Also known as amplification. 1
. The increase in a signal that is transmitted from one point to another through an amplifier. A material that exhibits gain rather than absorption, at certain frequencies for a signal passing through it, is known as an active medium. 2
. With reference to optical properties, the term may be defined in two ways: a. the relative brightness of a rear projection screen as compared with a perfect lambertian reflective diffuser; b. the ratio of brightness in footlamberts to incident illumination in footcandles. 3
. In a photodetector, the ratio of electron-hole pairs generated per incident photon.