High accuracy of radiometric calibration is a prerequisite and guarantee for quantitative remote sensing. The traditional calibration method based on radiation source could not meet the requirement for further application of remote sensing. Cryogenic radiometer is internationally recognized as a primary standard of light radiation with the highest accuracy, thus the radiation standard transfer chain which could be traced to cryogenic radiometer laid the foundation for quantitative remote sensing.
It has been demonstrated that the reflectance and the scattering effect of Brewster window plays a significant role in characterizing the absolute power of laser. As to the successive calibration in wide spectrum, the wavelength of the output beam from the tunable laser varies as the polarization state changes, thus it is essential to accurately measure the polarization state and carry out repeated adjusting in different wavelengths. However, the workload of this traditional experimental scheme is too large, and meanwhile the uncertainty increases due to the low repeatability of the optical path.
Researchers led by Prof. Xiaobin Zheng, from Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences designed and made use of a novel calibration optical path with the standard transfer detector which was placed inside the vacuum unit. This scheme theoretically eliminated the uncertainty caused by the window transmittance. It is reported in Chinese Optics Letters Vol.13, No.5, 2015.
In this study, a fine instrument with automatic reset technology was used to increase the accuracy of the cryogenic radiometer and standard transfer detector switching into the optical path, therefore, the uncertainty caused by spatial non-uniformity decreased. Two calibration optical paths, covering visible and infrared spectrum respectively, have been established, in which beam shaping, spatial filtering power stabling, and stray light suppression technology were put into use.
The novel optical scheme based on cryogenic radiometer for successive calibration in broad spectrum was introduced, which could efficiently reduce the window transmittance of cryogenic radiometer and therefore be used as a new calibration technique in this spectrum. At the meantime, the feasibility of cryogenic radiometer was improved. With the uncertainty being better than 10-4, this research result becomes a powerful support for the quantitative traceability of radiation standard.
Dr. Weiwei Pang, the first author of this article, believes that the improvement of the accuracy of the radiation standard and the standard transfer system, reduction of the standard transfer chain and the final enhancement of application level of radiation calibration for remote sensor would be urgent subjects worldwide.
As a matter of fact, the successive calibration experiments in ultraviolet-intermediate infrared band has not been carried out, especially those in infrared spectrum. In the next step, the research team would focus on the calibration process in the above band. Currently, a tunable laser has been equipped in their lab which covers the wavelength 350 nm~4.7 μm. Through the comparison of the cryogenic radiometer and synchrotron radiation source, the validation of the two methods to each other would be accomplished, which loads the foundation for a new technology of high precision radiation standard and transfer detector in ultraviolet band.
中国科学院安徽光学精密机械研究所的郑小兵研究员课题组设计和应用新型的真空仓内传递探测器定标光路，从原理上消除窗口透过率测量这一最大不确定性因素。相关实验结果发表在Chinese Optics Letters 2015年第5期上。
鉴于紫外~中红外谱段连续光谱定标、特别是在红外波段相对于低温辐射计的连续谱段定标研究工作尚未开展，未来这支科研团队将进行该谱段的连续光谱定标实验。目前，该实验室已配置波长覆盖范围约350 nm~4.7 μm的可调谐激光器。通过开展低温辐射计和同步辐射源之间的比对实验研究，实现两种不同工作体制辐射基准的相互验证，为紫外谱段高精度辐射计基准和传递探测器探索新的技术途径。