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Abstract
The vacuum radiance temperature standard facility (VRTSF) is used to determine the temperature traceability of calibration blackbodies accurately for infrared remote sensing. However, the internal surfaces of the VRTSF can emit thermal radiation, which introduces stray light and reduces measurement accuracy. Previously, background subtraction was used to eliminate stray light, without considering dynamic changes in the environment. Therefore, we propose a changeable veiling glare index (CVGI) to evaluate stray light performance based on the concept of the veiling glare index. We analyze the radiation model of the VRTSF system and simulate the attribution of stray light. CVGI can be improved considerably by cooling and by using suitable stop sizes. Experiments performed in the VRTSF show that the CVGI is suitable for analyzing stray light. Using the CVGI, we simulate a method that can significantly reduce stray light in the VRTSF.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Infrared remote sensors enable the temperature-related radiation emitted by objects to be measured from a long distance. Hence, they are used in a wide range of applications, such as monitoring of volcanic activity [1] and wild fires [2], soil moisture mapping [3], land surface separation [4], and weather forecasting [5–7]. These applications depend on the quantification of infrared remote sensors. For example, tracking the effects of climate change requires that the measurement data from infrared remote sensors be accurate within 0.1 K and that the stability over 10 years be better than 0.04 K; for measurement of sea surface temperature, the infrared sensors must be as accurate as 0.1 K and as stable as 0.01 K per decade [8]. Moreover, a higher level of quantification is needed because of the rapid development of infrared remote sensing [9].
To develop effective sensors for high-precision and high-quantification infrared remote sensing applications, it is necessary to develop high-precision calibration devices for such sensors. In this regard, laboratory calibration facilities are important in the calibration process. These facilities enable analysis of the traceability of laboratory calibration blackbodies and onboard calibration blackbodies. Several facilities have been developed internationally, such as the medium background infrared (MBIR) and low background infrared (LBIR) facilities at the National Institute of Standards and Technology [10,11], the reduced background calibration facility (RBCF) at Physikalisch-Technische Bundesanstalt (PTB) [12], and the medium-background facility (MBF) at the All-Russian Institute for Optophysical Measurements (VNIIOFI) [13]. Additionally, the China National Institute of Metrology (NIM) has developed a vacuum radiance temperature standard facility (VRTSF) [14] and used it for the calibration of Chinese Fengyun meteorological satellites [15].
To ensure the accuracy and stability of infrared remote sensing calibration devices, extensive research has been conducted on topics such as the stability and uniformity of vacuum changeable-temperature blackbodies [16–19], coating materials with high emissivity [20,21], and phase-fixed points [22,23]. Previous studies have mainly focused on redesigning the reference radiation sources to improve the performance of the calibration facilities. However, quantitative analysis of stray light remains inadequate. Stray light can affect the calibration accuracy directly as it can enter detectors along with the sample light. Therefore, it is necessary to perform stray light analysis for calibration systems.
In this study, we propose an evaluation index for quantifying infrared changeable stray light, namely, the changeable veiling glare index (CVGI). The CVGI is first derived and defined based on the concept of veiling glare index in visible spectral range. Then the CVGI is applied to the VRTSF for stray light analysis. The actual radiation model of VRTSF was built and the changeable stray light was separated. Changeable stray light, which changes with environmental temperature, causes the accuracy of the system to decrease. Besides, to verify the model, series of simulations by setting different stop sizes and temperatures were carried out. The corresponding experiments were conducted in the VRTSF system. The experiments results are consistent with the simulation results, which suggest that the CVGI can be used to analyze the stray light in VRTSF system. The CVGI results inform the proposed light suppression method and we put forward a method to reduce the stray light of VRTSF.
The remainder of this paper is structured as follows. Section 2 introduces the stray light evaluation method. Section 3 describes the CVGI simulation procedure and results of the VRTSF. The experimental CVGI measurements are presented in Section 4. Section 5 compares the simulated and experimental results and outlines the proposed stray light suppression method. Finally, Section 6 presents the conclusions.
