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Movie screenProjection screens are installations consisting of blank surface and a support structure used for projecting an image for the view of an audience. Projection screens may be permanently installed as in a movie theater, painted on the wall[http://specialsections.nypost.com/news/nypost/nyphome/20050521/p44.asp], semi-permanent or mobile, as in a conference room or other non-dedicated viewing space. 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. Flat or curved screens may be used depending on the optics used to project the image and the desired geometrical accuracy of the image production, flat screens being the more common of the two. Screens can be further designed for front or back projection, the more common front projection systems having the image source situated on the same side of the screen as the audience.
Different markets exist for screens targeted for use with digital projectors, movie projectors and slide projectors, although the basic idea for each of them is very much the same: front projection screens work on diffusely reflecting the light projected on to them, whereas back projection screens work by diffusely transmitting the light through them.
Screens by installation type
Permanently installed screens can often be found in venues that are used for continuous visual presentations, such as movie theaters. These can be pre-assembled or assembled on the spot.
Pull-down screens are a often used in multipurpose spaces where permanently installed screen would require too much space. These commonly use painted fabric that is rolled in the screen case when not used, making them somewhat more prone to damage than rigid screens.
Rigid wall-mounted screens maintain their geometry perfectly, which makes them suitable for applications that demand exact reproduction of image geometry, and a good choice for presentation where aesthetics are important.
Mobile screens usually use a pull-down screen on a free stand. These can be used when it is impossible or impractical to mount the screen to a wall or a ceiling, but the fabric of the screen can rarely stay immobile, giving imperfections to the projected image.
Home theater screens and gain
One of the most often quoted properties in a home theater screen is the gain. This is a measure of reflectivity of light compared to a screen coated with titanium dioxide, when the measurement is taken for light targeted and reflected perpendicular to the screen. Titanium oxide 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.
Frequently quoted gain levels of various materials range from 0.8 of light grey matte screens to 2.5 of the more highly reflective glass bead screens, some manufacturers claiming even higher numbers for their products. 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. Screens with higher gain will exhibit more mirror-like properties, namely a bright "hot spot" in the screen, an enlarged (and greatly blurred) reflection of the projectors lens. Opinions differ as to when this "hot spotting" begins to be distracting, but most viewers have trouble to notice as large as 30% difference in the image luminosity, unless presented with a test image and asked to look for variations in brightness.
In normal screens the greatest intensity of light will reflect at an equal and opposite angle to the angle of incidence, favouring ceiling-mounted projector setups as it will maximize the apparent screen brightness on the audience level. Glass-bead screens exhibit a phenomenon of retroreflection, where the highest intensity of the light is reflected back to where it came from. This is intended for setups where the image source is placed in the same direction from the screen as the audience. Users frequently report some hotspotting in such screens, although this type of screen is seen as desirable due to the high image intensity they can produce.
Screen geometry and optics
Square-shaped screens used for overhead projectors sometimes double as projection screens for digital projectors in meeting rooms, where space is scarce and multiple screens can seem redundant. These screens have an aspect ratio of 1:1 by definition. Other popular aspect ratios include 4:3 and a widescreen ratio of 16:9, which are often used as dedicated data projection and home cinema use, respectively.
Most image sources are designed to project a perfectly rectangular image on a flat screen. Optics designed for a curved screen will result in a pincushion effect when an image is projected on a flat screen.
Image brightness and contrast
Apparent contrast in a projected image - the range of brightness - is dependent on the ambient light conditions, luminous power of the projector and the size of the image being projected. A greater image will lead to less luminance (luminous power per unit solid angle per unit area) and thus a smaller contrast in the presense of ambient light. Some light will always be created in the room when an image is projected, increasing the ambient light level and thus contributing to the degradation of picture quality. This effect can be lessened by decorating the room with dark colours. The real-room situation is different from the contrast ratios advertised by manufacturers, who record the light levels with projector on full black / full white, giving as high contrast ratios as possible.
Manufacturers of home theater screens have attempted to resolve the issue of ambient light by introducing screen surfaces that direct more of the light back to the light source. The rationale behind this approach relies on having the image source placed near the audience, so that the audience will actually see the increased reflected light level on the screen. The level of omnidirectional light will be, in theory, unaffected by the reflective properties of the screen, whether highly directional or diffuse. Highly reflective screens tend to suffer from hot spots, when part of the screen seems much more bright than the rest: this is a result of the high directionality (mirror-likeness) of such screens. Screens with high gain also have a narrower usable viewing angle, as the amount of reflected light rapidly decreases as the viewer moves away from front of such screen.
A relatively recent attempt in improving the perceived image quality is the introduction of grey screens, which are more capable of darker tones in the presence of ambient light than their white counterparts. It can be disputed whether such screens increase the actual contrast ratio possible in the image, as a grey surface will necessarily reflect less light from the projected image, as well. 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.
At least one screen is designed to selectively reflect the narrow wavelengths of projector light while absorbing other wavelengths in the optical spectrum. This screen made by Sony [http://www.cdfreaks.com/news2.php?ID=9984] appears to be black when viewed in normal room light, and it is designed to be relatively unaffected by ambient light while still being capable of producing intense colors. It is unclear whether this approach protects the image from being degraded due to light that reflects from the screen to the room and back.
More reading:
[http://www.projectorcentral.com/projector_screens_gain.htm] - Projectorcentral.com - Screen gain, reflectiveness
[http://entertainment.howstuffworks.com/movie-screen.htm] - Howstuffworks.com - How movie screens work
[http://gothifi.com/hifishowroom/screentutor.htm] - Hifishowroom - on different screen types by gain and viewing angle
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