Half-gain is another important measurement, which tells you how much the screens "diffusion-particles" divert the light, when the image is watch from an angle to the screen. If you have a screen with peak-gain 3 (on axis), and you move away from axis, the quantity of light in the image will decrease. This can be measured with a photometer (lux-meter). When you move away from axis, you at a certain angle will reach gain 1,5, which will be the half-gain-angle for the screen.

Gain-curve illustrate how much light is transmitted/reflected by different viewing-angles of the screen going from centre(peak) to the "outskirt" (90°) and is measured with a photo-meter.

An example of af gain-curve for a diffusion-screen with peak-gain 3 can be: