal imaging performance of the thermal imager is excellent.
In one embodiment, the background clutter generating assembly 11 comprises, in order along the path of light propagation, a blackbody radiation source 111, an imaging light path 112, a DMD light modulator 113. The blackbody radiation source 111 may be denoted as a first blackbody radiation source 111.
Wherein, the blackbody radiation source 111 emits a first path of infrared light, and adjusts the temperature or intensity of the first path of infrared light.
Illustratively, the blackbody radiation source 111 may be a cavity blackbody, with an infrared radiation source disposed therein, the infrared radiation source having an adjustment knob that can adjust the intensity, brightness, and/or temperature of the radiation. The black body is used for isolating interference of external noise on the infrared radiation source.
As shown in fig. 1, blackbody radiation source 111 emits a first path of infrared light that propagates into imaging light path 112.
The imaging optical path 112 passes the first infrared light through a condenser to obtain an infrared image.
In one embodiment, imaging light path 112 employs a critical illumination mode to project a first path of infrared light emitted by blackbody radiation source 111 onto DMD light modulator 113. Wherein critical illumination refers to imaging the light source onto or near the projection object by a condenser lens.
Illustratively, the condenser lens may comprise a single-piece convex lens that images the first infrared light emitted by the blackbody radiation source 111, projected onto the surface of the DMD light modulator 113. The image projected on the surface of the DMD light modulator 113 is noted as an infrared image.
The DMD light modulator 113 obtains background clutter, modulates the infrared image with the background clutter to obtain a clutter background infrared image S1, and projects the clutter background infrared image S1 to the target generating component 12. The clutter background infrared image S1 may be noted as a first image.
In one embodiment, DMD light modulator 113 comprises video circuit 1131, an infrared digital micromirror array (hereinafter DMD) 1132, video circuit 1131 and infrared digital micromirror array (hereinafter DMD) 1132 are coupled to controller 152 in measurement and control assembly 15.
The input to the video circuit 1131 is background clutter, which is generated by the controller 152; the video circuit 1131 obtains background clutter, converts the background clutter into digital signals, and outputs the digital signals to an infrared digital micromirror array (DMD) 1132.
The driving voltage of DMD1132 is generated by controller 152. The DMD1132 obtains a digital signal, converts the digital signal into an analog signal under the action of a driving voltage, obtains a clutter image, modulates the clutter image and an infrared image to obtain a clutter background infrared analog image S1 (first image), and projects the first image to the target generating component 12.
In one embodiment, the target generating assembly 12 includes, in order, a blackbody radiation source 121 and a target plate 122 along the path of light propagation. The blackbody radiation source 121 may be denoted as a second blackbody radiation source.
Wherein the blackbody radiation source 121 emits a second infrared light, the intensity of which is adjusted to illuminate the target plate 122.
One surface of the target plate 122 facing the collimator 133 is plated with a reflective film layer with high reflectivity and a group of hollowed targets; the hollowed target transmits the second path of infrared light to generate target radiation; the reflective film layer reflects the first image to obtain background radiation. The background radiation and the target radiation are mixed together to obtain target radiation S4 which is projected to the collimation assembly 13.
In one embodiment, the number of hollowed-out targets may be multiple; preferably, the number of targets is 4, and the aspect ratio of each of the 4 targets is 7:1. 4 stripe-shaped target patterns can be produced. Illustratively, the blackbody radiation source 121 may be a cavity-type blackbody, having an infrared radiation source disposed therein, the infrared radiation source having an adjustment knob for adjusting the intensity, brightness, and/or temperature of the radiation. The black body is used for isolating interference of external noise on the infrared radiation source.
In one embodiment, the collimation assembly 13 includes, in order, a planar mirror 131, an off-axis parabolic mirror 132, and a parallel light pipe 133 along the path of light propagation.
