The light from the projector is focused on the screen. The diffusion layer scatters the light, thus producing an image.
In order to improve contrast the screen material is embedded with a dark tint which absorbs the ambient light striking the screen. At the same time it absorbs the light from the projector, reducing the light transmission and, therefore, the gain.
Screen gain in diffusion screens is controlled by adjusting the amount of tint and/or diffusing particles
Diffusion rear projection screen gains typically vary between 1 and 2.5. They are made from glass, rigid plastic or flexible plastic material. Being relatively cheap and easy they are therefore the cheapest to buy.
The main potential drawbacks with using diffusion screens come from uneven light distribution: ‘hot spot’ (poor brightness uniformity) and reduced viewing angles. This is because light dispersion from the screen is dependent on the angle of the light rays from the projector. These angles vary across the screen surface, with angles ranging from perpendicular at the centre to relatively acute in the corners. As a consequence the image is brightest in the centre, with dimmed corners.
This phenomenon is called ‘hotspot’ and is worse with short throw wide angle lenses because the angle of light striking the screen corners is greater the close the projector is to the screen.