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Optical reflection characteristic–based emissivity analysis of a pyramid array flat-plate blackbody for remote sensor calibration

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Abstract

The flat-plate blackbody (FPB) is the core device in infrared remote sensing radiometric calibration for providing accurate infrared radiation energy. The emissivity of an FPB is an important parameter that directly affects calibration accuracy. This paper uses a pyramid array structure based on the regulated optical reflection characteristics to analyze the FPB’s emissivity quantitatively. The analysis is accomplished by performing emissivity simulations based on the Monte Carlo method. The effects of specular reflection (SR), near-specular reflection (NSR), and diffuse reflection (DR) on the emissivity of an FPB with pyramid arrays are analyzed. In addition, various patterns of normal emissivity, small-angle directional emissivity, and emissivity uniformity are examined under different reflection characteristics. Further, the blackbodies with the NSR and DR are fabricated and tested experimentally. The experimental results show a good agreement with the corresponding simulation results. The emissivity of the FPB with the NSR can reach 0.996 in the 8–14 µm waveband. Finally, the emissivity uniformity of FPB samples at all tested positions and angles is better than 0.005 and 0.002, respectively. The standard uncertainty of experimental measurement of waveband emissivity and spectral emissivity are 0.47% and 0.38% respectively, and the simulation uncertainty is 0.10%.

© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

1. Introduction

Infrared remote sensing technology has a wide range of applications in many fields, including weather forecasting [1,2], national defense and military [3], climate monitoring [4], and disaster prevention and mitigation [5]. At present, the detection accuracy of infrared remote sensing loads is highly required. For example, to measure climate change, the sea surface temperature needs to meet the measurement accuracy of 0.1 K and stability of 0.04 K decade-1 [6]. The high quantization of infrared remote sensor puts forward new requirements for the calibration accuracy of ground calibration source [7,8]. The traditional cavity blackbody is already difficult to meet the minimum resolvable temperature difference of current large-aperture infrared remote sensing loads, and cannot meet the needs of high-precision calibrations [9]. A promising solution is to use a large-aperture flat-plate blackbody (FPB) for full-aperture laboratory calibration [1013], but the drawback of FPB is the relatively low emissivity. Now, Some FPBs made by HGH Infrared [14] Systems and Santa Barbara Infrared [15] have emissivity above 0.99. According to the constantly improving quantitative level, it is the current development trend to continuously improve the emissivity of FPB.

Emissivity is closely linked to calibration accuracy as a crucial calibration source parameter. However, emissivity can be enhanced by spraying high emissivity coating materials or using a machining array structure on the radiating surface of the blackbody [16]. Conventional coating materials scarcely that have been used in the aerospace field can meet the needs of high-precision measurements, but their emissivity is generally lower than 0.96 [17]. Kohei Mizuno et al. [18] increased the emissivity to 0.98–0.99 by generating smooth-surface carbon nanotube arrays. However, it has still been challenging to design carbon nanotube arrays with a uniform arrangement and high mechanical strength on the surface. Moreover, this defect makes the application to the radiating surface of an FPB difficult [19,20].

However, using an array structure on the radiating surface or constructing light traps at the bottom of the array structure can improve the absorption of light and enhance the blackbody emissivity. A blackbody developed by the Physikalisch-Technische Bundesanstalt uses an array of multiple independent pyramids [21]. In addition, constructing light traps at the bottom of a pyramid can reduce the aspect ratio of a microcavity structure [22]; while this scheme is feasible, the high aspect ratio of an independent structure and its combination structure can affect the calibration accuracy. Therefore, a better method could be to regulate the optical reflection characteristics of a radiating surface. Zhao et al. [23] demonstrated that a reflective surface with partial specular reflection could improve the emissivity of a blackbody cavity. Hao et al. [24] investigated the effect of optical properties of the surface on the emissivity of an on-board blackbody radiation source based on a ray trapping approach, which provided a new idea for designing high-emissivity blackbodies. The existing research on improving the blackbody emissivity has been mainly focused on cavity type blackbodies [2527] and V-groove structures [2830], while there has been little research on using the FPBs of radiating surface with pyramid arrays to achieve higher emissivity.