As shown in fig. 1, the plane reflector 131 is located inside the incidence notch at the middle section of the collimator tube 133, the off-axis parabolic reflector 132 is located at the bottom section of the collimator tube 131, and the off-axis parabolic reflector 132 and the plane reflector 131 are disposed in the collimator tube 133 and fixed in relative positions, so as to form a dual-mirror reflecting parallel radiation tube.
The planar mirror 131 obtains background radiation and target radiation from the target plate 122, reflecting the target radiation S4 to the off-axis parabolic mirror 132.
The off-axis parabolic mirror 132 obtains the background radiation and the target radiation emitted by the planar mirror 131 and reflects the background radiation and the target radiation into the collimator 133.
The collimator 133 modulates the background radiation and the target radiation into parallel radiation S5 to exit onto the thermal imager 14.
In one embodiment, the measurement and control assembly 15 includes a scanning radiometer 151 and a controller 152, the scanning radiometer 151 being disposed between the collimator 131 and the thermal imager 14.
The scanning radiometer 151 detects the temperature of the target radiation and the temperature of the background radiation in the target pattern against the clutter background, respectively.
The controller 152 includes, but is not limited to, a control device such as a computer, and the controller 152 is provided with a control module 1521 and a test module 1522.
Wherein the control module 1521 generates background clutter to the video circuit 1131; and control module 1521 generates a drive voltage input to DMD1132. The background clutter includes light, medium and heavy background clutter.
In one embodiment, the background clutter includes light, medium and heavy background clutter.
The test module 1522 obtains the temperature of the target radiation and the temperature of the background radiation, obtains the value of the minimum distinguishable temperature difference according to the adjustment of the temperature of the target radiation and the temperature of the background radiation, and determines the test result of the thermal imaging performance of the thermal imager.
In one embodiment, the test module 1522 includes a detection unit, a rank determination unit, and an analysis unit.
The detection unit detects the variance of pixel gray values of an image of target radiation and an image of background radiation in the target pattern, and determines a statistical variance; detecting the temperature of the background radiation and the temperature of the target radiation, and calculating the difference between the two to obtain a second temperature difference.
The grade determining unit determines a signal-to-noise ratio according to the second temperature difference and the statistical variance, and analyzes the contrast of brightness between the image of target radiation and the image of background radiation in the target pattern according to the grade of clutter of the target pattern and the second temperature difference; determining the value of the minimum resolvable temperature difference according to the temperature difference from zero to the image of the target radiation which can be resolved by human eyes when the contrast of the brightness is gradually increased; when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent; when the value of the minimum distinguishable temperature difference is larger than the set threshold value, the thermal imaging performance of the thermal imager is poor.
According to the thermal imaging performance test system under the background clutter interference, the test system is required to be adjusted before testing, so that a test system light path is obtained.
FIG. 2 is a diagram of a thermal imaging performance test system under background clutter interference according to an embodiment of the present application. As shown in fig. 2, the collimator 133 has an off-axis parabolic mirror 132 and a plane mirror 131 built therein, and the relative positions are fixed; the DMD light modulator 113 is parallel to the collimator 133, and the incident direction of the blackbody radiation source 111 makes an angle of about 45 ° with the DMD light modulator 113; the target plate 122 is spaced apart from the collimator 133 by a certain distance and is at an angle to the collimator 133. The DMD light modulator 113 is adjustable in both the direction of incident radiation and the direction of target, and the thermal imager 14 is positioned at a location parallel to the spatial irradiance of the radiation S5 by the collimation assembly 13.
In the test system light path diagram shown in fig. 2, the blackbody radiation source 111 emits a first path of infrared light to propagate to the convex lens, which projects the first path of infrared light onto the DMD light modulator 113 by means of critical illumination; the DMD light modulator 113 projects the clutter background infrared image S1 to the target plate 122, and background radiation is obtained through reflection; the blackbody radiation source 121 emits a second path of infrared light to irradiate the target plate 122, and a hollowed target on the target plate 122 transmits the second path of infrared light to generate target radiation; the background radiation and the target radiation are mixed together to obtain target radiation S4, the target radiation S4 is emitted to the plane mirror 131, reflected by the plane mirror 131, reaches the off-axis parabolic mirror 132, and then is emitted to the thermal imager 14 to form the parallel radiation S5 after being reflected by the off-axis parabolic mirror 132.