2. Derivation of the changeable veiling glare index (CVGI)
In the visible band, the veiling glare index (VGI) is often used to quantify the stray light performance of optical systems [24–26]. As per ISO 9358, the VGI is the ratio of the irradiance at the center of the image of a small, circular, perfectly black area superimposed on an extended field of uniform radiance, to the irradiance at the same point of the image plane when the black area is removed [27]. The VGI is generally used to evaluate the stray light from surface light sources, such as the environmental and atmospheric backgrounds. The VGI can be expressed as [28]
where Lout and Lin are the irradiance produced by the stray light and the signal light at the image plane, respectively. The VGI can be measured with the black spot method, which requires an extended light screen with uniform brightness and an ideal black spot [29]. The ideal black spot is placed on the extended light screen and the illuminance at the center of the image is formed by the black spot. Then, the black spot is removed and the illuminance on the image is measured again. The VGI is the ratio of the illuminance measurements with and without the black spot.However, in the infrared band, the VGI cannot be used, because the ideal black spot is not easy to obtain. Any body above 0 K emits thermal radiation. Infrared optical systems are affected by excessive stray light from their surroundings. However, measurement of stray light is difficult because it can become separated in infrared optical systems. A part of the stray light remains unchanged during the measurement process and can be eliminated via background subtraction. However, the other part changes owing to the environmental temperature. Therefore, it is the stray light that changes in response to environmental changes that affects the system most notably. To evaluate the suppression of this stray light more accurately, we propose the changeable veiling glare index (CVGI), which is defined as
where $L^{\prime}_{out}$ is the changeable stray light radiation at the focal plane. The changeable stray light is defined as the stray light that changes in response to environmental changes.3. Simulation of the CVGI
3.1 Calculation procedure
The stray light analysis is based on the Monte Carlo principle with software TracePro, which uses ray tracing to calculate the CVGI. There are four steps: first, building the mechanical structure and optical system models; second, determining the surfaces of the radiation sources; third, setting the surface properties of critical surfaces according to the scattering model; and fourth, statistically analyzing the energy of each critical surface in order to calculate the CVGI. The simulation framework is shown in Fig. 1.
By simulation, the CVGI can be obtained using
where $L^{{\prime}i}_{out}$ is the energy of the stray light radiation incident on the receiving surface (i represents different critical surfaces) and Lin is the energy of the signal radiation on the receiving surface.To obtain the VGI of the infrared optical system, it is necessary to introduce a constant factor G, which represents the constant stray light originating from the background radiation in the detector and spectrometer. Although G is difficult to obtain by simulation, it exists in real systems. Assuming that, under equivalent conditions, the simulated and experimental CVGIs are equal, then G can be obtained using the following equations:
The VGI of measurement can be expressed as:
where Lmeas_out and Lmeas_in are the spectral responses of the measured stray light and signal light radiation, respectively.The CVGI of measurement is
where ${L_{meas\_out}^{\prime}}$ is the effective changeable stray light in measurement. The relationship between t Lmeas_out and ${L_{meas\_out}^{\prime}}$ can be expressed as Using Eqs. (4)–(7), G can be obtained and is used to calculate the CVGI of experiments for the same system.3.2 Simulation results
The mechanical structure and optical system models of the VRTSF system are shown in Fig. 2 [18]. Stop 1 is located on the right of the optical tubes (“Stop 1” in Fig. 2). Stops can also be placed at positions S2, S3, and S4. The device includes a vacuum low-background cabin, an optical path switching cabin, a vacuum-changeable medium-temperature blackbody (VMTBB), a vacuum mercury fixed-point blackbody source (VMFPBB), a liquid-nitrogen blackbody (LNBB), a vacuum cooling optical system, a Fourier-transform infrared spectrometer (FTIR), a data acquisition system, and other necessary facilities. The specifications of the VRTSF system are shown in Table 1.
Table 1. Specifications of the VRTSF system.
The testing sample or blackbody is placed in the vacuum low-background cabin. The spectral radiation from the blackbody radiation source or the sample is reflected by the plane mirror, then passes through three optical tubes cooled with liquid-nitrogen, before converging on the FTIR spectrometer after being reflected by the off-axis ellipsoidal mirror.
The properties of the surfaces defined by the scattering model, the custom material library, and the default material library used in the simulation are listed in Table 2. The properties of these materials are average empirical values whose waveband range from 1850cm-1 to 650 cm-1 (or 5.4 µm to 15.4 µm).
Table 2. Properties of the surfaces in the optical and mechanical systems.
Through backward ray tracing, the critical surfaces of the system can be determined, including the blackbody cavity (radiation source), the stop of the blackbody, the inner surfaces of the three optical tubes, the inner surfaces of the square cabin, and the stop in front of the off-axis ellipsoidal mirror (stop 1). The temperature and surface materials of the critical surfaces were set and simulated, respectively, as shown in Table 3.
Table 3. Temperature and emissivity of the critical surfaces.
