Several different types of screens are available for rear projection, and they offer very different properties in the projected image. First of all, the difference between optical screens and diffusion screens needs to be explained. An optical screen is a screen with lens elements integrated in the screen. A diffusion screen is a screen containing light dispersion particles.
The optical screen (on the left of Figure 4.25) is designed to ‘catch’ all the light emitted by the projector, and forward it into the viewing area in a certain way
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FIGURE 4.21 Cube display wall. (See colour insert.) |
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FIGURE 4.22 Inside of a cube.
(decided by the design criteria of the screen). The diffusion screen (on the right of Figure 4.25) can do nothing but diffuse the light emitted by the projector. The degree of diffusion is determined by the amount and characteristics of the diffuser particles. Diffusion screens are available in large formats (typically 300 to 400 inches) and with different optical specs (high gain and reduced viewing angles, low gain and high viewing angles, and tinted models with low gain for enhanced contrast).
Optical screens are available in limited sizes (typically 70 to 200 inches) and offer a better compromise of brightness, viewing angle, and contrast than diffusion screens. This is simply because all the light emitted by the projector ‘is being utilised’. Optical rear projection screens are available in two basic different designs: singleelement screens and two-element screens. The two-element screen has more surfaces that serve as lenses, and therefore it provides higher optical efficiency at the screen edges. Figure 4.26 shows that the single-element screen suffers from reflection light loss at the screen edge/corner, especially with very short projection distances.
