Different markets exist for screens targeted for use with digital projectors, movie projectors, overhead 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
In commercial movie theaters, screen is a reflective surface which may be either aluminized (for high contrast in moderate ambient light) or a white surface with small glass beads (for high brilliance under dark conditions). The screen also has hundreds of small, evenly spaced holes in order to allow the passage of air to and from the speakers and subwoofer which often are directly behind it.
Rigid wall-mounted screens maintain their geometry perfectly just like the big movie screens, which makes them suitable for applications that demand exact reproduction of image geometry. Such screens are often used in home theaters, along with the pull-down screens.
Pull-down screens are 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.
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 if there are currents of air in the room, giving imperfections to the projected image.
Screen 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 magnesium carbonate[1] or titanium dioxide[2], 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. 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.
Curved screens can be made highly reflective without introducing any visible hot spots, if the curvature of the screen, placement of the projector and the seating arrangement is designed correctly. The object of this design is to have the screen reflect the projected light back to the audience, effectively making the entire screen a giant "hot spot". If the angle of reflection is about the same across the screen, no distracting artefacts will be formed.
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. If the audience stays relatively close to the projector, 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.
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 larger screen size means less luminance (luminous power per unit solid angle per unit area) and thus less contrast in the presence 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 projector 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.
Highly reflective flat 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. Because of the said effect, these screens are also less vulnerable to ambient light coming from the sides of the screen, as well.
Grey screens
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. A matte grey screen would have no advantage over a matte white screen in terms on contrast; contemporary grey screens are rather designed to have a gain factor similar to those of matte white screens, but a darker appearance. They will reflect less light from the projector to the room and vice versa, thus decreasing the reflected ambient light both ways, while maintaining an acceptable image brightness. Many screen manufacturers thus appropriately call their grey screens "high-contrast" models.
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 many different luminosities as "white", as long as the visual clues present in the environment suggest such an interpretation. A grey screen may thus succeed almost as well in delivering a bright-looking image, or fail to do so in other circumstances.
Compared to a white screen, a grey screen reflects less light to the room and less light from the room, making it increasingly effective in dealing with the ambient light originating from the projector. Ambient light originating from other sources may reach the eye immediately after having reflected from the screen surface, giving no advantage over a white high-gain screen in terms of contrast ratio. 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.
Selectively reflective screens
Certain screens are designed to selectively reflect the narrow wavelengths of projector light while absorbing other wavelengths in the optical spectrum. One screen made by Sony [3], which appears grey when viewed in normal room light, is designed to reduce the effect of ambient light.[4] This works by preferentially absorbing ambient light while preferentially reflecting the projector's red, green and blue light.[5] 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.[citation needed] A contrast enhancing screen has been introduced by Dai Nippon Printing (DNP) and Screen Innovations which is based on thin layers of black absorbing louvers rather than wavelength selective properties[6].
Non-solid screens
FogScreen is a non-solid screen created with water and ultrasound. [7] [8]
References
- ^ Da-Lite. Angles of View
- ^ Projector Central [1] Screen gain
- ^ Seán Byrne. Sony introduces black projector screen for well-lit up areas. June 24, 2004.
- ^ Gizmodo. Picture of Sony's Black Backed Projection Screen. June 29, 2004.
- ^ Gizmodo. Sony's high contrast screen
- ^ Ray Adkins. Screen Innovations Visage Review
- ^ Gizmag. News article on Fogscreen
- ^ DJ Zone. Article on FogScreen
See also
External links