The simulation waveband ranges from 1850cm-1 to 650 cm-1. One of the cross-section diagrams of VRTSF with ray tracing is shown in Fig. 3. Following the ray tracing simulation, the response of the MFPBB and the total response of the stray light was obtained. The statistical results are shown in Fig. 4. For the mercury fixed-point blackbody, the signal response remains invariant irrespective of cooling. From Fig. 4, the main sources of stray light are identified as the blackbody stop, the square cabin, and stop 1. It is evident that cooling can reduce the stray light significantly.
Fig. 4. Statistical results of simulation. (a)–(e) Total irradiance on the receiving surface (TIRS, unit: W) of different radiation sources, with and without cooling. (f) CVGI calculated using the data in (a)–(e).
It can be seen that cooling reduces the CVGI significantly. When the diameter of stop 1 is 90 mm, cooling reduces the CVGI from 85.6% to 49.9%, representing a reduction of 35.7%. As the diameter of stop 1 increases, the extent by which the CVGI is reduced decreases, for example, for diameter of 148 mm, cooling reduces the CVGI by 5.1%. In the case of non-cooling, the CVGI increases sharply as the stop size decreases, whereas for cooling, the change is small. The results suggest that the effect of cooling on the CVGI is significantly greater than that of the stop size.
4. Experimental results
4.1 Experimental determination of the VGI
The structure of the VRTSF is compact and is in a vacuum and low temperature state during measurement, which requires the instrument to have high stability. The CVGI on the VRTSF cannot be measured, because the changeable stray light is not separated. Therefore, a radiation model of the VRTSF is required. Based on the radiation model, the VGI in the experiments was measured. The original experimental data were obtained using the existing experimental equipment, and the VGI can be calculated via data processing.
For an ideal radiation model of the VRTSF, the detector only receives infrared radiation from standard blackbody radiation sources or samples. Therefore, for a standard blackbody source, the radiation received by FTIR is
In practice, the internal components of the system generate spontaneous infrared radiation and reflect or scatter the infrared radiation from other objects. Thus, the radiation received by FTIR is
The detector was cooled with liquid-nitrogen during the measurement to reduce noise interference as much as possible. The spectral radiance of the background noise in the system and spectrometer can be expressed as
Using Eqs. (1) and (7), the VGI associated with the measurement can be expressed as
As shown in Fig. 5, the theoretical spectral response of the mercury fixed-point blackbody is much greater than that of the liquid-nitrogen blackbody. For the experimental measurement results, the difference in the radiance response of the mercury fixed-point blackbody and the liquid-nitrogen blackbody can be regarded as the real response of the mercury fixed-point blackbody or the testing sample. The radiance response of the liquid-nitrogen acts as the total response of the total stray light.
According to Eq. (9), the measurement signal of the mercury fixed-point blackbody can be expressed as
4.2 Experimental device
The CVGI measurements were performed using the VRTSF. The system structure is shown in Fig. 2. The experimental measurement process was as follows. First, the system device was adjusted so that the blackbody or sample was in a vacuum state, and the optical path switching part was cooled by liquid-nitrogen. After achieving system stability, the plane mirror was rotated, and the signal was acquired using FTIR. By switching the mirror, different blackbodies were tested. The measurement order was: the vacuum-changeable medium-temperature blackbody, the liquid-nitrogen blackbody, the vacuum mercury fixed-point blackbody and the liquid-nitrogen blackbody again. Two groups of data were obtained in a cycle according to the measurement order. The FTIR scans were repeated 30 times for each measurement, and a total of five experiments were carried out. The differences in the setup of each experiment are summarized in Table 4.
Table 4. Experimental setup conditions.
4.3 Experimental results
Experimental data was collected in the 1850–650 cm-1 (or 5.4 µm to 15.4 µm) waveband. By calculating the VGI of the experiments, the spectral curve of the VGI can be obtained, as shown in Fig. 6.
It can be seen that the measured VGI of the system is related to the waveband. As the wavenumber decreases or the wavelength increases, the VGI increases. In addition, as the diameter of stop 1 increases, the VGI decreases. This indicates strongly that cooling is also a good way to reduce stray light.
5. Discussion
5.1 Comparison of simulation and experiments
To obtain the CVGI from the experiments, it is necessary to combine the simulated and experimental data. From Eqs. (3)–(5), supposing that the diameter of stop 1 is 148 mm, the average CVGI of the simulation and the experiment is equal. Then, the CVGI corresponding to other stop diameters can be obtained using the resulting calculations of G. The comparison between the experimental and simulated CVGIs and the average VGI of the experiment are shown in Fig. 7.
