Abstract: A method and apparatus for measuring the frequency of radiation is described comprising a pair of filters (2,3) or a beam splitter (30) responsive to radiation from a source of radiation (4) for providing a first output having a magnitude which increases and a second output having a magnitude which decreases with an increase in the frequency of said radiation, a divider (11) for providing a ratio of the magnitudes of said first and second outputs and a comparator (14) and memory (17) responsive to the ratio from said divider (11) for providing an output corresponding to the frequency of said radiation.

Abstract: An apparatus for testing the optical output of light-emitting devices in the presence of ambient light, with means for compensating for said ambient light, is described. The d.c. and periodic components of the ambient light are measured and stored during a time when the apparatus is not being subjected to radiation from the device under test (DUT). When radiation from the DUT is received, the resulting signal is processed with the stored ambient light intensity data, and an output proportional to the intensity of the radiation from the DUT and compensated for the ambient light is provided.