Considering the current demands for high emissivity, this paper adopts a pyramid array structure based on regulated optical characteristics to analyze blackbody emissivity. The optical transmission model of an FPB with a pyramid array structure is designed. The normal emissivity, directional emissivity with a small angle, and emissivity uniformity of an FPB are simulated under different reflection conditions. Finally, four FPB samples are fabricated, and their waveband emissivity and spectral emissivity are tested experimentally. The experimental results show good agreement with the simulated results.

2. Simulation principle and optical transmission model of FPB

2.1 Simulation principle

Monte Carlo method calculate the directional effective emissivity of blackbody cavities by means of ray tracing [31]. The light incident on the blackbody radiation surface interacts with the cavity wall numerous times due to the surface of the microcavity structure and high emissivity coating. Then, the light is absorbed by the radiation surface, and a small portion of it continues to reflect until the power of the reflected light is less than the minimum power threshold, at which point ray tracing is terminated. Finally, the energy of the light received from the radiation surface on the receiving surface is counted. The calculation model simulated by Monte Carlo methods is as follows:

$${P_{light}} = \sum\limits_{i = 1}^N {{P_i}} , $$
where Plight is the total power of the incident light, Pi is the power of the i ray and N is the total number of incident rays. After several interactions between the incident light and the radiation surface, a small part of the light escapes from the radiating surface. The power Preflect of the reflected light received by the receiving surface is:
$${P_{reflect}} = {\rho _e} \times {P_{light}}, $$
where ${\rho _e}$ is the effective reflectivity. Following Kirchhoff's law, when a blackbody is in thermal radiation equilibrium, the normal emissivity is numerically equal to the absorptivity, and the sum of absorptivity and reflectivity is 1 [32]. The normal emissivity of a blackbody is given by:
$${\varepsilon _e} = 1 - \frac{{{P_{reflect}}}}{{{P_{light}}}}. $$

Through the simulation of ray tracing method, the total power of the radiated light can be obtained, and the effective emissivity of the radiation can be obtained from Eq. (3).

2.2 Optical transmission model of FPB

The basic principle of an FPB based on optical reflection characteristic is shown in Fig. 1. In this model, a pyramid array structure is combined with the blackbody radiation surface to form a surface structure composed of multiple cavity structures. Under the joint effect of the blackbody cavity and optical properties of the regulated surface, the light is absorbed and reflected in multiple directions in the blackbody. The majority of the light is absorbed, and only a tiny piece of the light ultimately escapes from the mouth of the hole structure, ensuring high emissivity.

 figure: Fig. 1.

Fig. 1. The optical transmission model of the FPB based on optical reflection characteristic.

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In the model, the reflection of light can be divided into three main categories: specular reflection (SR), near-specular reflection (NSR) and diffuse reflection (DR). SR follows the law of reflection, and the angle of incidence is equal to the angle of reflection. The power distribution of NSR is usually in the form of Gaussian scattering. DR is Lambertian diffuse reflection.

The reflectance ${\rho _e}$ can be further expressed as follows:

$${\rho _e} = {\rho _{specular}} + {\rho _{nearspecular}} + {\rho _{diffuse}}, $$
where ${\rho _{specular}}$, ${\rho _{nearspecular}}$, and ${\rho _{diffuse}}$ are the specular reflectance, near specular reflectance, and diffuse reflectance, respectively [33].

The three reflected optical paths involved in Eq. (4) are related to different reflection types, according to which the parallel light uniformly incident from the blackbody top behaves differently. In the SR, the light is symmetrically distributed, reflecting the radiation surface, and the reflected light power is concave in the center but convex around the symmetrical distribution. In the NSR, the reflected light power poles are gently clustered toward the center. Lastly, in the DR, the closer the position is to the bottom of the cavity, the more difficult it is to reflect after repeated reflection and absorption; namely, the reflected light power poles are in the center, and the overall reflected power is distributed in a peak-like pattern. The optical path diagram and the reflected light received power distribution of the above three reflection types are shown in Fig. 2.

 figure: Fig. 2.

