Projected image from a video projector in a home cinema.
A video projector takes 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 widely used for conference room presentations, classroom training, home theatre and live events applications.
Overview
A video projector, also known as a Digital Projector, may be built into a cabinet with a rear-projection screen (rear-projection TV, or RPTV) to form a single unified display device, now popular for “home theater” applications.
Common display resolutions for a portable projector include SVGA (800×600 pixels), XGA (1024×768 pixels), 720p (1280×720 pixels), and 1080p (1920×1080 pixels).
The cost of a device is not only determined by its resolution, but also by its light output, acoustic noise output, contrast, and other characteristics. While most modern projectors provide sufficient light for a small screen at night or under controlled lighting such as in a basement with no windows[1], a projector with a higher light output (measured in lumens, abbreviated “lm”) is required for a larger screen or a room with a higher amount of ambient light. A rating of 1500 to 2500 ANSI lumens or lower is suitable for smaller screens with controlled lighting or low ambient light.[1][2] Between 2500 and 4000 lm is suitable for medium-sized screens with some ambient light or dimmed light. Over 4000 lm is appropriate for very large screens in a large room with no lighting control (for example, a conference room). Projected image size is important; because the total amount of light does not change, as size increases, brightness decreases. Image sizes are typically measured in linear terms, diagonally, obscuring the fact that larger images require much more light (proportional to the image area, not just the length of a side). Increasing the diagonal measure of the image by 25% reduces the image brightness by 35%; an increase of 41% reduces brightness by half.
Projection technologies
A Zenith 1200 CRT Projector based home theater. Circa 2006.
- CRT projector using cathode ray tubes. This typically involves a blue, a green, and a red tube. Minimal maintenance is required (unlike projectors that use expensive lamps which must be periodically replaced after they burn out). This is the oldest system and 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
- LCD projector[3] 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 minimized this.
- 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 artifact. Systems with 3 DMDs never have this problem, as they display each primary color simultaneously.
- LCOS projector using Liquid crystal on silicon.
- D-ILA JVC’s Direct-drive Image Light Amplifier based on LCOS technology.
- LED Use an array of Light Emitting Diodes as the light source, negating the need for lamp replacement.
Obsolete technologies
DIY video projectors
With a growing DIY-community and an increasing interest in low cost devices, some hobbyists have endeavored to build their own projectors from kits, sourced components, or from scratch. Through the internet, the community is now able to obtain plans to construct DIY-video projectors.[4][5] and there is a growing trend toward building them for domestic requirements 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.
See also
Major manufacturers
References
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