Fig. 3 is a pre-test preparation flowchart of the test system in the embodiment of the present application. As shown in fig. 3, steps S301-S302 are performed prior to the formal test.
S301, system scaling; system calibration is performed using a standard blackbody radiation source and scanning radiometer 151.
In one embodiment, the following steps S3011-S3012 are included.
S3011, calibrating the scanning radiometer 151 with a standard blackbody radiation source.
Standard blackbody radiation sources refer to standard certified blackbody radiation sources having a temperature range of 50-1000 c with a precision +/-0.2 c.
Illustratively, the scanning radiometer 151 may be calibrated with the standard blackbody radiation source 30 emitting thermal radiation at a temperature of 30 ℃.
For example, the linearity of a scanning radiometer may be calibrated with thermal radiation of a plurality of temperature values emitted by a standard blackbody radiation source.
S3012, calibrating the thermal radiation values of the first blackbody radiation source 111 and the second blackbody radiation source 121 with the calibrated scanning radiometer 151.
Illustratively, the intensity or brightness value of the thermal radiation of the first blackbody radiation source 111 and the intensity or brightness value of the thermal radiation of the second blackbody radiation source 121 are calibrated, respectively, according to the measurement result of the scanning radiometer 151.
S3013, measuring the heat radiation value of each component in the test system by using the calibrated scanning radiometer 151, and calibrating the uniformity and the temperature difference of the heat radiation value of the whole test system provided by the embodiment of the application, thereby obtaining the constant of the whole test system provided by the application. The constants of the whole test system comprise thermal sensitivity and temperature measurement precision.
S302, selecting the spatial frequency of the test system provided by the application, and scanning the scanning radiometer at least on four spatial frequencies, wherein the spatial frequency is the spatial frequency meeting the working requirement of the thermal imager.
In one embodiment, scanning by the scanning radiometer 151 to obtain the minimum resolvable temperature difference of the test system provided herein is performed at least at four spatial frequencies, the spatial frequencies being selected to reflect the operational requirements of the thermal imager 14.
Illustratively, the value of the spatial frequency may be selected to be 0.2f when scanning radiometer 151 scans 0 、0.5f 0 、1.0f 0 And 1.2f 0 Wherein f 0 For a characteristic frequency of 1/(2 x DAS), DAS is the thermal imager detector size versus its objective angle-of-field, meaning spatial resolution/IFOV, IFOV is the angle that can be imaged on a single pixel, expressed in milliradians mrad because the angle is too small. The effect of the IFOV on the detector and lens is that the lens is unchanged, the higher the pixel, the smaller the IFOV. Whereas the pixels are unchanged, the smaller the field angle, the smaller the IFOV. Meanwhile, the smaller the IFOV, the clearer the imaging effect. When the temperature difference between the target radiation and the background radiation gradually increases from zero until the observer confirms that the 4 stripe-shaped target patterns displayed by the thermal imager 14 can be resolved, the temperature difference between the target radiation and the background radiation becomes the Minimum Resolvable Temperature Difference (MRTD) at the spatial frequency.
The spatial frequency varies from small to large, e.g. from 0.2f 0 Becomes 0.5f 0 This can be achieved by adjusting the opening angle (mrad) of the objective lens from small to large.
Illustratively, when the thermal imager 14 is used for severe weather conditions or low temperature differential targets, a lower spatial frequency should be selected.
Steps S301 to S302 above are preparation work before measurement.
In one embodiment, after system calibration is completed and the spatial frequency of the thermal imager is determined, the temperature of the target radiation and the temperature of the background radiation are obtained by the test module 152 in the measurement and control assembly 15, the value of the minimum resolvable temperature difference is obtained according to the adjustment of the temperature of the target radiation and the temperature of the background radiation, and the test result of the thermal imaging performance of the thermal imager is determined.
