A target full-scale BRDF testing method

Abstract

The present invention discloses a target full-size BRDF test method, in which a laser light source emits a light beam with a certain divergence angle, which is modulated into a periodically changing light signal of a certain frequency by a chopper, and the light beam is expanded and collimated by a beam expander, so that the light beam after removing the background stray light becomes a parallel light of a certain size, which is incident on the surface of an optical sample to remove the scattering of the background stray light. The target to be measured is placed on a lifting platform, and the test equipment measures the irradiance at different angles around the target to be measured, so that the distance between the target detection point and the photosensitive surface of the detector at different times is the same, and by changing the height of the lifting platform, the scattered light measurement on different circumferences centered on the target element to be measured is realized. The scattered light at different positions in space is measured, thereby realizing BRDF measurement.

Description

Technical Field

[0001] The invention relates to the technical field of infrared radiation measurement, and in particular to a target full-size BRDF testing method. Background Art

[0002] At present, when infrared radiation measurement equipment performs dynamic measurement, the focal plane detector receives the radiation energy E in the air. t In addition, it is inevitable to receive stray energy from the environment. The stray energy includes three parts: atmospheric radiation E a (Atmospheric scattering and scattering of clouds, smoke, and haze), radiation reflected by the target (sun, moon, clouds, ground radiation) E p Taking into account the attenuation of the atmosphere, the energy E received by the device is: E = τ1E t +E a +τ2E P , where τ1 is the attenuation of the infrared radiation energy of the flying target by the atmosphere, and τ2 is the attenuation of the radiation reflection energy by the atmosphere. It can be seen that a large part of the energy measured by the detector in the infrared radiation is composed of the environmental reflection energy. Therefore, measuring the direct solar radiation and the scattered radiation from the sky is the focus of the environmental radiation measurement, while the measurement of the reflection characteristic parameters of the target is the focus of the target's own radiation parameter test.

[0003] At present, due to the lack of a unified and effective physical modeling method, the reflection characteristic parameters of the target can only be obtained through experimental testing, and then the appropriate BRDF function is selected to describe it according to the data characteristics. The differences between different materials are very large, and the selected BRDF function may also be different. Therefore, the BRDF characteristic test of the target is the key to building the target reflection radiation model. However, due to the complex material composition of the test target, the large size of the target, the lack of whole machine testing methods, and the difficulty of testing, the domestic target radiation characteristic measurement, such as the infrared characteristic measurement method of aerial targets, lacks the measurement of target BRDF characteristic parameters, making it difficult to eliminate the error caused by the reflected radiation energy in the infrared characteristic test of the field target.

[0004] From the above, we can see that the intrinsic infrared physical parameters of the target and the environmental radiation parameters such as BRDF parameters are closely related. The measurement methods of related parameters need to be further improved. Otherwise, even for the same type of targets, their test data will be affected by the environment and target status, and their differences will be large. The test data will be difficult to use in model calibration and it will be difficult to support model calibration and simulation verification. Summary of the invention

[0005] In view of the above problems, the present invention proposes a target full-size BRDF testing method, and the technical solution adopted is:

[0006] A target full-size BRDF test system, characterized in that it includes a laser, a chopper, a beam expander and an infrared test device, wherein the laser is used to emit a laser beam, the chopper is used to modulate a received beam signal to modulate it into a periodically changing light signal, the beam expander is used to expand and collimate the modulated beam, and convert the beam with background stray light removed into parallel light, and the infrared test device is used to measure the irradiance of the object to be tested.

[0007] A target full-size BRDF testing method, characterized by comprising the following steps:

[0008] Step 1: Place the target to be measured on a lifting platform with a turntable below it;

[0009] Step 2: Use a laser to emit a beam with a divergence angle of 0-30°;

[0010] Step 3: using a chopper to modulate the light beam in step 2 into a periodically changing light signal of one of short wave, medium wave or long wave;

[0011] Step 4: The modulated optical signal is expanded and collimated by a beam expander to obtain a parallel light beam with background stray light removed, and the parallel light beam is incident on the surface of the target to be measured;

[0012] Step 5: Adjust the angle of the target to be measured by rotating the turntable, and adjust the height of the target to be measured by the lifting platform;

[0013] Step 6: Use infrared testing equipment to measure the irradiance of the target at different angles, and make the distance between the target detection point and the detector photosensitive surface at different times the same, so as to obtain the scattered light measurement values ​​on different circles centered on the target, and realize the measurement of scattered light at different positions in space.

[0014] Furthermore, the step of measuring the irradiance of the target to be measured in step 6 is:

[0015] Step 61: Create a target BRDF model based on the test light source:

[0016] Assume the incident radiation brightness L is:

[0017] L=L e +L s (1)

[0018] Among them, L e To test the direct radiation brightness of the light source, L s is the background radiation brightness. The background infrared radiation is the superposition of the environmental scattering and the atmospheric thermal radiation.

[0019] Then the total radiation brightness of the target to be measured in the band is:

[0020]

[0021] Among them, E 直 is the direct radiance of the light source on the target, θ i is the target observation pitch angle, φ i is the target observation azimuth, f r is the target surface material spectral BRDF;

[0022] Step 62: Measure the radiation brightness of the target using formula (2), and use the modified five-parameter model to perform data fitting calculation to obtain the spectral BRDF of the target surface material. The modified five-parameter model formula is:

[0023]

[0024] Among them, θ i is the incident pitch angle, θ r is the reflected pitch angle, is the relative azimuth; k b , k d , b, a, k r is a parameter to be determined; is the masking function. Usually, the masking function is not considered. is the distribution function of the surface normal of the small source on the target surface; exp[b·(1-cosγ) a ] is an approximate description of the Fresnel reflection function.

