It is basically possible to project onto any surface which is not totally black. Projecting on scenery is quite common in theatre, but when a
clear
picture is called for, it is better to project on a flat unarticulated surface with a smooth texture. Although plain cloth, white scrim or a plastered surface can be adequate in many cases, specially designed
projection
screens, for either front or rear projection, maximize the brightness and sharpness of the projection.
One of the major factors effecting the quality of the
projection
is the luminance of the
projection
screen - the amount of light reflected (or transmitted, in the case of rear
projection
screens) from the
projection
screen. This is measured in footcandles (lum/ft).
Projections in theatre are often hindered by other lights on the stage. Even when the luminaires are meticulously focused off the screen, light will reflect off actors' bodies, the
stage
floor and the set. The more light there is on stage, the fuzzier the projected
image
will be. Projection screens are designed so as not to reflect the light of the
projection
in a uniform pattern, but rather to concentrate the reflected light at a smaller angle towards the spectators. This is described by the screen gain. The more light is reflected towards the viewer the higher the screen gain will be.
A front
projection
screen which spreads and reflects the light equally in all directions (in theory), has a screen gain of 1. But if the screen reflects more light in one direction than another, the gain is greater than when measured in the specific direction. When the screen gain is 2, the
image
projected towards the spectators is twice as strong as that of a screen with a screen gain of 1. High-gain
projection
screens can be viewed only within a small viewing angle while low-gain screens can be viewed within wider viewing angle . The higher the gain of a
projection
screen the better it rejects incident light which is reflected in a different direction than the main projection.