Fig. 2. Three types of reflective light paths and power distribution.

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3. Emissivity simulation of FPB

3.1 Emissivity simulation model

To evaluate the effect of reflective properties of a blackbody, two physical models of the blackbody, namely models in ideal and non-ideal states, are constructed, as shown in Fig. 3(a). In the ideal model (IM), the blackbody surface represents a perfect pyramid array; in contrast, in the non-ideal model (N-IM), micro-platforms are formed at the top and the bottom of the pyramid due to inaccuracies during machining and the buildup of the sprayed high emissivity coating. The length d of the micro-platforms is 0.5 mm. The simulation model size is 80 mm ×80 mm and the size of the pyramid is expressed by the base-height ratio:

$$f = D/H, $$
where f is the base-height ratio, D is the length of the bottom side of the pyramid, and H is the height of the pyramid. The coating emissivity of a radiant surface is denoted by ${\boldsymbol{\varepsilon}}$0. The detailed emissivity simulation is performed for different positions and angles of an FPB. As shown in Fig. 3(b) and Fig. 3(c), the simulation angles are θ = 0°, ± 1°, ± 3°, ± 5°. Four test regions of pyramids are selected: the bottom center, the bottom corner, the side, and the top. The software LightTools was used for the simulation. Figure 3(d) and Fig. 3(e) show the schematic diagrams of ray tracing at different positions and angles, respectively.

 figure: Fig. 3.

Fig. 3. Illustration of the emissivity simulation model and simulation process. (a) IM and N-IM model of pyramid arrays; (b) different test angles of FPB; (c) different test positions of FPB; (d) schematic diagrams of ray tracing at different positions; (e) schematic diagrams of ray tracing at different angles; (f) simulation framework flow.

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The basic steps of the simulation were as follows. First, The FPB model was constructed to set the size of the structure. Second, the light source and receiving surface were constructed, and their size and location were determined. Next, the number of rays involved in ray tracing, the total power of the rays, and the relative ray power threshold were set. After that, the surface reflection characteristics of the FPB model were set. Finally, the ray tracing was started, and the total power of the reflected rays received by the receiving surface was counted to calculate the emissivity of the FPB model. The simulation framework is shown in Fig. 3(f), and the parameters used in the simulation are listed in Table 1.

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Table 1. Emissivity simulation settings

3.2 Simulation results

3.2.1 Normal emissivity of FPB with different reflection characteristics

To explore the relationship between the reflection characteristics and the emissivity of the radiation surface, various combinations of SR, NSR, and DR at different ratios were analyzed. The coating emissivity versus the surface reflectance at different ratios is shown in Fig. 4. For the IM with a coating emissivity of 0.90, the emissivity at the DR-100% was 0.9693, and as the percentage of the NSR increased by 20%, the emissivity improved by approximately 0.3%. When the coating emissivity increased to 0.98, the NSR-100% could still improve the emissivity by 0.4% compared to the DR-100%, as shown in Fig. 4(a). The results of the N-IM in the comparison experiment are shown in Fig. 4(b). Under the reflection characteristics of NSR-100%, which enhanced the absorption of the cavity material, the emissivity was the highest. These results indicated that the surface reflection performances contributed greatly to a higher blackbody emissivity.

 figure: Fig. 4.

Fig. 4. Normal emissivity of the blackbody with different reflection characteristics. (a) IM simulation results; (b) N-IM simulation results.

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3.2.2. Normal emissivity at different positions of pyramid

As mentioned above, surface reflectance characteristics can improve blackbody emissivity. Thus, this study analyzed the emissivity and emissivity uniformity at different positions of a pyramid for three reflection types, and the emissivity uniformity was represented by the difference between the maximum and minimum. The results were shown in Figs. 5(a)–5(c). The settings of the three different surface characteristics are given in Table 2.

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Table 2. Surface settings of the simulation.

