KEYWORDS: Emissivity, Black bodies, Monte Carlo methods, Reflection, Optical surfaces, Coating, Specular reflections, Infrared radiation, Ray tracing, Remote sensing
As one of the commonly used blackbody types for infrared radiation calibration, the emissivity level of surface blackbody is limited by the surface coating and surface microstructure array. To study the emissivity of surface coatings and the influence of surface microstructure on the emissivity of surface blackbodies, we establish optical and mechanical models of four different array concave cavity surface blackbodies, including triangular concave cavity, quadrangular concave cavity, pentagonal concave cavity, and hexagonal concave cavity arrays. Based on the Monte Carlo ray tracing method, simulation calculations of blackbody emissivity are carried out, with a focus on analyzing the effects of array concave cavity structure type, optical properties of blackbody surface coatings, and height of concave cavity structure on the emissivity of surface source blackbodies. The simulation results show that, under the condition of 100% diffuse reflection coating on the surface and constant concave cavity edge length, for the four types of concave cavity arrays, an increase in coating emissivity or array height can promote the emissivity of the blackbody. When the length of the concave cavity remains constant and the surface emissivity remains constant, the higher the proportion of near specular reflection, the higher the normal emissivity. When the surface optical properties are the same and the array height is the same, for the four structures, the triangular concave cavity structure surface blackbody has a higher emissivity. The simulation results can provide theoretical basis and data support for the design of high emissivity surface blackbody for infrared radiation calibration.
KEYWORDS: Emissivity, Black bodies, Coating, Optical microcavities, Calibration, Reflection, Infrared sensors, Infrared radiation, Monte Carlo methods, Ray tracing
The surface-source blackbody is widely used in infrared remote sensing and other fields as a calibration source to calibrate infrared loads by providing accurate infrared radiation, and the effective emissivity of the surface-source blackbody is one of the important parameters for the calibration of infrared devices, which directly determines the accuracy of calibration. In this paper, two microarray structure models, ideal and non-ideal cases, are established around the effective emissivity of the surface-source blackbody radiating surface, and simulation experiments based on Monte-Carlo method (MCM)are carried out. The effective emissivity of the blackbody radiation source is analyzed to be influenced by the geometrical parameters of the microcavity structure, the emissivity of the surface coating and different angles. The effective emissivity increases with the decrease of the base height ratio of the pyramidal microcavity structure, and the emissivity of the surface coating material is linearly increasing, and the linearity of the two models is better than 0.9997. The effective emissivity of the ideal and non-ideal models has a small trend in the vertical direction and horizon direction angle 0-±10°, and the standard deviation is less than 0.15%, which has high stability. The results of the study can provide theoretical guidance for the enhancement of the effective emissivity of the surface-source blackbody.
It is crucial to obtain infrared emission coating materials with emissivity close to one unit for remote sensing temperature measurement, earth climate monitoring and other fields. In order to obtain large-area uniform high emissivity coating, multi-layer spraying and multi-walled carbon nanotubes doping were used to enhance the emissivity of commercial black paint NVC811-21 from 8μm to 14μm. Compared with the coating directly sprayed with NVC811-21, the emissivity of the coating in the normal band from 8μm to 14μm in the atmosphere can be increased from 0.965 to 0.978. In addition, from the spectral emissivity of the coating in this band, the doping of carbon nanotubes can improve the emissivity near 10μm of the coating. For NVC811-21, this improves the uniformity of the spectral emissivity of the coating material. Due to the application of NVC811-21 in the actual service of the on-orbit blackbody, this paper's preparation method may help improve the emissivity of the on-orbit blackbody and the measurement accuracy of satellite remote sensors.
Radiation temperature measurement is a non-contact temperature measurement, which has important applications in quantitative remote sensing, industrial thermal monitoring, biomedical engineering and military field. The infrared radiation of an object is directly proportional to its emissivity, which is an important parameter that affects radiation temperature measurement. In order to obtain the spectral emissivity of an object, this paper proposes a method for measuring spectral emissivity based on the radiation at multiple temperatures. Based on Planck's law of radiation, the expression of spectral emissivity is theoretically given by deriving the relationship between spectral emissivity, contact temperature and radiation. The simulation is carried out based on theoretical derivation. The spectral emissivity of three samples is simulated. The waveband of the samples is 8-14μm, and the spectral emissivity does not change with temperature. Two algorithms are used to avoid the problem of singular values in direct calculation. Based on the constrained linear least-squares method, the average relative errors of the three samples are 7.0%, 7.2%, and 6.2%. The maximum relative errors are 22.1%, 18.9% and 15.0%. Based on the improved constrained linear least-squares method, the average relative errors of the three samples are 2.2%, 1.1%, and 3.0%, and the maximum relative errors are 6.7%, 3.2%, and 4.2%. The simulation results verify the feasibility of inversion of spectral emissivity at multiple temperatures. The results show that the improved constrained linear least-squares method has smaller average relative errors.
Field radiometric calibration is an important technical means to ensure the observation accuracy of satellite remote sensors on-orbit operation. Accurately obtaining the spectral radiometric characteristics of the site is of great significance to improve the accuracy of field radiometric calibration. In order to better meet the field calibration requirements of thermal infrared band satellite remote sensors, a Multi-channel Self-calibration Infrared Radiation Thermometer (MSIRT) with automatic observation capabilities has been developed, which is used to measure the atmospheric downward radiance and ground surface radiance in field. In this paper, the structure, working principle and laboratory calibration of the MSIRT was introduced. In optical design, the automatic switching of four spectral channels in the range of 8 ~ 14μm was realized by using the filter wheel, and the instrument was equipped with an ambient temperature blackbody and a heated blackbody, respectively. The real-time calibration of the detector can be realized through the alternate measurement of high and low temperature blackbody, which ensures the high precision measurement requirements. The developed instrument also has the ability of automatic observation and remote data transmission. In the laboratory calibration of MSIRT, the liquid bath blackbody was used as the standard radiation source, and the calibration uncertainty was 0.5 %, corresponding uncertainty of the equivalent radiance temperature was 0.3 K (@ 303 K, 11 μm). The experimental results show that the MSIRT can meet the measurement requirements of high precision, and has important applications in the field calibration of thermal infrared satellite remote sensor.
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