Broadband bidirectional reflectance distribution function measurement method and system for tracing to direction hemispherical reflectivity

Abstract

The invention provides a broadband bidirectional reflectance distribution function (BRDF) measurement method and system of source-to-direction hemispherical reflectance (DHR), belongs to the technical field of bidirectional reflectance distribution function measurement, and is used for reflection spectrum distribution measurement of materials at different wavebands and detection angles. Existing researches on the aspects of BRDF measurement, modeling, application and the like mainly aim at visible light and near-infrared bands, spectrum BRDF measurement is directly achieved through a broadband light source, the method has the advantages of being rapid, simple and convenient, however, when an incident signal is decomposed to a wavelength point, energy is weak, and the measurement signal-to-noise ratio is affected. According to the invention, the BRDF measurement absolute value is traced to the direction hemisphere reflectivity by using the relation between the BRDF and the reflectivity and adopting a BRDF relative measurement method, so that the measurement of incident light and a detection solid angle is avoided. In the aspect of detection, a band-pass spectrum is selected by using an optical filter, and broadband BRDF measurement is carried out. According to the method and the system, complete measurement of the BRDF is realized, and the method and the system have important significance in the fields of infrared remote sensing and radiation temperature measurement.

Description

(1) Technical field

[0001] This paper proposes a broadband bidirectional reflectance distribution function (BRDF) measurement method and system that traces the source to directional hemispherical reflectance, belonging to the field of BRDF measurement technology. This method and system utilizes the relationship between BRDF and reflectance to trace the absolute value of the BRDF measurement to directional hemispherical reflectance, avoiding the measurement of incident light and detection solid angle. Instead, it uses filters to select the bandpass spectrum to perform broadband BRDF measurements. (2) Background technology

[0002] The bidirectional reflectance distribution function (BRDF) is defined as the ratio between the radiance of reflected light and the irradiance of incident light at various angles in a hemispherical space on the surface of a material. It can fully describe the spatial scattering characteristics of the material surface and is a key basic optical parameter in computer graphics algorithms, remote sensing imaging, radiometry and other fields.

[0003] Traditional research on BRDF measurement, modeling and application mainly focuses on the visible and near-infrared bands. The measurement systems are mostly narrow-band laser light sources, which have the characteristics of high power and stable wavelength, greatly improving the signal-to-noise ratio of reflected light, but the cost is extremely high and can only cover a few bands. Using a broadband light source to directly achieve spectral BRDF measurement has the advantages of being fast and simple, but its optical path diverges and the incident light is difficult to obtain, making absolute BRDF measurement difficult. If relative measurement is considered, a BRDF standard sample is required. Due to the difficulty of mid-infrared BRDF measurement, there is a lack of corresponding BRDF standard samples in the infrared band. The present invention utilizes the relationship between BRDF and reflectivity, and adopts the BRDF relative measurement method to trace the absolute value of the BRDF measurement to the directional hemispherical reflectivity, avoiding the measurement of incident light and detection solid angle. In terms of detection, a filter is used to select a bandpass spectrum to carry out broadband BRDF measurement. (3) Summary of the invention

[0004] In response to the problem that the traditional BRDF measurement band is limited and the incident light intensity for broadband BRDF measurement is difficult to obtain, the present invention proposes a broadband bidirectional reflectance distribution function (BRDF) measurement method and system that traces the source to the directional hemispherical reflectance (DHR). This method considers relative measurement and uses the relationship between BRDF and reflectance to trace the absolute value of BRDF measurement to the directional hemispherical reflectance, avoiding the measurement of incident light and detection solid angle, and using filters to select the bandpass spectrum to carry out broadband BRDF measurement.

