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, and home theatre applications.
A video projector may also 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 1000 to
1500 ANSI lumens or lower is suitable for smaller screens
with controlled lighting or low ambient light.[1][2] Between
1500 and 3000 lm is suitable for medium-sized screens with
some ambient light or dimmed light. Over 3000 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 per cent; an increase of 41 per cent reduces brightness
by half.
Projection technologies
- 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. CRT projectors are
suitable only for fixed installations.
- 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 minimized this.
- DLP projector using Texas Instruments’ DLP technology.
This uses one, two, or three microfabricated light valves
called a 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. Systems with 3 DMDs never have this problem.
More recent projectors with higher speed (2x or 4x) and
otherwise optimised color wheels have minimized this artifact.
- LCOS projector using Liquid crystal on silicon.
- D-ILA JVC’s Direct-drive Image Light Amplifier
based on LCOS technology.
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