The reproduction quality of projection surfaces or screens is determined by the reflection behavior of the projection screen, its luminous efficacy and the viewing angle. The characteristic value for the reflection behavior is the gain factor or luminance factor. It is a dimensionless value that reflects the reproduction properties of front and rear projections and can be used as a selection criterion for the various projection surfaces.
The gain factor is the ratio of projected light falling on the projection screen to the light reflected from it. It is a numerical value from which the reflectance behavior with respect to a reference surface can be seen. As a reference, the luminance factor uses a flat standard white projection surface. For curved surfaces, the gain value changes because the reflected projection light is brighter at certain viewing angles and darker at others. In general, it depends on the viewing angle, but is specified independently of it. This means that the gain factor also says something about the light distribution. In addition to the luminance factor, there is another index that takes into account the projection direction of the projection surface: the luminance factor indicator matrix. This is a diagram showing the gain factor as a function of the radiation direction.
As far as the gain factors are concerned, at small gain factors between 0.7 and 1.1, the light from the projection screen is reflected approximately equally in all directions, as in the case of the diffuse projection screen. The viewing angle is correspondingly large. With higher luminance factors between 1.5 and 2.0, which corresponds to light amplification by light bundling, the reflected light is concentrated in a main viewing axis. The viewing angle is relatively narrow, and a strong reduction in brightness can be seen when viewed from the side.
Rear projections have higher gain factors. They are between 3 and 4 and higher, which is also due to the reduced influence of ambient light.