1. The branch of science and technology that (a) is devoted to the measurement of electromagnetic radiation in the infrared, visible, and ultraviolet regions, i.e., the optical spectrum, of the electromagnetic spectrum and (b) includes the science and technology devoted to light sources, photodetectors, and the effects of radiation on materials. Note: Although photometry was originally devoted to the effects of electromagnetic radiation on the human eye, the area of interest has been expanded to include the science and technology devoted to (a) light sources, such as fiber optic light sources, (b) photodetectors, such as fiber optic photodetectors, and (c) the effects of radiation on materials. 2. The branch of science and technology that (a) is devoted to the measurement of the effects of electromagnetic radiation on the eye, (b) is an outgrowth of the psychophysical aspects induced by the radiation, and (c) includes the determination of visual effectiveness by considering response to various levels of incident radiation, i.e., irradiance, and the sensitivity of the eye to various frequencies, i.e., various wavelengths. See also effect,  electromagnetic radiation,  fiber optic light source,  fiber optic photodetector,  incidence,  incident,  infrared,  irradiance,  level,  light source,  optical spectrum,  photodetector,  radiation,  radiometry,  response,  sensitivity,  ultraviolet,  visible spectrum,  visual,  wavelength.