Realization of a photometric scale based on cryogenic radiometry

A standard of illuminance responsivity has been realized on a set of photopically-corrected detectors. It is based on the use of a cryogenic radiometer for the measurement of radiant power in terms of electrical standards and on a spectral responsivity scale established on silicon photodiodes in a "trap" configuration to evaluate the photopic response of the detectors. The spectral responsivity of the photometers was obtained from the trap detector by comparison using a spectrophotometer beam which underfilled both. The spatial responsivity of the photometers was found to be non-uniform by up to 1 % and this is a significant source of uncertainty in the realization. The measurements of aperture area used visual identification of the edges and were corrected for a bias in the visual technique that was demonstrated by checks with a similar-sized ring gauge robust enough for contact measurement.