From Fig. 7, it can be seen that the simulated and experimental CVGIs increase as the stop diameter decreases. Both the simulation and the experiments clearly show that cooling reduces stray light. As shown in Fig. 7, the simulated CVGI is larger than that of the experiments without cooling. However, this trend is reversed when cooling is used. There are some possible reasons for this. First, though we adopt the properties of surfaces, the real properties in VRTSF system may be different. Second, the simulation is an ideal process, for which the cooling temperature is set to 77.15 K. In experiments, it may not be possible to cool to the temperature. Though Platinum resistance thermometers are set to monitor the temperatures and the temperatures are used to simulate stray light. There are still some key positions inner the system we can’t put thermometers, such as the stop 1 and blackbody stop. Third, the simulation didn’t consider the response of detector. These difference in simulation and experiment cause the errors. Although there are some differences between the simulation and experiments, they show good consistency with respect to the same variable changes. In Fig. 7, the variation in the average VGI of the experiment is smaller than that in the CVGI. It means that though the stray light was reduced, the VGI could not represent the changes. Therefore, the CVGI can present the stray light performance of the system clearly. Moreover, the CVGI represents the level of changeable stray light, which causes more errors than the fixed stray light. For optical systems that reduce stray light by background deduction, the CVGI is useful and can be used to estimate the stray light suppression.
5.2 Uncertainty of the CVGI in experiment and simulation
Several factors contribute to the combined standard uncertainty of simulation method and experimental method. In experimental method, the combined standard uncertainty can be expressed as
Table 5. Combined uncertainty of the CVGI in the experiment.
For simulation, the main sources of uncertainty are the properties of surfaces and the number of rays. The uncertainty results of simulation are shown in Table 6.
Table 6. Combined uncertainty of the CVGI in the simulation.
5.3 Stray light suppression method
The VRTSF system is a high-precision vacuum low-background calibration system and has been used to calibrate Chinese meteorological satellites for several years. Therefore, the analysis and suppression of stray light is important for upgrading the system.
Based on the CVGI, we can propose a stray light suppression method, which involves setting series of stops, the cooling, and the system coating. There are four stops set in the optical tubes. The optical tubes, stops, and square cabin are cooled with liquid-nitrogen. The surfaces of the stops were covered with high-emissivity paint. The location of the stops in the optical system is shown in Fig. 2. The sizes of the stops are 148 mm, 120 mm, 90 mm, and 60 mm, corresponding to the locations of stop 1, S2, S3, and S4, respectively. The simulated CVGI of the stray-light suppression method was 11.7%, as shown in Table 7. Therefore, the stray light is effectively reduced compared to the original system, which had a CVGI of 39.5%. By simulation, the VRTSF system has a lower CVGI with the stray light suppression method than that without this method.
Table 7. Simulation results of the stray light suppression method.
6. Conclusions
The VRTSF is a key device in infrared remote sensing calibration. The stray light in the VRTSF is nonnegligible and can reduce the signal-noise ratio and increase the uncertainty of the system. In previous work, we canceled out the stray light by subtracting the measured signal and liquid-nitrogen blackbody signal. However, in actual measurements, not all optical or mechanical components are cooled with liquid nitrogen, and the environmental conditions may vary. Therefore, in this study, the stray light was separated and analyzed.
Based on the real radiation model of the system, this paper analyzes the stray light sources of the VRTSF and proposes a new evaluation index, namely the CVGI. This evaluation index can represent the stray light that changes with environmental fluctuations. If the changeable stray light is suppressed efficiently, the stability of the system can be promoted. By performing simulations, the CVGI can be calculated for different stop diameters and temperatures. The same conditions were used for the experiments. A series of corresponding experiments were conducted using the VRTSF system. The experiment and the corresponding simulation show consistency in the CVGI results. Thus, we proposed a stray-light suppression method based on the CVGI, which can, theoretically, reduce the stray light compared to the original system.
The CVGI could be suitable for suppressing stray light in infrared systems, although complete suppression is economically unviable. Background subtraction is a common method widely used in infrared calibration. If the background is kept stable, accurate measurement can be ensured.
Funding
National Key Research and Development Program of China (2018YFB0504700, 2018YFB0504702); National Natural Science Foundation of China (12075229, 61871034).
Disclosures
The authors declare no conflicts of interest.
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