Fig. 4a is a flow chart of a test module of the test system in the embodiment of the present application. As shown in fig. 4a, the test module 152 performs the following steps S401 to S404 to realize the test of the thermal imaging performance under the background clutter interference.
S401, determining the level of background clutter.
In one embodiment, step S401 is implemented by the following steps S4011-S4013.
S4011, detecting variance of pixel gray values of an image on the thermal imager 14, and determining a statistical variance SV.
In one embodiment, the image on the thermal imager 14 may be divided into M cells, approximately twice the size of the image of the target radiation (image of a single strip target), and the variance σ of pixel gray values for each of the M cells is calculated i Then, the average value of the variance sums of all M units is calculated and the statistical variance SV value is obtained:
s4012, detecting the temperature of background radiation and the temperature of target radiation, and calculating the difference between the two to obtain a second temperature difference; determining a signal-to-noise ratio (SCR) value according to the second temperature difference and the Statistical Variance (SV) value, wherein the SCR value is as follows:
in the formula (2), T max Is the maximum radiation intensity value, mu, of the target radiation b Is the background radiation average radiation intensity value and SV is the statistical variance value.
S4013, determining the level of the background clutter according to the magnitude of the signal-clutter ratio SCR value.
In one embodiment, background clutter can be generalized into 1 three classes according to signal-to-clutter ratio SCR values: mild clutter, SCR >10; medium clutter, 1< scr <10; and heavy clutter, SCR <1.
S402, generating the background clutter of the level according to the level of the background clutter.
Based on the three levels of background clutter, for each spatial frequency of target radiation, the controller 152 generates a mild, moderate, and severe clutter background at that spatial frequency, and the thermal imaging performance test is performed on each of the three clutter backgrounds.
S403, analyzing thermal imaging performance under background clutter interference, including: according to the clutter level of the target pattern and the second temperature difference, analyzing the contrast of brightness between the image of target radiation and the image of background radiation in the target pattern; determining the value of the minimum resolvable temperature difference according to the temperature difference from zero to the image of the target radiation which can be resolved by human eyes when the contrast of the brightness is gradually increased; when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent; when the value of the minimum distinguishable temperature difference is larger than the set threshold value, the thermal imaging performance of the thermal imager is poor.
For example, if the resolution is low, i.e. the contrast in brightness between the image of the target radiation and the image of the background radiation in the target pattern in the observer's view angle is low, the individual strip-like target images almost overlap, with an infinitely long observation by the observer on the display screen. When the contrast of the brightness between the image of the target radiation and the background radiation is gradually increased from zero to the point that the observer confirms the target pattern capable of distinguishing four strips, the temperature difference between the target radiation and the background radiation becomes the minimum distinguishable temperature difference under the spatial frequency; when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent; when the value of the minimum distinguishable temperature difference is larger than the set threshold value, the thermal imaging performance of the thermal imager is poor.
FIG. 4b is a flow chart illustrating a thermal imaging performance under background clutter interference according to an embodiment of the present application. As shown in fig. 4b, step S403 may be implemented by the following steps S4031-S4034.
S4031, a spatial frequency is determined at which the target radiation can be clearly imaged on the thermal imager 14.
In one embodiment, the observer-adjustable thermal imager's field angle (mrad) may start with low frequency target radiation, such as 0.2f 0 The target radiation is imaged clearly on the thermal imager 14.
S4032, determining a first temperature difference between the target radiation and the background radiation, the first temperature difference being a temperature difference at which the target image displayed on the thermal imager 14 just can resolve the black-and-white pattern.
In one embodiment, the observer may reduce the temperature difference between the target radiation and the background radiation until just the black and white pattern is resolved, the target plate 122 is fixed, and the temperature difference between the target radiation and the background radiation at this time is recorded, thereby obtaining a first temperature difference between the target radiation and the background radiation.
