光谱辐射照度量值复现中计算大口径固定点黑体拐点温度的新方法

我国光谱辐射照度基准量值复现的理论基础是普朗克定律，它揭示了波长、温度和光谱辐射照度之间存在着精确的定量关系。采用比较法、通过卤钨灯来进行光谱辐射照度量值的保存与传递。其中，黑体的温度测量在量值复现中是最主要的不确定来源。以往都是采用变温黑体作为量值复现的辐射源，其温度通过高温计溯源至温标固定点黑体。为满足我国对地观测、气象遥感、应对气候变化、海洋水色等领域对光谱辐射照度高精度的测量需求，中国计量科学研究院NIM建立了一套14 mm大口径钨碳-碳WC-C高温固定点黑体系统，作为基准辐射源直接复现光谱辐射照度量值，进一步缩短了量值溯源链，减少了温度测量误差。实验中高温计用来测量黑体的相对温度，采用固定点熔化温坪曲线的拐点POI值校准后得到温度绝对值。因此拐点POI的合理评估计算就十分重要。与小口径WC-C固定点黑体不同，大口径固定点的熔化温坪曲线的持续时间更长，但温度起伏也更大，因此广泛用于小口径固定点的传统拐点POI算法不再适用。提出了一种可筛选多次拟合法，用于计算大口径固定点黑体的拐点温度。采用3 mm小口径WC-C和Re-C固定点对新方法的有效性进行验证，结果表明新方法与三种传统方法的平均值的最大偏差为-0.007和-0.001 K，在500 nm引入的光谱辐射照度测量误差分别为0.002 2%和0.000 3%。进一步采用14 mm大口径WC-C大口径固定点对新方法的稳健性进行验证。分析比较了筛选条件、数据平滑处理、拟合范围对拐点POI计算结果的影响，新方法和三种传统POI计算方法的最大影响量分别为0.001和0.633 K，在500 nm引入的光谱辐射照度不确定度分别为0.000 3%和0.20%。所提出的新方法能够有效减小外界因素引入的温度误差，进一步提升光谱辐射照度量值的复现准确度，更适用于大口径高温固定点黑体拐点温度的计算。

A New Method for Inflection Point Temperature Calculation of Large-Area High-Temperature Fixed-Point Blackbody Used in Spectral Irradiance Scale Realization

The theoretical basis of the national primary standard apparatus of spectral irradiance in China is Planck’s law revealing a real quantitative relationship between wavelength, temperature and spectral irradiance. The spectral irradiance comparison method is used to preserve and transfer the spectral irradiance standard by halogen tungsten lamp. Moreover the temperature measurement of a blackbody is the main source of uncertainty in the realization of spectral irradiance. For a long time in the past, a variable high-temperature blackbody was used as the primary radiation source for realization of spectral irradiance scale, and the temperature measurement of the blackbody was realized by a pyrometer traceable to the fixed-point temperature scale blackbodies of NIM. In order to meet the needs of high accuracy measurement of spectral irradiance in the fields of earth observation, meteorological remote sensing, climate change monitoring and ocean color detection in China, National Institute of Metrology (NIM) established a 14 mm diameter WC-C high-temperature fixed-point blackbody (HTFP) system, which was used as the primary radiation source to realize spectral irradiance scale directly. This method can shorten the traceability chain and reduce the temperature measurement error. In the experiment, the data obtained by a pyrometer are only the relative distribution of the blackbody temperature rather than the absolute value. In order to obtain the absolute temperature of a WC-C HTFP blackbody which can be used for the realization of spectral irradiance, it is necessary to use the point of inflection (POI) temperature of the melting temperature plateau curve for comparison calibration. So it is important to calculate and evaluate the POI temperature reasonably. Unlike a small-area WC-C HTFP, the melting temperature plateau curve of a large-area WC-C HTFP has a longer duration and has greater temperature variation, so traditional POI calculation methods, which are widely used in small-area HTFPs, are no longer applicable. So this paper proposed a selective multiple fit methods calculating the POI of a large-area WC-C HTFP with a 14 mm inner diameter. The influences of selective criterion, data smoothing and fitting range on the calculation results of POI were investigated. The maximum discrepancy between the new and traditional methods was 0.001 and 0.633 K, introducing 0.000 3% and 0.20% spectral irradiance measurement errors at 500 nm respectively. Using small-area WC-C and Re-C fixed-points with 3 mm inner diameters to investigate the validity of the new method. The results showed that the maximum discrepancy between the new method and the average value of the three traditional methods was -0.007 and -0.001 K, introducing 0.002 2% and 0.000 3% spectral irradiance measurement errors at 500 nm respectively. Compared with the three traditional methods, the new method can effectively reduce the temperature error and improve the realization accuracy of spectral irradiance. It is more suitable to calculate the POI temperature of a large-area WC-C HTFP blackbody.