低温辐射计黑体腔吸收率分析

Absorptance Analysis of Blackbody Cavity in Cryogenic Radiometer

低温辐射计是目前国际上光辐射功率计量中准确度最高的测量系统,其光辐射测量不确定度可达10-5量级,目前国内仅有少数研究机构从国外引进低温辐射计开展计量研究,亟待发展国产低温辐射计替代进口产品。由于低温辐射计采用低温超导状态下的电替代测量原理进行光辐射功率测量,发展低温辐射计的难点之一在于研制黑体吸收腔这一核心光辐射接收器件,并要求黑体腔在各波长下的吸收率都要达到0.9999以上。为研制超高光谱吸收率的黑体吸收腔,系统性分析了各影响黑体腔光谱吸收率因素,在此基础上利用蒙特卡罗光线追迹方法重点研究了光谱波长、腔体长度、黑材料漫反射系数、黑材料吸收率和入射光空间位置等对斜底黑体腔光谱吸收率的影响。研究结果表明:在300~1100nm波长范围内黑体腔吸收率与其内壁涂黑材料的吸收率呈正相关,且在300~1000nm范围内的吸收率都达到了0.9999以上,其中在700nm处的吸收率取得最大值0.9999415,表明采用该类型黑材料的黑体腔只在300~1000nm范围内满足低温辐射计设计要求,后续需要根据仿真和测试结果对低温辐射计在不同波长下的光电不等效性进行修正;在黑体腔结构和口径确定的情况下,黑体腔吸收率将随腔长增加而逐渐升高,在40mm后变化趋缓,并在65mm后逐渐趋于平衡,考虑到低温辐射计低温舱对腔体尺寸的限制,认为腔体长度与口径之比为6.5时较为合适;黑体腔吸收率还受黑材料的漫反射系数影响,随着黑材料漫反射系数的提高,腔体吸收率呈现近似线性下降,所以在选择黑体腔涂黑材料时,在吸收率等指标相同的情况下应尽量选择镜面吸收黑;黑材料吸收率从0.8到1的变化过程中,腔体吸收率提升了0.05个百分点,且黑材料吸收率为0.92时腔体吸收率可达到0.9999以上,表明黑材料在其有效工作波长范围内任一点的光谱吸收率都要大于0.92;腔体吸收率还受入射光投射的空间位置影响,光线位置越靠近斜底腔顶点处,腔体吸收率越高,但整体吸收率变化不明显,光线位置对腔体吸收率影响只有不到0.004个百分点,几乎可以忽略,认为斜底腔不同位置处的吸收率是均匀的。研究结果对低温辐射计黑体腔研制有一定参考价值。

Cryogenic radiometer is currently the world’s most accurate measurement system in optical radiation power metrology, and the uncertainty of measurement is up to on the order of 10 to the-5 th. At present, there are only a few research institutes in the country which have imported cryogenic radiometers from abroad to carry out metrological study. A domestically-made cryogenic radiometer is urgently developed to replace imported products. Because the electrical substitution measurement principle in low-temperature superconducting state is used for optical radiation power measurements, one of the difficulties in the development of cryogenic radiometers lies in the development of a core light radiation receiving device-blackbody absorption cavity. The cavity requires a high spectral absorption over 0.999 9 at each wavelength. For the development of blackbody cavity with high absorption rate, the factors affecting the absorption rate of the blackbody cavity are given systematic analysis in this study. Based on the above, the influences of spectral wavelength, cavity length, diffuse reflectance of dark material, absorption rate of dark material and spatial position of incident light are mainly researched by Monte Carlo ray tracing method. The results showed that the absorption rate of blackbody cavity is positively associated with the absorption rate of dark material. The cavity has a high absorption rate over 0.9999 in the range of 300 nm-1000 nm, and gets the maximum value 0.999 941 5 at 700 nm. Above results indicate that the blackbody cavity meets the design requirements of cryogenic radiometer in the range of 300~1 000 nm. In the later design, the photo-electrical non-equivalent of low-temperature radiometer at different wavelengths should be corrected based on simulation and test results. If the structure and aperture are fixed, the absorption rate of blackbody cavity will gradually increase as the cavity length increases, which changes slowly at 40 mm and gradually tends to balance after 65 mm. Considering the limitation cryogenic radiometer’s cryogenic cabin, it is considered that the best ratio of the cavity length to the aperture is 6.5. The absorption rate of blackbody cavity is also affected by the diffuse reflectance of dark material. With the increase of the diffuse reflection of the black material, the absorption rate of cavity shows an approximately linear decrease. As a result, it is best to choose mirror absorption black material, when the absorption rate is the same. During the change of the absorption rate of black material from 0.8 to 1, the absorption rate of cavity increases by 0.05%. When the black material absorption rate is 0.92, the cavity absorption rate can reach over 0.999 9, which indicates that the black material should have a spectral absorbance over than 0.92 at any point within its effective operating wavelength range. The absorption rate of cavity is also influenced by the spatial position of incident light, which will get higher when the light is closer to the vertex of the inclined bottom cavity. However, the influence of light position on the absorption rate of the cavity is less than 0.004%, which is almost negligible. It is considered that the absorption rate of the inclined bottom cavity is uniform. The research results will have certain reference value for the development of cryogenic radiometer blackbody cavity.