[0005] The system is divided into BRDF two-dimensional distribution high-resolution measurement and DHR measurement device. The BRDF two-dimensional distribution high-resolution measurement device includes a broadband high-power light source, a shaping optical path, a sample to be measured, a bandpass filter, an infrared detector, a data acquisition card and a host computer. The sample is installed on a robotic arm, and the detector is installed on a circular track. The rotation angle of the robotic arm and the circular track is controlled to realize the measurement of any BRDF four-angle parameters; the DHR measurement device includes a silicon carbon rod, an infrared standard diffuse reflection gold plate and a calibration sample, a reverse gold-plated integrating sphere and a Fourier spectrometer. The silicon carbon rod generates a broadband infrared light source, and the gold-plated integrating sphere homogenizes the reflected light so that the reflected light is integrated within the spatial range. Then, a Fourier spectrometer is used to directly detect the reflected light, and the infrared standard diffuse reflection gold plate and the calibration sample are measured respectively to obtain the spectral DHR. The infrared spectral reflectance measurement value of the calibration sample can be traced back to the national standard. Then, continuous scanning measurement is performed using a BRDF two-dimensional distribution high-resolution measurement device to obtain a high-resolution BRDF distribution with a fixed incident angle. Interpolation is performed in two-dimensional space to obtain a two-dimensional distribution with uniform reflection zenith and azimuth angles. The relationship between BRDF and infrared hemispherical spectral reflectance is the integral of BRDF over the π space on the sample surface to calculate the incident light intensity. Finally, the incident angle is measured using the developed system. and reflection angle The reflected signal under the BRDF is obtained, avoiding the measurement of the incident light intensity parameters and the detection solid angle, and completing the accurate measurement of the BRDF. After the angle is detected, the filter wheel is used to change different filters to achieve BRDF measurement under different bandwidths. The specific measurement steps are as follows: Step 1: According to the broadband BRDF measurement method and system for tracing back to the directional hemispherical reflectivity described in claim 1, it is characterized in that the BRDF measurement is combined with the DHR measurement by utilizing the relationship between BRDF and reflectivity to solve the problem of difficulty in measuring the incident light of the broadband light source, introduce the incident projection factor, comprehensively consider the blackbody spectrum, atmospheric transmittance, detector spectral response and other factors, obtain the incident light intensity parameter by spectral integration, avoid the measurement of the detection solid angle, and achieve accurate BRDF measurement; BRDF is defined as the ratio between the reflected brightness and the incident illuminance, expressed as:

[0006]

[0007] In the actual measurement process, brightness is expressed as the ratio of illuminance to the product of the direction cosines of the solid angle projection, which is expressed as:

[0008]

[0009] Among them, I and U are the incident light intensity and reflected light intensity detected respectively, Ω r To detect the solid angle, it is expressed as:

[0010]

[0011] Where A is the detector aperture area, and D is the distance from the center of the sample to the center of the detector aperture. Accurately measure each component of equation (2) to obtain the BRDF measurement value.

[0012] When the four angles of the BRDF are fixed, the infrared radiation emitted by the broadband light source passes through the air and the bandpass filter and directly illuminates the detector surface. After being collected by the data acquisition card, the detector light intensity signal is obtained, which is the incident signal, expressed as:

[0013]

[0014] Among them, κ λ1-λ2 is the detection spectrum scale factor, λ is the spectrum wavelength, S λ1-λ2 (λ) is the system spectral response function, which is expressed as the product of the blackbody radiation spectrum, the atmospheric transmission spectrum, the filter spectrum, and the detector spectral response. B (λ) is the blackbody spectral radiation intensity distribution, covering the entire band of 3 to 14 μm, T A (L,λ) is the air transmittance when the light passes through a distance of L. The absorption section of H2O molecules in the atmosphere will be greatly reduced. S (λ) is the reflectivity of the sample being tested, and its spectral distribution is related to the actual characteristics of the sample being tested. λ1-λ2 (λ) is the transmittance of the selected filter. The bandwidth of the bandpass filter is the bandwidth of the broadband BRDF to be measured. For example, when measuring the 3-5um broadband BRDF, the transmittance of the bandpass filter is higher than 80% at 3-5um, and is cut off in the 2-3 and 5-15um bands. D (λ) is the spectral responsivity of the detector, and the MCT detector is selected. In the process of incident light intensity calibration, the spectral information of the above parameters is required, among which M B (λ)R D (λ) and T λ1-λ2 (λ) Measure the spectrum using a spectrometer. A (L,λ) is obtained by combining the atmospheric component concentrations and the HITRAN database.