The results indicated that under the same surface settings, the smaller the f value was, the more obvious the blackbody cavity effect and the higher the emissivity when at the constant DR and NSR ratios were. For the case of f1 = 6:9-IM, compared to the emissivity uniformity under the DR-100% model, the uniformity was improved by 48.3% and 95.7% under the DR-50% and NSR-50% reflection and the NSR-100% reflection, respectively. In addition, the emissivity uniformity of the N-IM was improved to a certain extent, indicating that when the specular reflection on the surface optical properties was considered, the light reflection path within the radiation surface could be optimized, thereby increasing the absorption of the light.

The emissivity trends at different positions were the same for the three scaled surface properties for the IM. However, for the N-IM, the emissivity gradually increased from the bottom to the top of the pyramid under the NSR-100% reflection, as shown in Fig. 5(c). A comparison of the results obtained under the NSR-100% reflection and the DR-100% reflection indicated that when f1 = 6:9-N-IM, the increase in emissivity was mainly concentrated on the pyramid surface and the pyramid top, achieving the values of 3.4% and 5.6%, respectively. The pyramid bottom center showed a lower increase in emissivity. The light distribution at the top and the bottom center of the pyramid were analyzed from the perspective of light reflection. In this test, the light irradiated at the top, and the light on the surface of type I: DR-100% was reflected more easily out of the microcavity structure than that on the surface of type II:NSR-100%. However, the light on the surface of type II: NSR-100% was reflected at a specific angle and entered the bottom of the cavity showing multiple reflections and absorption before reflecting out of the microcavity, as shown in Fig. 5(d). As shown in Fig. 5(e), the light hit the pyramid bottom center, and the blackbody cavity effect was obvious, with the light reflecting irregularly between the type I cavities and adjacent surfaces. For type II, the light easily escaped from the microcavity structures after reflection.

 figure: Fig. 5.

Fig. 5. Normal emissivity at different positions of the FPB. (a) Emissivity at DR-100%; (b) emissivity at NSR-50% and DR-50%; (c) emissivity at NSR-100%; (d) schematic diagram of light tracing at the pyramid top under type I: DR-100% and type II: NSR-100%; (e) schematic diagram of light tracing at the pyramid bottom center under type I: DR-100% and type II: NSR-100%.

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Consequently, when the blackbody surface has a certain emissivity, the higher the proportion of the NSR and the lower the proportion of the DR are, the higher the normal emissivity and the better the uniformity of the normal emissivity will be.

3.2.3. Directional emissivity of FPB at small angles

Considering the uniformity, the simulation of small-angle directional emissivity was performed at the maximum emissivity value, and the obtained results are presented in Fig. 6. As shown in Fig. 6(a), at f1 = 6:9-N-IM, the emissivity decreased with the tilt angle. Namely, at a larger angle (${\beta}$ = 5°), the emissivity variation was 0.14%. In addition, from f1 to f2, the emissivity variation almost quadrupled, while the emissivity uniformity decreased. As the NSR ratio increased, the emissivity became more uniform. Figure 6(b) and Fig. 6(c) demonstrate that emissivity variation reduced at the NSR-50% and tended towards stability at the NSR-100%.

 figure: Fig. 6.

Fig. 6. Directional emissivity at different angles. (a) directional emissivity at DR-100%; (b) directional emissivity at NSR-50% and DR-50%; (c) directional emissivity at NSR-100%; (d) light scattering intensity results for DR-100%; (e) light scattering intensity results for NSR-100%.

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The light scattering intensity calculation was conducted to analyze variations in uniformity. Under the DR-100% mode, the light tilt angle changed from 0° to 5°, and the change in the distribution of scattered particles received at the receiver was larger, as shown in Fig. 6(d). Further, as presented in Fig. 6(e), the surface light absorption rate increased at a larger NSR ratio, reducing the light scattering intensity. However, as the tilt angle continued to increase, the variation in scattered particles on the receiver reduced, which proved that increasing the NSR ratio optimized the uniformity of the emissivity. The simulation results suggest that increasing the NSR ratio can improve emissivity and its uniformity in a small angle range. However, this can reduce the calibration error caused by a difference in the orientation angle between the calibration instrument and the blackbody.

