A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. All video projectors use a very bright light to project the image, and most modern ones can correct any curves, blurriness, and other inconsistencies through manual settings. Video projectors are used for many applications such as conference room presentations, classroom training, and concerts. In schools and other educational settings, they are sometimes connected to an interactive whiteboard. In the late 20th century they were commonplace in home cinema but later lost market share to large LCD television screens.
A video projector, also known as a digital projector, may project onto a traditional reflective projection screen, or it may be built into a cabinet with a translucent rear-projection screen to form a single unified display device.
Common display resolutions for contemporary (as of 2012) portable projectors include SVGA (800×600 pixels), XGA (1024×768 pixels), 720p (1280×720 pixels), and 1080p (1920×1080 pixels).
The cost of a device is determined by its resolution and its light output. A projector with a higher light output (measured in lumens, “lm”) is required for a larger screen or for a room with a larger amount of ambient light. For example, a light output of approximately 1500 to 2500 ANSI lumens is suitable for small screens viewed in rooms with low ambient light; approximately 2500 to 4000 lm is suitable for medium-sized screens with some ambient light; over 4000 lm is needed for very large screens or for use in rooms with no lighting control such as conference rooms.
A few camcorders have a built-in projector suitable to make a small projection; a few more powerful "pico projectors" are pocket-sized, and many projectors are portable.
CRT projector using cathode ray tubes. This typically involves a blue, a green, and a red tube. This is the oldest system still in regular use, but falling out of favor largely because of the bulky cabinet. However, it does provide the largest screen size for a given cost. This also covers three-tube home models which, while bulky, can be moved (but then usually require complex picture adjustments to get the three images to line up correctly).
LCD projector using LCD light gates. This is the simplest system, making it one of the most common and affordable for home theaters and business use. Its most common problem is a visible “screen door” or pixelation effect, although recent advances have reduced the severity of this effect.
DLP projector using Texas Instruments’ DLP technology. This uses one, two, or three microfabricated light valves called digital micromirror devices (DMDs). The single- and double-DMD versions use rotating color wheels in time with the mirror refreshes to modulate color. The most common problem with the single- or two-DMD varieties is a visible “rainbow” which some people perceive when moving their eyes. More recent projectors with higher speed (2x or 4x) and otherwise optimised color wheels have lessened this effect. Systems with 3 DMDs never have this problem, as they display each primary color simultaneously.
LCoS projectors (liquid crystal on silicon). Such projectors often process light in the Fourier domain, which enables correction of optical aberrations using Zernike polynomials. Some commercially available technologies include:
D-ILA JVC’s Direct-drive Image Light Amplifier based on LCoS technology.
SXRD Sony’s proprietary variant of LCoS technology.
LED projectors use one of the above-mentioned technologies for image creation, with a difference that they use an array of Light Emitting Diodes as the light source, negating the need for lamp replacement.
Hybrid LED and laser diode system developed by Casio. Uses a combination of Light Emitting Diodes and 445 nm laser diodes as the light source, while image is processed with DLP (DMD) chip.
Laser diode projectors have been developed by Microvision and Aaxa Technologies. Microvision laser projectors use Microvision's patented laser beam-steering technology, whereas Aaxa Technologies uses laser diodes + LCoS.
Eidophor oil-film projectors
LIA (light image amplifier) light valves
Schmidt-CRT, developed by Kloss Video
Talaria oil-film projectors
Some hobbyists build a do-it-yourself (DIY) projector for low cost. They build their projectors from kits, sourced components, or from scratch. Through the internet, they obtain plans to construct them for domestic and classroom use. The DIY-projectors are now being used both in developed countries and in developing countries in the interest of providing education as well as for entertainment purposes.
