Absolute cryogenic radiometer is built based on a new theory of electrical-substitution measurement, which is for measuring the radiant power by using the equivalent electrical power and has recently served as a primary standard for radiant power measurements. This study aims to design and implement a cryogenic radiometers to measure the optical power in a range from <inline-formula><tex-math id="M1">\begin{document}$0.1\;{\text{μ}}{\rm{W}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.png"/></alternatives></inline-formula> to 2 mW, which can substitute for the imported products. Intensive experiments are performed to study the thermal circuit of cryogenic radiometer, and systematically analyze the influences of cavity assembly and heat link materials on the responsivity and thermal time constant of cryogenic radiometer. On this basis, the thermo-structure mechanical parts are developed, which are comprised of a blackbody cavity, heat link and heat sink. Both the heat sink and the blackbody cavity are made of OFHC copper that is plated with gold. All surfaces are highly polished and reflective to reduce any radiative effects. The absorptance of the cavity can reach up to 0.999995 at 633 nm. And then, a characteristic parameters’ test system of cryogenic radiometer is built. Through optimizing the temperature control system and improving the design of the heat sink, the standard deviation of the heat sink can be kept under 0.2 mK for 30 min. By using that test system, the responsivity and thermal time constant of cryogenic radiometer with four different kinds of heat link materials (OHFC copper, 6061 Al, SS304 stainless steel, and polyimide) are tested experimentally. The experimental results show that the responsivity and thermal time constant are 35.5 K/W and 23 s for OHFC copper, 318.9 K/W and 106 s for 6061 Al, 434.8 K/W and 297 s for SS304 stainless steel, 714.8 K/W and 506 s for polyimide. As the thermal conductivity of heat link material changes, the two parameters of responsivity and thermal time constant will simultaneously change significantly. The responsivity and thermal time constant are a pair of mutually constrained parameters, and temperature stability is an important parameter for designing the thermo-structure. After increasing the responsivity, it will not only significantly increase the measurement time and resource consumption, but also affect the temperature control stability, and hence limiting the measurement accuracy. All the test data indicate that the characteristic parameter of cryogenic radiometer can be adjusted by changing the material and structure of heat link. The obtained results will have a certain reference value for the index distribution of cryogenic radiometer characteristic parameters and designing the next generation of absolute cryogenic radiometers.
