= Cryogenic radiometer
A <cryogenic radiometer> uses low-<temperature> operation to improve <electrical substitution radiometry> through particular material and thermal properties. Lower <heat capacity> of a copper receiver permits a substantial cavity without proportionately slow relaxation; cold shields reduce unwanted <thermal radiation>, and heater leads with <superconductivity> can suppress resistive lead heating. These features do not make <radiometric absorptance> exactly one or abolish <optical-electrical heating non-equivalence>.<Spectroradiometric Detector Measurements: Ultraviolet, Visible, and Near-Infrared Detectors for Spectral Power by Thomas C. Larason and others>[§5.2, printed pp. 30–32: cryogenic advantages and remaining window/non-equivalence corrections]{ref}
Cold surroundings matter as much as a cold receiver. In a simplified gray-surface model, net outward <thermal radiation> is $\epsilon\sigma A(T^4-T_s^4)$, using <emissivity> $\epsilon$, <area> $A$, shield <temperature> $T_s$ and the <Stefan-Boltzmann constant> $\sigma$. Cold shields reduce the incoming term as well as the receiver's emission; cooling the receiver alone while leaving it exposed to warm surroundings does not remove incoming radiation. An entrance window still provides an optical and thermal path. <Vacuum> reduces gas-mediated <heat transfer>, while an intentional <radiometer thermal link> remains necessary. This model explains specific losses, rather than promising a universal improvement factor or perfect accuracy.
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