[0015] When a blackbody light source passes through the air and is reflected by the sample, the reflected light intensity is obtained, which can be expressed by combining equations (2) and (4):

[0016]

[0017] Then, the BRDF in this band is expressed as:

[0018]

[0019] Assume the incident projection factor is:

[0020]

[0021] Then formula (6) is expressed as:

[0022]

[0023] The solid angle and projection angle cosine values ​​are obtained through geometric measurement. To obtain the BRDF value at a fixed angle, the incident light intensity is required. When a broadband light source is used as the BRDF measurement light source, the incident light intensity is used as a fixed parameter and calibrated with a standard diffuse reflectance sample and its infrared spectral emissivity value before actual measurement.

[0024] Step 2: A broadband BRDF measurement method and system for tracing back to directional hemispherical reflectivity according to claim 1, characterized in that the incident light is measured indirectly, and the relationship between the BRDF and the infrared hemispherical spectral reflectivity is used to calibrate the incident light intensity. The infrared hemispherical spectral reflectivity is the integral of the BRDF in the space π on the sample surface, expressed as:

[0025]

[0026] Use dedicated measurement equipment to measure DHR. After obtaining the spectral DHR, combine it with the system spectral response function to obtain the spectral integral DHR, which is expressed as:

[0027]

[0028] When using a BRDF measurement device, the reflection signal is directly measured, which is the spectral integrated reflection value under the fixed BRDF four angles. After obtaining the complete reflection distribution in the hemispherical space, the measured value is integrated in two dimensions in the hemispherical space, which is the product of the incident light intensity and the spectral integrated DHR. Therefore, the spectral integrated DHR is expressed as:

[0029]

[0030] The two spectral integral DHR expressions of equations (10) and (11) are equivalent, so the incident projection factor η is expressed as:

[0031]

[0032] When calibrating the incident light intensity of BRDF, a special calibration sample is used to carry out high-resolution measurement of BRDF two-dimensional distribution and DHR measurement. First, the directional hemispherical reflectance of the sample is measured using the DHR measurement device, and the spectrum obtained is the above formula. Then, the incident angle is fixed at BRDF by continuous scanning measurement. The high-resolution distribution of BRDF at this time is measured by setting the discrete single-point position of the robotic arm and continuously scanning the circular track. After obtaining the high-precision scanning distribution, it is interpolated in the two-dimensional space to obtain a two-dimensional distribution with uniform distribution of reflection zenith and azimuth, which is Substituting each measured value into the above formula, we can obtain the incident projection factor η λ1-λ2 The incident projection factor is obtained by measuring the gold-plated diffuse reflection standard sample before measuring the BRDF of the sample under test. Afterwards, when measuring the sample under test, the sample reflection signal detected by the MCT is synchronously collected by the data acquisition card, and the detection signal is demodulated on the host computer using the digital orthogonal demodulation algorithm to obtain the reflection signal amplitude information, which is the detected reflection value. η λ1-λ2 Substitute it into formula (6) as a parameter to calculate, thereby realizing broadband BRDF measurement. (IV) Description of the accompanying drawings

[0033] Figure 1 The invention is a typical structure and flow chart of a broadband bidirectional reflectance distribution function measurement method and system that can be traced back to the directional hemispherical reflectivity, and is composed of the following parts: a broadband light source (1), a shaping optical path (2), a sample to be measured (3), a calibration sample (4), a bandpass filter (5), an infrared detector (6), a silicon carbon rod (7), a gold-plated integrating sphere (8), a standard gold plate (9), a spectrometer (10), and a host computer (11).

[0034] Figure 2 This is a diagram of a broadband BRDF high-resolution measurement device.

[0035] Figure 3 is the directional hemispherical reflectivity of the calibration sample.

[0036] Figure 4 It is the interpolation distribution diagram of the reflected signal in different bands. (V) Specific implementation methods

[0037] The present invention will be further described below with reference to examples.