[0025] The beneficial effects of the present invention are:

[0026] The target full-size BRDF test method proposed in the present invention uses a turntable as a bearing base to achieve lifting and horizontal rotation of large aircraft and obtain the reflection values ​​of the aircraft test parts at various angles. The five-parameter model is used for data fitting calculation to achieve the calculation of the BRDF characteristics of each part, solving the problems caused by irregular results and inconsistent materials in the detection of large aircraft and other targets. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the target BRDF model based on the test light source;

[0028] Figure 2 This is a schematic diagram of BRDF characteristic test; DETAILED DESCRIPTION

[0029] In order to enable ordinary technicians in the field to better understand the technical solution of the present invention, the technical solution of the present invention is further described below from the perspective of supporting theory, embodiments, and drawings.

[0030] 1. Using BRDF function as background

[0031] Bidirectional Reflectance Distribution Function can characterize basic optical properties. It characterizes the distribution of reflected energy in the hemispherical area of ​​the optical surface for incident light from a certain direction. This reflection phenomenon is related to the roughness of the component surface, the wavelength of the radiated light, and polarization. The spectral characteristics and reflection characteristics of the light beam are described by projecting it onto a surface of a material with sufficient area and relatively homogeneous properties. The bidirectional reflectance distribution function can be obtained according to the radiation amount at this time.

[0032] Assume that the incident radiation brightness L is:

[0033] L=L e +L s (1)

[0034] L e To test the direct radiation brightness of the light source, L s is the background radiation brightness, which is the background infrared radiation composed of the scattering of the environment and the superposition of atmospheric thermal radiation.

[0035] Combined with Figure 1 , then the total radiation brightness of the target in the band is:

[0036]

[0037] Among them, E 直 is the direct radiance of the light source on the target, θ i is the target observation pitch angle, φ i is the target observation azimuth, f r is the target surface material spectral BRDF.

[0038] According to the radiation characteristics of the target skin, a modified five-parameter model is proposed to perform data fitting calculation:

[0039]

[0040] Among them, θ i is the incident pitch angle, θ r is the reflected pitch angle, is the relative azimuth; k b , k d , b, a, k r is a parameter to be determined; is the masking function. Usually, the masking function is not considered. is the distribution function of the surface normal of the small source on the target surface; exp[b·(1-cosγ) a ] is an approximate description of the Fresnel reflection function;

[0041]

[0042] Among them, the first term represents the coherent scattered component (specular reflection component) of the target surface, and the second term represents the non-coherent scattered coherent component (diffuse reflection component);

[0043] 2. BRDF testing method proposed in the present invention

[0044] like Figure 2 As shown in the figure, the BRDF measurement system is a laser light source that emits a light beam with a certain divergence angle, which is modulated by a chopper into a periodically changing light signal of a certain frequency. The light beam is expanded and collimated by a beam expander, so that the light beam without background stray light becomes parallel light that matches the size of the object being measured, and is incident on the surface of the optical sample to remove the scattering of the background stray light.

[0045] The specific steps of the test method proposed in the present invention are as follows: first, the target to be tested is placed on the lifting platform in the middle; second, the irradiance at different angles is measured around the target to be tested by the infrared test equipment, so that the distance between the detected point on the target to be tested and the photosensitive surface of the infrared test equipment at different times is the same; finally, by changing the height of the lifting platform, the scattered light measurement on different circumferences centered on the target to be tested is realized, so as to measure the scattered light at different positions (different heights and angles) in space, and then realize BRDF measurement. The model previously established by formula (3) can be corrected by BRDF to avoid the deviation caused by secondary radiation such as sunlight.

[0046] The above describes the new algorithm of the present invention, the technical theory behind the algorithm, the main advantages and examples. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A target full-size BRDF testing method, characterized in that: The following steps are involved: Step 1: Place the target to be measured on a lifting platform with a turntable below it; Step 2: Use a laser to emit a beam with a divergence angle of 0-30°; Step 3: using a chopper to modulate the light beam in step 2 into a periodically changing light signal of short wave, medium wave or long wave; Step 4: The modulated optical signal is expanded and collimated by a beam expander to obtain a parallel light beam with background stray light removed, and the parallel light beam is incident on the surface of the target to be measured; Step 5: Adjust the angle of the target by rotating the turntable and adjust the height of the target by the lifting platform; Step 6: Use infrared testing equipment to measure the irradiance of the target under test at different angles, and make the distance between the target detection point and the photosensitive surface of the detector at different times the same, so as to obtain the scattered light measurement values ​​on different circumferences centered on the target under test, and realize the measurement of scattered light at different positions in space; The steps for measuring the irradiance of the target to be measured in step 6 are as follows: Step 61: Create a target BRDF model based on the test light source: Assume that the incident radiation brightness for （1) in, To test the direct radiation brightness of the light source, is the background radiation brightness. The background infrared radiation is the superposition of the environmental scattering and the atmospheric thermal radiation. Then the total radiation brightness of the target to be measured in the band is: （2） in, is the direct radiance of the light source on the target, is the target observation pitch angle, is the target observation azimuth, is the target surface material spectral BRDF; Step 62: Measure the radiation brightness of the target using formula (2), and use the modified five-parameter model to perform data fitting calculation to obtain the spectral BRDF of the target surface material. The modified five-parameter model formula is: （3） in, is the incident pitch angle, is the reflected pitch angle, is the relative azimuth; , , , , is a parameter to be determined; is the masking function. Without considering the masking function, let =1; is the distribution function of the small surface source normal of the target surface; is an approximate description of the Fresnel reflection function.

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