In one embodiment, the observer may rotate the adjustment knob to change the intensity, brightness, and/or temperature of the first infrared light and the second infrared light, respectively, while adjusting the target plate 122 so that the thermal imager 14 can just resolve the black-and-white pattern, and record the temperature difference between the target radiation and the background radiation at that time, thereby obtaining the first temperature difference between the target radiation and the background radiation.
S4033, generating a background clutter of the current level to enable the thermal imager 14 to display the target pattern under the clutter of the level.
In one embodiment, the observer may cause the controller 152 to generate background clutter at light, medium, and heavy clutter levels based on the selected spatial frequency and the temperature of the target radiation at that time and the temperature of the background radiation, and the thermal imager 14 generates a target pattern under clutter in response to the selected spatial frequency and the temperature of the target radiation at that time.
5a-5c, FIG. 5a shows a target pattern corresponding to a light clutter at a certain spatial frequency according to an embodiment of the present application; FIG. 5b is a corresponding target pattern for medium clutter at a spatial frequency according to an embodiment of the present application; FIG. 5c is a diagram of a target pattern for a corresponding heavy clutter at a spatial frequency in an embodiment of the present application.
In one embodiment, for target patterns under light clutter, the controller 152 controls the DMD light modulator 113 to regulate the DMD1133 to modulate the clutter image and the infrared image to output a light first image so that the thermal imager 14 can display target patterns under the level of clutter. The observer adjusts the various control and viewing distances to the optimum and then fixes the position and parameters of the various components of the instrument.
S4034, obtaining a change value of the contrast of the target pattern under the clutter of the grade according to the change value of the difference value of the temperature of the target radiation and the background radiation, and determining the value of the minimum distinguishable temperature difference under the clutter of the grade.
In one embodiment, the temperature difference of the target radiation and the background radiation is adjusted and the hot bar temperature difference and the cold bar temperature difference are recorded, respectively.
Wherein, when the target radiation is higher than the temperature of the background radiation, the positive temperature difference is marked as a hot rod or a white rod, and when the temperature of the target radiation S3 is lower than the temperature of the background radiation S2, the negative temperature difference is marked as a cold rod or a black rod.
The temperature difference above 75% of the area of each target and 75% of the area between the two bars is referred to as the hot bar (or white bar) temperature difference.
The temperature difference below 75% of the area per bar and 75% of the area between the bars is referred to as the cold bar (or black bar) temperature difference.
In one embodiment, the temperature difference between the target radiation and the background radiation may be increased, the temperature difference between the target radiation S3 and the background radiation S2 may be gradually decreased, the measurement may be continued, the temperature of the blackbody radiation source 121 illuminating the target plate 122 may be adjusted from below the set background temperature to above the background temperature, the black and white pattern of the target pattern displayed by the calorimeter 14 may be resolved, and the hotbar temperature difference may be recorded. And continuing to reduce the temperature difference until the cold rod appears, and recording and judging whether the temperature difference is the cold rod temperature difference. The interpretation is based on the fact that 75% of observers can separate the images.
S4035, repeating the above steps S4031 to S4034 for the target pattern under moderate and severe clutter.
In one embodiment, the above-described S4031-S4035 processes are repeated for target patterns under clutter at other prescribed spatial frequencies.
S404, calculating a hot rod temperature difference and a cold rod temperature difference average value, and calculating and drawing an MRTD (f) curve to obtain a test result of the thermal imaging performance of the thermal imager 14.
In one embodiment, the MRTD (f) value of the thermal imager 14 is calculated by taking the average of the absolute values of the hot bar temperature difference and the cold bar temperature difference and correcting for the transmittance of the collimation assembly 13 and the emissivity of the target generator.
