UV radiometers are used in many areas. There are many kinds of UV light sources with different peak wavelength and different wavelength range. The broadband UV radiometers are wildly used due to easy to use and low cost. However, there are some obvious disadvantages for the broadband radiometers. They cannot distinguish the spectral characteristics of UV sources. That will cause the spectral mismatch measurement error for the UV broadband radiometers calibration. Recently, the fiber spectroradiometer plays a more and more important role in this area. The fiber spectroradiometer is more portable and low cost compared to the double grating spectroradiometer. We can obtain the spectral characteristics and any UV irradiance using the fiber spectroradiometer. However, for most fiber spectroradiometers, we cannot use them to replace the UV broadband radiometers for the absolute irradiance measurement. There are four key effects for that. The first one is the stray light. Stray light effect is obvious for the fiber spectroradiometer, especially in the UV wavelength range. The second one is the temperature effect. The third one is the non-linearity effect. The fourth one is the bandwidth effect. This effect will cause the measurement error for the spectral distribution of the UV source. In this paper, we research the four factors that reduce the measurement accuracy of the fiber spectroradiometer in UV wavelength range.
In order to achieve the goal of spectral radiance (SR) and spectral irradiance (SI) calibration with an uncertainty of less than 1.0%, NIM set up a MC-C large-area high-temperature fixed point blackbody(HTFP BB) as the new generation of SR and SI reference source, which is composed of large WC-C fixed point cell and BB3500MP. It can be used directly for the SR and SI realization, thereby further reducing measurement uncertainty. The HTFP BB with WC-C fixed point has excellent stability, reproducibility and repeatability in the high temperature range of more than 3020K, which greatly improves the detection capability in the ultraviolet range (UV). Accurate measurement of melting temperature of HTFP BB is an important source of error, which is determined as the point of inflection (POI) of melting plateau. At the same time, POI is also an important reference point used in comparison experiments. Different calculation methods introduce different degrees of errors, which are critical. This paper studied the current three POI numerical calculation methods, namely “differential+second-order fitting” (DSF), “third-order direct fitting” (TDF), “histogram+Gaussian fitting” (HGF). The numerical calculation of the POI was performed on the data of WC-C14 and Re-C measured at NIM and WC-C14 and WC-C10 measured at VNIIOFI by the above three methods. Combining the fitting results and correlation coefficients to explore the characteristics of each method. Based on the experimental results, a more reasonable calculation method is proposed to reduce the calculation error of the POI of the previous data to less than 5 mK.
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