4. Experimental results of FPB emissivity

4.1 FPB design and preparation

Two different surface-treated FPB samples, corresponding to two states of NSR and DR with different base-height ratios, were prepared to verify the simulation result. Blackbodies I and III were coated with high emissivity paint Nextel 811-21 [34] and the surface reflection was mainly DR. Blackbodies II and IV were oxidized and blackened and the surface reflection was approximately NSR. The sample size was 80 mm × 80 mm, and f1 and f2 meant the pyramid structure's base-height ratio. W was 7.5 mm away from the tops of adjacent pyramids. The size d of micro platform was 0.5 mm. The Pt100 platinum resistance thermometer hole was reserved at the bottom of the blackbody. The picture of the FPB samples is presented in Fig. 7.

 figure: Fig. 7.

Fig. 7. The picture of fabricated FPB samples.

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4.2 Emissivity measurement method

The experiment aimed to measure the blackbody emissivity using emissivity measurement devices for controlled environment radiation [35]. The principle of test was as follows.

The observed radiation signal of the sample includes both the sample’s own radiation signal and the environmental radiation signal reflected by the sample.

$${L_\lambda }(T )= {\varepsilon _{sample}}(\lambda ){L_p}({\lambda ,{T_{sample}}} )+ ({1 - {\varepsilon_{sample}}(\lambda )} ){L_\lambda }({{T_{bg}}} ), $$
where ${\boldsymbol{\varepsilon}}$sample, Tsample are the emissivity and temperature of the sample separately. Tbg denotes the ambient temperature. When the sample is in front of the hot disk, the hot disk produces the majority of the sample’s environmental radiation, and the environmental radiance can be expressed as:
$$\begin{array}{l} {L_\lambda }({{T_{bg}}} )= F{\varepsilon _{disk}}(\lambda ){L_P}({\lambda ,{T_{disk}}} )+ ({1 - {\varepsilon_{sample}}(\lambda )} )({1 - {\varepsilon_{disk}}(\lambda )} ){L_\lambda }({{T_{bg1}}} )\\ + ({1 - {\varepsilon_{sample}}(\lambda )} )({1 - F} ){L_\lambda }({{T_{bg}}} )\end{array}, $$
where F is the sample's radiation view factor of the disk, and Tdisk is the temperature of the hot disk. The temperature of the hot plate is set to Tdisk1 and Tdisk2, respectively, and the radiance of the sample at temperatures T1 and T2, L$_{\lambda}$ (T1) and L$_{\lambda}$ (T2), can be calculated by Sakuma-Hattori equation [36]. Following that, the emissivity of the sample can be obtained by
$${\varepsilon _{sample}} = \frac{{[{{L_\lambda }({{T_2}} )- {L_\lambda }({{T_1}} )} ]- F{\varepsilon _{disk}}[{{L_P}({\lambda ,{T_{disk2}}} )- {L_P}({\lambda ,{T_{disk1}}} )} ]}}{{[{{L_P}({{T_{sample2}}} )- {L_P}({{T_{sample1}}} )} ]- F{\varepsilon _{disk}}[{{L_P}({\lambda ,{T_{disk2}}} )- {L_P}({\lambda ,{T_{disk1}}} )} ]}}. $$

The measurement device is shown in Fig. 8. The emissivity measurement process is as follows:

  • (1) The sample was located in front of the normal temperature disk. By adjusting the adjustable sample table, the position of the sample was controlled, so that the center point of the radiation thermometer with the laser was located at a specific position.
  • (2) The temperature of the normal temperature disk was kept at 293 K and the hot disk was heated to 370 K.
  • (3) The emissivity of the standard sample was measured. After all components’ temperatures have stabilized, the temperatures of the normal temperature disk, the sample, the room, and the radiation thermometer will be recorded. The hot disk was then moved to the position where the normal temperature disk was, and the measurement was resumed.
  • (4) Compare the emissivity of the standard sample to ensure the stability of the device measurement. The blackbody to be tested was repeated 5 times to calculate the emissivity.
  • (5) By controlling the adjustable sample table, setting different angles and repeating the above steps, the directional emissivity can be measured.
  • (6) When measuring the spectral emissivity, the FTIR spectrometer was selected for the reception of the radiation signal.

 figure: Fig. 8.

Fig. 8. Emissivity measurement devices.