ΔT 1 ＝T 1 -T 0 ,ΔT 2 ＝T 2 -T 0 (4)
Wherein: phi is the thermal imager constant, related to the MTF of the thermal instrument, the transmittance of the collimation assembly 13, and the emissivity of the radiation of the target generator; t (T) 0 Is an effective ambient temperature; t (T) 1 The temperature of the target radiation when the stripe pattern can be distinguished for the human eye; t (T) 2 The temperature of the target radiation when the black stripe pattern can be distinguished for the human eye; delta T 1 The minimum temperature difference (T) between the target radiation and the background radiation when the white stripe pattern is distinguishable for the human eye 1 ＞T 0 )；ΔT 2 To distinguish the temperature difference (T) between the target radiation and the background radiation when the black stripe pattern is recognized for human eyes 2 ＜T 0 ). In this application, the target generator is referred to as the blackbody radiation source 121.
MRTD (f) values under different spatial frequencies and different clutter degrees are recorded respectively, are filled in a table 1, and are plotted (specific data in the table are omitted), so that a test result is obtained.
Table 1 thermal imager MRTD test record table
The thermal imaging performance of the thermal imager is excellent when the value of the minimum resolvable temperature difference is equal to or lower than the set threshold value, and is poor when the value of the minimum resolvable temperature difference is higher than the set threshold value. Therefore, the quality of the resolving power of the thermal imager is judged through the numerical value of the MRTD, and the stronger the resolving power of the thermal imager under the clutter background, the more advanced the equipment performance is.
Those of ordinary skill would further appreciate that the elements and algorithm steps of the examples described in connection with the embodiments disclosed herein may be embodied in electronic hardware, in computer software, or in a combination of the two, and that the elements and steps of the examples have been generally described in terms of function in the foregoing description to clearly illustrate the interchangeability of hardware and software. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the solution. Those of ordinary skill in the art may implement the described functionality using different approaches for each particular application, but such implementation is not to be considered as beyond the scope of the present application.
The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied in hardware, in a software component executed by a processor, or in a combination of the two. The software components may be disposed in Random Access Memory (RAM), memory, read Only Memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.
The foregoing detailed description has further been provided for the purpose of illustrating the embodiments of the present application, and is to be understood that the foregoing detailed description is merely illustrative of the embodiments of the present application and is not intended to limit the scope of the embodiments of the present application, but is to be construed as including any modifications, equivalents, improvements or etc. that fall within the spirit and principles of the embodiments of the present application.

Claims (9)

1. A system for testing thermal imaging performance under background clutter interference, the system comprising:
the background clutter generation assembly is used for modulating the first path of infrared light and the background clutter to obtain a first image; projecting the first image; the intensity of the first path of infrared light is adjustable; the target generation assembly is used for receiving the first image and reflecting the first image through the target plate to obtain background radiation; obtaining a second path of infrared light, transmitting the second path of infrared light through a target plate to generate target radiation, and emitting the background radiation and the target radiation together; the intensity of the second path of infrared light is adjustable, and the intensity difference between the first path of infrared light and the second path of infrared light is a first temperature difference;
the collimation component is used for modulating the background radiation and the target radiation through double-mirror reflection to obtain parallel radiation;
the thermal imager is used for receiving the parallel radiation and displaying the parallel radiation as a target pattern under clutter; the contrast of brightness between the image of the background radiation and the target radiation in the target pattern varies with the adjustment of the first temperature difference;
the measurement and control assembly is used for detecting the difference value between the temperature of the background radiation and the temperature of the target radiation to obtain a second temperature difference, wherein the second temperature difference changes along with the adjustment of the first temperature difference; determining a value of a minimum resolvable temperature difference according to the second temperature difference and the target pattern, wherein the minimum resolvable temperature difference is a temperature difference when the contrast of brightness between an image of target radiation and an image of background radiation is gradually increased from zero to an image of target radiation which can be resolved by human eyes;
the measurement and control assembly comprises a controller, wherein the controller comprises a test module, the test module is used for obtaining the temperature of the target radiation and the temperature of the background radiation, and obtaining the value of the minimum distinguishable temperature difference according to the adjustment of the temperature of the target radiation and the temperature of the background radiation, so as to obtain the test result of the thermal imaging performance of the thermal imager;
