Primary Standard Cryogenic Radiometer The basis for the traceability of all optical radiation standards, assuring accuracy into the hands of the end user - the customer. NPL's cryogenic radiometer is the primary standard for the measurement of optical radiant power. It uses the electrical substitution technique, whereby the optical power incident on an absorbing cavity is compared with the electrical power required to heat the cavity to the same temperature. High Tc superconducting leads to the cavity heater ensure true equivalence of electrical and optical power. The cavity is made of electroformed copper to reduce its mass and is coated internally with NPL super black. The design of the cavity in combination with operation at helium temperatures ensures an optimal response to incoming ra

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