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4.3 Emissivity measurement results

4.3.1 Waveband emissivity measurement results

(I) Normal emissivity of FPB at different positions

The test results of four samples for the normal emissivity at various positions are presented in Fig. 9(a). For samples I and III with high-emissivity coating, the normal emissivity was the highest at the pyramid bottom center and the lowest at the pyramid top. The average normal emissivity values of samples I and III were 0.9896 and 0.9911, and their emissivity uniformity values were 0.0039 and 0.0046, respectively. However, the extreme positions of samples II and IV with the reflection characteristic regulation were opposite to those of samples I and III, with average normal emissivity values of 0.9939 and 0.9951 and emissivity uniformity values of 0.0033 and 0.0029, respectively. In general, the normal emissivity of samples showed large fluctuations between the pyramid top and the pyramid bottom. The normal emissivity of samples II and IV based on the reflection characteristic regulation was higher than that of samples I and III with high-emissivity coating. Moreover, samples II and IV optimized the uniformity of the normal emissivity.

 figure: Fig. 9.

Fig. 9. Test results of the emissivity of the FPB samples I–IV. (a) Test results of normal emissivity at different positions; (b) test results of directional emissivity at different inclination angles.

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(II) Directional emissivity of FPB at different angles

The emissivity of samples I–IV was measured in small-angle directions, including 1°, 3°, and 5° offsets to the left and 1°, 3°, and 5° offsets to the right, and the measurement position was selected at the point with the highest emissivity. The results are represented in Fig. 9(b). The directional emissivity at small angles differed by 0.0007 and 0.0013 for samples I and III, respectively. The standard deviation of the directional emissivity fluctuation of blackbodies II and IV in the small-angle range was under 0.02% and mostly the same. The changes of samples II and IV were smaller than samples I and III in the measurement results of directional emissivity, and the emissivity of all four blackbodies varies within 0.04%.

4.3.2 Spectral emissivity measurement results

The spectral emissivity of samples I–IV was measured in the wavelength range of 8–14 µm in the atmosphere, and the results are shown in Fig. 10. The results indicated that samples I and III, which were sprayed with a high-emissivity coating, had high emissivity and stability within the wavelength range, with an average emissivity of 0.9905 and 0.9917, respectively. Also, the standard deviation of these samples was better than 0.04%. The average emissivity of sample II and sample IV with reflection characteristic regulation is higher than 0.996, which has lower emissivity stability compared with sample I and sample III in the wavelength of 8–14 µm. As shown in Fig. 10(b) and Fig. 10(d), samples II and IV were stable in the wavelength range of 8–11 µm, achieving average emissivity values of 0.9962 and 0.9972 and standard deviation values of 0.042% and 0.039%, respectively. When the wavelength ranged from 11 to 14 µm, the emissivity showed a slow downward trend. The emissivity change in this waveband range was related to the emissivity of the radiant surface material, which was consistent with the conclusion of Monte and Hollandt [37]. Even after repeated absorption and reflection, it was difficult to improve the spectral emissivity of this spectrum segment. This phenomenon indicated that blackbodies II and IV had good spectral selectivity, high emissivity, and high stability only in a specific spectral range.

 figure: Fig. 10.

Fig. 10. Spectral emissivity results of samples I–IV.

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5. Discussion

5.1 Comparison of simulation and experiments

Considering the surface optical characteristic parameters of samples I–IV based on light absorption, the emissivity was simulated under the atmosphere. The simulation and experimental results were compared. The comparison results of samples I and II are shown in Table 3, and those of samples III and IV are shown in Table 4. The results show that the overall maximum difference between the experiment and simulation results was within 0.5%, which proved that the experimental results were almost the same as the simulation results. The surface of samples I and III was sprayed with a high-emissivity coating, and the surfaces were close to the DR type. Additionally, the surfaces of samples II and IV were polished and then oxidized and blackened. and their surfaces were close to the NSR type. The emissivity of the surface coating of samples I and III was higher than that of samples II and IV, but the oxidized blackened blackbody close to the NSR-100% (i.e., samples II and IV) had higher emissivity and better emissivity uniformity than the other type of blackbody. The results of the experiments and simulations of directional emissivity at small angles show that its changes were small with the increase of the angle.

