A Radiometric Realization of the Photometric Units

Abstract

A description is given of the experimental procedures used to transfer the photometric scales of the National Measurement Laboratory to a radiometric base. Tungsten halogen lamps and gas-filled tungsten-filament lamps, operated at the distribution temperature 2856 K, are calibrated as standards of illuminance by using an electrically calibrated radiometer to measure the irradiance when a filter simulating the V (λ) function is interposed between the lamp and radiometer. A value of 682 lm W^-1 has been adopted for K_m, the maximum spectral luminous efficiency of radiation, because the measurements have shown that this value gives continuity with the Laboratory's previous scale of luminous intensity. It is expected that the photometric base unit will eventually be redefined so as to provide an exactly known, internationally agreed value for K_m.