[0038] The structure of the present invention is as follows Figure 1As shown in FIG, a broadband light source (1) emits incident light of a wide spectrum, which is adjusted and converged by a shaping optical path (2), and the shaped light beam is irradiated on the surface of the sample to be tested (3) and the calibration sample (4). The sample is mounted on a six-axis robotic arm, and the reflected light is detected by a bandpass filter (5) and an infrared detector (6). The bandpass filter and the infrared detector are mounted on a circular guide rail, and the reflected signal of the detection surface is collected by a data acquisition card and uploaded to the host computer (11); a silicon carbon rod (7) generates a broadband infrared light source, and a gold-plated integrated circuit is used. The sphere (8) is used to homogenize and collect the reflected light, and the infrared standard diffuse reflectance gold plate (9) and the calibration sample (4) are measured respectively. The reflected light is detected by Fourier spectrometer to trace the directional hemispherical reflectance measurement value of the calibration sample to the national standard. Then, the BRDF measurement device is used for continuous scanning measurement to obtain the BRDF high-resolution distribution with a fixed incident angle. The two-dimensional distribution with uniform distribution of reflection zenith and azimuth is obtained by interpolation in two-dimensional space. The incident light intensity is calculated using the relationship between BRDF and infrared hemispherical spectral reflectance. Finally, the incident angle is measured using the developed system. and reflection angle The reflected signal under the condition of IR is detected, avoiding the measurement of incident light intensity parameters and detection solid angle, completing the accurate BRDF measurement. After the angle is detected, the filter wheel is used to change different filters to achieve BRDF measurement under different bandwidths.

[0039] The broadband light source in this example uses a high-temperature blackbody radiation source. The light directly emitted by the radiation source is divergent light, and its ability to reach the sample and detector is too low. In order to improve the detection signal capability, a double concave mirror is used to constrain the beam size. A chopper is set at a position with a smaller beam cross-section to modulate the blackbody radiation intensity with a modulation frequency of 200Hz. This allows the light beam to illuminate the center of the sample under test and converge to the detector position. The distance from the blackbody furnace to the center of the sample surface is 1.4m. The sample is mounted on a six-axis robotic arm gripper. The center position of the sample surface is fixed. As the six-axis robotic arm rotates, the angle between the sample and the incident light changes. The detection module is mounted on a circular track and rotates continuously or discretely. The detection module includes a filter wheel and an infrared detector. 3-5μm, 5-9μm and 8-14μm filters are installed on the filter wheel. By switching the filters through the filter wheel, broadband BRDF distributions under different bandwidths are obtained. The response bandwidth of the infrared detector covers 2-16μm. The measurement device is shown in the figure. Figure 2 The detected sample reflection signal and chopper monitoring signal are collected synchronously by the data acquisition card. The detection signal is demodulated on the host computer using the digital orthogonal demodulation algorithm to obtain the reflection signal amplitude information, which is the detected reflection value U λ1- System spectral response function S λ1-λ2 (λ) is calculated by measuring the optical spectral characteristics of each module.

[0040] Use infrared spectrometer to measure the optical characteristics of the module, including the blackbody radiation source spectrum M B (λ), filter transmittance T λ1-λ2 (λ) and detector spectrum. The air transmission spectrum T is calculated using the air gas component concentration parameters and the HITRAN database A (L,λ). By combining the spectral characteristics of the above modules, the distribution S of the system spectral response function in three bands is obtained. 3-5 (λ), S 5-9 (λ) and S 8-14 (λ).

[0041] The DHR of the calibration sample is measured using a directional hemispherical reflectivity standard device, such as Figure 3 Substitute the DHR curve and the system spectral response function of the three bands calculated in step 1 into formula (10) to obtain the system spectral response integral values ​​of the three filter corresponding bands. The BRDF measurement device developed was used to measure the BRDF two-dimensional distribution of the three filters in the corresponding bands. The incident zenith angle and azimuth angle were fixed at 0°, the reflection zenith angle covered -90° to +90°, and the azimuth angle interval was 10°. Three filters were installed in front of the detector, and the ring track continuously rotated and scanned to obtain the reflected signal azimuth scanning curve group. Difference in the range, obtain a two-dimensional distribution such as Figure 4 As shown. Thus, the integral of the two-dimensional distribution in the hemispherical space is calculated Combined with The incident projection factor η is obtained by spectral integration DHR and formula (12) λ1-λ2 In this example, the sample to be tested is high-temperature alloy MA956, which has diffuse reflection characteristics. During each measurement, the sample is fixed, and a black body radiation source is used as the incident light source. Using the developed system, the angle of the mechanical arm is rotated to adjust the incident angle. To measure the angle, the detector is installed on a circular track with a diameter of 300mm. When the detector rotates, the detection signal and the chopping modulation signal are synchronously sampled by the data acquisition card. The detection signal is demodulated by the digital orthogonal demodulation algorithm on the host computer to obtain the reflection signal amplitude information, which is the detected reflection value. After the angle is detected, the filter wheel is used to change different filters, and the BRDF values ​​measured in different bands are obtained using formula (8), thereby realizing BRDF measurement under different bandwidths.