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Table 3. Comparison between experimental and simulation results of blackbodies I and II.

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Table 4. Comparison between experimental and simulation results of blackbodies III and IV.

5.2 Uncertainty of the emissivity in experiment and simulation

Several factors contribute to the combined standard uncertainty of simulation method and experimental method. The combined standard uncertainty can be expressed as:

$${U_c} = \sqrt {u_1^2 + u_2^2 + u_3^2 + \ldots .u_8^2 + u_9^2}$$

The basic equation of the experimental method is shown in Eq. (8). The sources of uncertainty in experimental measurement of sample emissivity mainly include sample temperature measurement(u1), hot disk temperature measurement(u2), room temperature(u3), hot disk temperature uniformity(u4), hot disk emissivity(u5), radiation thermometer(u6), the calculated radiation view factor(u7) [35], measurement repeatability(u8) and long-term stability(u9).

There is a temperature difference between the surface temperature of the sample and the sample contact temperature, which will affect the accuracy of temperature measurement. Thus, the uncertainties of the blackbody contact temperature and the hot disk temperature measurement are evaluated as 30 mK. Room temperature changes at 0.5 K. The temperature uniformity of the hot disk affects the amount of background radiation, which is 1.56 K at 370 K. The disk emissivity is simulated using Steep 3 based on the Monte Carlo method with an error of less than 0.02 in the wavelength range 8-14 µm. The calibration uncertainty of radiation thermometer (TRT) is 0.1 K and the NEDT of the FTIR spectrometer is less than 15 mK, which will directly affect the measurement of the radiation signal. The radiation view factor is calculated by the concentric circle model of the blackbody and the hot disk, and the effect is within 0.02. The sample emissivity is measured several times and the repeatability is calculated. The long-term measurement stability of the device is better than 0.01% [38]. The components of the uncertainty of waveband emissivity are shown in Table 5, and the comprehensive standard uncertainty is 0.47% (k = 1).

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Table 5. Combined uncertainty of the waveband emissivity in the experiment.

The difference between the spectral emissivity uncertainty and the waveband emissivity uncertainty consists of two main parts: (1) The spectral emissivity is measured using the FTIR spectrometer for signal measurement while the band emissivity is measured using the radiation thermometer. (2) The repeatability of the emissivity measurements is inconsistent, and the repeatability of the emissivity measurements is plotted in Fig. 10. For the spectral emissivity, four wavelengths of 8 µm, 10 µm, 12 µm and 14 µm were selected for the uncertainty evaluation. The comprehensive standard uncertainty of spectral emissivity is 0.38% (k = 1), as shown in Table 6.

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Table 6. Combined uncertainty of the spectral emissivity in the experiment.

For simulation, the main sources of uncertainty are the emissivity of surface coatings (us)and the number of rays (uN). The uncertainty results of simulation are shown in Table 7. The combined standard uncertainty in the simulation is 0.1% (k = 1).

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Table 7. Combined uncertainty of the emissivity in the simulation.

6. Conclusion

This paper uses a pyramid array structure based on regulated optical characteristics to analyze blackbody emissivity. The simulation experiment based on the Monte Carlo method is performed. The results show that when the surface emissivity of the blackbody remains unchanged, the smaller the base-to-height ratio is, the higher the ratio of the NSR, normal emissivity of a blackbody, and emissivity uniformity in the small angle will be. An FPB with an oxidized black surface and mainly the NSR and an FPB coated with a high-emissivity paint and mainly the DR are fabricated and tested. Experimental and simulation results show that the FPB of the surface reflection component with NSR has a better light absorption ability, which has higher emissivity and excellent emissivity uniformity in the range of 8–14 µm. The results indicate that the emissivity can reach 0.996, and the uniformity of normal emissivity at different positions is better than 0.005; the uniformity of small-angle directional emissivity is 0.002. The standard uncertainty of experimental measurement of waveband emissivity and spectral emissivity are 0.47% and 0.38% respectively, and the simulation uncertainty is 0.10%. The proposed scheme can improve the measurement accuracy of the radiation value and can meet the requirement for high calibration precision. This work provides a new idea for improving the emissivity of FPBs used in remote sensor calibration and further promotes the improvement of infrared remote sensing calibration accuracy.

