What is the Half Gain Angle?

Half gain is the standard used to measure the brightness performance of a projection screen when the viewer is looking at the screen from an angle. A projection screen’s peak brightness occurs when the viewer is perpendicular to the center of the screen. As the viewer moves to the side, away from the center of the screen, the brightness of the image will decline. When the brightness drop off reaches 50% of peak gain, that measurement is called the half gain.
 
Half gain is how we define the viewing cone. Half gain is expressed in degrees of arc from the centerline of an image. This is a key consideration in screen fabric selection. The half gain measurement is commonly used to describe the limits in a planned viewing area where excellent performance can be expected.

The wider the viewing cone, often the more susceptible to loss in contrast due to ambient light influence on black levels. When projector power is limited, the use of an elevated gain surface may have a consequence in limiting the viewing cone.