wherein, the test module includes:
a detection unit for detecting the variance of pixel gray values of an image of target radiation and an image of background radiation in the target pattern, and determining a statistical variance; obtaining the temperature of the background radiation and the temperature of the target radiation, and calculating the difference between the two to obtain the second temperature difference;
the grade determining unit is used for determining a value of a signal-to-noise ratio according to the second temperature difference and the statistical variance and determining the grade of clutter of the target pattern according to the value of the signal-to-noise ratio;
an analysis unit, configured to analyze a contrast of brightness between an image of target radiation and an image of background radiation in the target pattern according to the level of clutter of the target pattern and the second temperature difference; determining a value of a minimum resolvable temperature difference according to a temperature difference from zero to an image which can be resolved by human eyes according to the contrast ratio of the brightness; when the value of the minimum distinguishable temperature difference is equal to or lower than a set threshold value, the thermal imaging performance of the thermal imager is excellent;
wherein said determining the statistical variance comprises:
dividing an image on a thermal imager into M units;
calculating a variance of pixel gray values of each of the M cells
And (3) according to the following formula, averaging the sum of variances of the M units to obtain a statistical variance SV value:
wherein the determining the value of the signal-to-noise ratio according to the second temperature difference and the statistical variance comprises:
the signal-to-noise ratio SCR is calculated according to:
wherein T is max Is the maximum radiation intensity value, mu, of the target radiation b Is the background radiation average radiation intensity value.
2. The test system of claim 1, wherein the background clutter generating assembly comprises, in order along the path of light propagation:
the first blackbody radiation source is used for emitting the first infrared light and adjusting the intensity of the first infrared light;
the imaging light path is used for obtaining an infrared image from the first path of infrared light through the collecting lens;
and the DMD light modulator is used for obtaining the background clutter and modulating the infrared image and the background clutter to obtain the first image.
3. The test system of claim 2, wherein the DMD light modulator comprises:
the video circuit is used for obtaining the background clutter and converting the background clutter into digital signals to be output;
the infrared digital micro-mirror array is used for obtaining the digital signal, converting the digital signal into a clutter image under the action of driving voltage, and modulating the clutter image and the infrared image to obtain the first image.
4. The test system of claim 1, wherein the target generating assembly comprises, in order along the path of light propagation:
the second blackbody radiation source is used for emitting the second path of infrared light and adjusting the intensity of the second path of infrared light;
the target plate comprises a group of hollowed targets and a reflecting film layer, wherein the hollowed targets are used for transmitting the second path of infrared light to generate target radiation, and the reflecting film layer is used for reflecting the first image to obtain background radiation.
5. The test system of claim 4, wherein the number of hollowed-out targets is a plurality, and wherein each hollowed-out target in the plurality of hollowed-out targets has an aspect ratio of 7:1.
6. The test system of claim 1, wherein the collimating assembly comprises, in order along the path of light propagation:
the plane reflector is used for obtaining the background radiation and the target radiation and reflecting the background radiation and the target radiation out together;
an off-axis parabolic reflector for obtaining the background radiation and the target radiation, and reflecting the background radiation and the target radiation into a collimator;
the collimator is used for modulating the background radiation and the target radiation into parallel radiation and emitting the parallel radiation;
the plane reflector is positioned in the incidence notch at the middle section of the collimator, and the off-axis parabolic reflector is positioned at the bottom section of the collimator and has a fixed relative position.
7. The test system of claim 3, wherein the measurement and control assembly further comprises:
a scanning radiometer for detecting the temperature of the target radiation and the temperature of the background radiation in the target pattern under the clutter background respectively;
the controller also comprises a control module, wherein the control module is used for generating the background clutter to be input into the video circuit and generating a driving voltage to be input into the infrared digital micro-mirror array.
8. The test system of claim 7, wherein the background clutter comprises light, medium and heavy background clutter.
9. The test system of claim 7, wherein the scanning of the scanning radiometer is performed at least at four spatial frequencies, the spatial frequencies being those that meet the operating requirements of the thermal imager.
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