Funding

National Key Research and Development Program of China (2022YFF0610800, 2022YFF0610801); National Natural Science Foundation of China (12075229, 62031018).

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (10)

Fig. 1.
Fig. 1. The optical transmission model of the FPB based on optical reflection characteristic.
Fig. 2.
Fig. 2. Three types of reflective light paths and power distribution.
Fig. 3.
Fig. 3. Illustration of the emissivity simulation model and simulation process. (a) IM and N-IM model of pyramid arrays; (b) different test angles of FPB; (c) different test positions of FPB; (d) schematic diagrams of ray tracing at different positions; (e) schematic diagrams of ray tracing at different angles; (f) simulation framework flow.
Fig. 4.
Fig. 4. Normal emissivity of the blackbody with different reflection characteristics. (a) IM simulation results; (b) N-IM simulation results.
Fig. 5.
Fig. 5. Normal emissivity at different positions of the FPB. (a) Emissivity at DR-100%; (b) emissivity at NSR-50% and DR-50%; (c) emissivity at NSR-100%; (d) schematic diagram of light tracing at the pyramid top under type I: DR-100% and type II: NSR-100%; (e) schematic diagram of light tracing at the pyramid bottom center under type I: DR-100% and type II: NSR-100%.
Fig. 6.
Fig. 6. Directional emissivity at different angles. (a) directional emissivity at DR-100%; (b) directional emissivity at NSR-50% and DR-50%; (c) directional emissivity at NSR-100%; (d) light scattering intensity results for DR-100%; (e) light scattering intensity results for NSR-100%.
Fig. 7.
Fig. 7. The picture of fabricated FPB samples.
Fig. 8.
Fig. 8. Emissivity measurement devices.
Fig. 9.
Fig. 9. Test results of the emissivity of the FPB samples I–IV. (a) Test results of normal emissivity at different positions; (b) test results of directional emissivity at different inclination angles.
Fig. 10.
Fig. 10. Spectral emissivity results of samples I–IV.

Tables (7)

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Table 1. Emissivity simulation settings

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Table 2. Surface settings of the simulation.

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Table 3. Comparison between experimental and simulation results of blackbodies I and II.

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Table 4. Comparison between experimental and simulation results of blackbodies III and IV.

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Table 5. Combined uncertainty of the waveband emissivity in the experiment.

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Table 6. Combined uncertainty of the spectral emissivity in the experiment.

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Table 7. Combined uncertainty of the emissivity in the simulation.

Equations (9)

Equations on this page are rendered with MathJax. Learn more.

P l i g h t = i = 1 N P i ,
P r e f l e c t = ρ e × P l i g h t ,
ε e = 1 P r e f l e c t P l i g h t .
ρ e = ρ s p e c u l a r + ρ n e a r s p e c u l a r + ρ d i f f u s e ,
f = D / H ,
L λ ( T ) = ε s a m p l e ( λ ) L p ( λ , T s a m p l e ) + ( 1 ε s a m p l e ( λ ) ) L λ ( T b g ) ,
L λ ( T b g ) = F ε d i s k ( λ ) L P ( λ , T d i s k ) + ( 1 ε s a m p l e ( λ ) ) ( 1 ε d i s k ( λ ) ) L λ ( T b g 1 ) + ( 1 ε s a m p l e ( λ ) ) ( 1 F ) L λ ( T b g ) ,
ε s a m p l e = [ L λ ( T 2 ) L λ ( T 1 ) ] F ε d i s k [ L P ( λ , T d i s k 2 ) L P ( λ , T d i s k 1 ) ] [ L P ( T s a m p l e 2 ) L P ( T s a m p l e 1 ) ] F ε d i s k [ L P ( λ , T d i s k 2 ) L P ( λ , T d i s k 1 ) ] .
U c = u 1 2 + u 2 2 + u 3 2 + . u 8 2 + u 9 2
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