Projection screen
Projection screens may be permanently installed on a wall, as in a movie theater, mounted to or placed in a ceiling using a rollable projection surface that retracts into a casing (these can be motorized or manually operated), painted on a wall,[1] or portable with tripod or floor rising models as in a conference room or other non-dedicated viewing space.
[2] Uniformly white or grey screens are used almost exclusively as to avoid any discoloration to the image, while the most desired brightness of the screen depends on a number of variables, such as the ambient light level and the luminous power of the image source.
The screen also has hundreds of small, evenly spaced holes to allow air to and from the speakers and subwoofer, which often are directly behind it.
In the latter screens, the fabric can rarely stay immobile if there are currents of air in the room, giving imperfections to the projected image.
Titanium dioxide is a bright white colour, but greater gains can be accomplished with materials that reflect more of the light parallel to projection axis and less off-axis.
Very high gain levels could be attained simply by using a mirror surface, although the audience would then just see a reflection of the projector, defeating the purpose of using a screen.
Many screens with higher gain are simply semi-glossy, and so exhibit more mirror-like properties, namely a bright "hot spot" in the screen—an enlarged (and greatly blurred) reflection of the projector's lens.
This is possible because humans have greater sensitivity to contrast in smaller details, but less so in luminosity variations as great as half of the screen.
Glass-bead screens exhibit a phenomenon of retroreflection; the light is reflected more intensely back to its source than in any other direction.
This differs from semi-specular screens where the hot spot's location varies depending on the viewer's position in the audience.
Retroreflective screens are seen as desirable due to the high image intensity they can produce with a given luminous flux from a projector.
If the audience stays relatively close to the projector, a curved screen may be used instead without visible distortion in the image geometry.
Viewers closer or farther away will see a pincushion or barrel distortion, and the curved nature of the screen will become apparent when viewed off-axis.
The real-room situation is different from the contrast ratios advertised by projector manufacturers, who record the light levels with projector on full black / full white, giving as high contrast ratios as possible.
A relatively recent attempt in improving the perceived image quality is the introduction of grey screens, which are more capable of darker tones than their white counterparts.
This decreases the luminance (brightness) of both the projected image and ambient light, so while the light areas of the projected image are dimmer, the dark areas are darker; white is less bright, but intended black is closer to actual black.
Grey screens are designed to rely on powerful image sources that are able to produce adequate levels of luminosity so that the white areas of the image still appear as white, taking advantage of the non-linear perception of brightness in the human eye.
People may perceive a wide range of luminosities as "white", as long as the visual clues present in the environment suggest such an interpretation.
The potential improvement from a grey screen may thus be best realized in a darkened room, where the only light is that of the projector.
Partly fueled by popularity, grey screen technology has improved greatly in recent years.
[9] In an optimally configured system, projection screen surface and the real image plane are made to coincide.
