Video and displays
Screen gain
The ratio of light a projection screen reflects toward the viewer compared with a reference matte white surface, with 1.0 as unity.
Screen gain compares a screen's on-axis brightness with a perfectly diffusing white reference. A gain of 1.0 reflects evenly in all directions; 1.3 sends more light back toward the center of the room; 0.8 absorbs some light, usually to deepen blacks.
Higher gain buys brightness without a bigger projector, but the extra light comes from a narrower reflection cone, so side seats get a dimmer picture and the center can hot-spot. Grey low-gain screens trade brightness for contrast in rooms with some ambient light.
The half-gain angle is where brightness falls to half the on-axis value. Gains of 1.0 to 1.3 keep a cone wide enough for a row of seats; above about 1.5 it tightens noticeably. Gain multiplies directly into the foot-lambert estimate.