Full Title

Photometry, e.g. photographic exposure meter (spectrophotometry G01J3/00; specially adapted for radiation pyrometry G01J5/00 ) > by comparison with reference light or electric value > intensity of the measured or reference value being varied to equalise their effects at the detectors, e.g. by varying incidence angle > using separate light paths used alternately or sequentially, e.g. flicker > using electric radiation detectors