[0042] The above description of the present invention and its embodiments is not limited thereto, and the accompanying drawings are only one embodiment of the present invention. Without departing from the purpose of the present invention, any structure or embodiment similar to the technical solution designed without creativity shall fall within the scope of protection of the present invention.

Claims

1. A broadband bidirectional reflectance distribution function (BRDF) measurement method and system traceable to directional hemispherical reflectivity, the system is divided into a BRDF two-dimensional distribution high-resolution measurement and a DHR measurement device, the BRDF two-dimensional distribution high-resolution measurement device includes a broadband high-power light source, a shaping light path, a sample to be measured, a bandpass filter, an infrared detector, a data acquisition card and a host computer, the sample is installed on a robotic arm, and the detector is installed on a circular track, and the rotation angle of the robotic arm and the circular track is controlled to realize the measurement of any BRDF four-angle parameters; the DHR measurement device includes a silicon carbon rod, an infrared standard diffuse reflection gold plate and a calibration sample, an inverted gold-plated integrating sphere and a Fourier spectrometer, the silicon carbon rod generates a broadband infrared light source, the gold-plated integrating sphere homogenizes the reflected light so that the reflected light is integrated in the spatial range, and then the Fourier spectrometer is used to directly detect the reflected light, respectively. The gold plate and the calibration sample are reflected to obtain the spectral DHR, and the infrared spectral reflectance measurement value of the calibration sample can be traced back to the spectral DHR; continuous scanning measurement is performed through the BRDF two-dimensional distribution high-resolution measurement device to obtain the BRDF high-resolution distribution with a fixed incident angle, and interpolation is performed in the two-dimensional space to obtain a two-dimensional distribution with uniform distribution of reflection zenith and azimuth angles. The relationship between BRDF and infrared hemispherical spectral reflectance is used, and the infrared hemispherical spectral reflectance is the integral of BRDF in the space π on the surface of the sample to calculate the incident light intensity; the developed system is used to measure the reflection signal under the incident angle and reflection angle, avoiding the measurement of the incident light intensity parameters and the detection solid angle, and completing the accurate BRDF measurement. After the angle is detected, the filter wheel is used to change different filters to achieve BRDF measurement under different bandwidths.

2. A broadband BRDF measurement method and system for tracing back to directional hemispherical reflectivity according to claim 1, characterized in that: The relationship between BRDF and reflectivity is used to combine BRDF measurement with DHR measurement to solve the problem of difficult incident light measurement of broadband light sources. The incident projection factor is introduced, and factors such as blackbody spectrum, atmospheric transmittance, and detector spectral response are comprehensively considered. The incident light intensity parameter is obtained by spectral integration, avoiding the measurement of detection solid angle and achieving accurate BRDF measurement. BRDF is defined as the ratio between reflected brightness and incident illuminance, expressed as: In the actual measurement process, brightness is expressed as the ratio of illuminance to the product of the direction cosines of the solid angle projection, which is expressed as: Among them, I and U are the incident light intensity and reflected light intensity detected respectively, Ω r To detect the solid angle, it is expressed as: Where A is the detector aperture area, and D is the distance from the center of the sample to the center of the detector aperture. Accurately measure the components of formula (2) to obtain the BRDF measurement value. When the four BRDF angles are fixed, the infrared radiation emitted by the broadband light source passes through the air and the bandpass filter and directly illuminates the detector surface. After being collected by the data acquisition card, the detector light intensity signal is obtained, which is the incident signal, expressed as: Among them, κ λ1-λ2 is the detection spectrum scale factor, λ is the spectrum wavelength, S λ1-λ2 (λ) is the system spectral response function, which is expressed as the product of the blackbody radiation spectrum, the atmospheric transmission spectrum, the filter spectrum, and the detector spectral response. B (λ) is the blackbody spectral radiation intensity distribution, covering the entire band of 3 to 14 μm, T A (L,λ) is the air transmittance when the light passes through a distance of L. The absorption section of H2O molecules in the atmosphere will be greatly reduced. S (λ) is the reflectivity of the sample being tested, and its spectral distribution is related to the actual characteristics of the sample being tested. λ1-λ2 (λ) is the transmittance of the selected filter. The bandwidth of the bandpass filter is the bandwidth of the broadband BRDF to be measured. For example, when measuring the 3-5um broadband BRDF, the transmittance of the bandpass filter is higher than 80% at 3-5um, and is cut off in the 2-3 and 5-15um bands. D (λ) is the spectral responsivity of the detector. When using MCT detector, the spectral information of the above parameters is required during the incident light intensity calibration process. B (λ)R D (λ) and T λ1-λ2 (λ) Measure the spectrum using a spectrometer, T A (L,λ) is obtained by combining the atmospheric component concentration and the HITRAN database. When the blackbody light source passes through the air and is reflected by the sample, the reflected light intensity is obtained, which can be expressed by combining equations (2) and (4): Then, the BRDF in this band is expressed as: Assume the incident projection factor is: Then formula (6) is expressed as: The cosine values ​​of the solid angle and projection angle are obtained through geometric measurement. To obtain the BRDF value at a fixed angle, the incident light intensity needs to be obtained. When a broadband light source is used as the light source for BRDF measurement, the incident light intensity is considered as a fixed parameter and is calibrated in combination with a standard diffuse reflectance sample and its infrared spectral emissivity value before actual measurement.

3. A broadband BRDF measurement method and system for tracing back to directional hemispherical reflectivity according to claim 1, characterized in that: The incident light intensity is calibrated by indirectly measuring the incident light and utilizing the relationship between BRDF and infrared hemispherical spectral reflectance. The infrared hemispherical spectral reflectance is the integral of BRDF over the space π on the sample surface, expressed as: DHR is measured using dedicated measurement equipment and combined with the system spectral response function to obtain the spectral integral DHR, which is expressed as: When using a BRDF measurement device, the reflection signal is directly measured, which is the spectral integrated reflection value under the fixed BRDF four angles. After obtaining the complete reflection distribution in the hemispherical space, the measured value is integrated two-dimensionally in the hemispherical space, which is the product of the incident light intensity and the spectral integrated DHR. Therefore, the spectral integrated DHR is expressed as: The two spectral integral DHR expressions of equations (10) and (11) are equivalent, so the incident projection factor η is expressed as: When calibrating the incident light intensity of BRDF, a special calibration sample is used to carry out high-resolution measurement of BRDF two-dimensional distribution and DHR measurement. First, the directional hemispherical reflectivity of the sample is measured using the DHR measurement device. The spectrum obtained is the above formula. Then, the incident angle is fixed at BRDF by continuous scanning measurement. The high-resolution distribution of BRDF at this time is measured by setting the discrete single-point position of the robotic arm and continuously scanning the circular track. After obtaining the high-precision scanning distribution, interpolation is performed in two-dimensional space to obtain a two-dimensional distribution with uniform distribution of reflection zenith and azimuth, which is Substituting each measured value into the above formula, we can obtain the incident projection factor η λ1-λ2 The incident projection factor is obtained by measuring the gold-plated diffuse reflection standard sample before measuring the BRDF of the sample under test. Afterwards, when measuring the sample under test, the sample reflection signal detected by the MCT is synchronously collected by the data acquisition card. The detection signal is demodulated on the host computer using the digital orthogonal demodulation algorithm to obtain the reflection signal amplitude information, which is the detected reflection value. η λ1-λ2 Substitute it into formula (6) as a parameter to achieve broadband BRDF measurement.

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