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Overview
Although Plasma and LCD flat panel televisions seem to be
all the rage these days, with growing numbers of consumers
reaching into their pockets to buy the stylish sets, they
are still out of reach of most average household budgets.
As a result, the rear-projection TV (also referred to as an
RPTV), with its lower pricing yet high performance, is taking
center stage in the consumer television market. The following
outlines the technology and basic elements of a rear-projection
television.
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Definition
Of Rear-Projection
The actual term "rear-projection" comes from the
fact that the image is projected and reflected onto the screen
from behind the screen, unlike traditional video and film
projection in which the projector itself is placed in front
of the screen, such as in a movie theater.
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The
Elements Of Rear-Projection Television
There are five basic elements in the construction of a rear-projection
television. First, there is the type of projector technology
used to produce a video image. Second, the type of lens assembly
used to magnify the projected image. Third, the necessity
to employ a mirror to reflect the projected image. Fourth,
the screen upon which the reflected image is presented. Fifth,
the sealed box that contains all of the previous elements.
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Video
Projection Technology
Currently, there are three major basic types of projection
technology commonly used in rear-projection televisions in
today: CRT, LCD, and DLP. There are also variations of these
three types that are not widely used at this time (including
D-ILA and LCOS). However, an overview of the three major video
projection technologies will be presented.
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The
CRT (Cathode Ray Tube) Projection System
When rear-projection televisions first arrived on the scene,
television technology was based on the Cathode Ray Tube (CRT).
In a CRT-based rear-projection television, three small CRTs
(one for each primary color), coupled with a light magnifying
lens, projects a color image onto a mirror and is then reflected
onto a screen. With the proper video processing circuitry,
CRT size, and lens combination, CRTs can produce excellent
high resolution images.
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Advantages
Of CRT Video Projection
1. A CRT is basically a large vacuum tube
in which an electron beam, emanating from a single point in
the neck of the tube, scans the face of the tube very rapidly,
which, in turn lights up phosphors on the tube's surface in
order to create an image. A CRT produced image is not limited
to a fixed pixel field, as are other rear projection types
(to be discussed, in more detail, later in this article).
This makes the CRT-based video projection possibly the best
option where the flexibility of displaying variable resolutions
is the main consideration.
2. CRT-based rear-projection
technology can produce the blackest blacks of all projection
types as well as the full range of color, and brightness,
giving a CRT projector the ability to produce the most film-like
images of projectors for home use (up to this point).
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Limitations
Of CRT Video Projection
1. A CRT projection television is rather
large. A CRT-based projection television can take up a lot
of space, in terms of cabinet depth, not only in comparison
to Plasma and LCD flat panel sets, both also in comparison
to DLP and LCD rear-projection units.
2. A CRT-based rear-projection
television has to be converged properly for best image reproduction.
Since the modern CRT-based rear-projection television houses
three projection tubes (red, green, and blue), the tubes need
to be aligned so that the projected image does not exhibit
color halos and the colors are mixed correctly. If one projection
tube fails, all three have to be replaced in order to provide
the correct color and brightness balance.
3. Although all projection
televisions generate some heat, this is a challenge in CRT-based
projection sets, due to the fact that three separate projection
tubes must be cooled and kept at an even temperature. This
is accomplished by liquid gels developed for this purpose.
Although very rare, the liquid gels casings can leak, thus
causing the television to malfunction.
4. CRT projection
televisions are more susceptible to image burn-in from continuously
displayed station logos and window box lines resulting from
extensive display of 4x3 programming on 16x9 CRT projection
sets.
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LCD
Video Projection Technology Overview - The LCD (Liquid Crystal
Display) Projection System
The second type of rear-projection television technology to
consider is LCD (Liquid Crystal Display) projection (not to
be confused with LCD flat panel television). Unlike a CRT
projection television, the LCD-based rear projection television
is not based on the traditional projection tube. A rear-projection
LCD television basically works by passing a powerful light
source through a transparent LCD chip made up of individual
pixels (which displays the moving video image) and projecting
that image through a magnifying lens, to a mirror, which then
reflects that image, onto a screen.
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Advantages
Of LCD Video Projection
1. What makes the LCD projector assembly
very practical is that it is compact, since the LCD chip is
very small.
One LCD chip is hundreds of times smaller than the three projection
tubes needed in CRT-based rear-projection televisions. This
means that LCD-based rear-projection televisions can be made
a lot thinner and lighter than traditional CRT-based rear-projection
sets. Although you can't hang it on the wall like you can
with an LCD flat panel or Plasma Television, you can still
save a lot of floor space, and spend less money than you would
buying that stylish LCD flat panel or Plasma set. In addition,
since these are projection sets, you can get one in larger
screen sizes than you can with either LCD or Plasma flat panel
types.
2. Other advantages
of rear-projection LCD technology are its high contrast and
brightness capability, as well as lower power consumption.
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Limitations
Of LCD Video Projection
1. A LCD projection television can often
times exhibit what is called "the screen door effect".
Since the screen is made up of individual pixels, the pixels
can be visible on a large screen, thus giving the appearance
of viewing the image through a "screen door".
2. LCD rear-projection
televisions, although appearing to be simpler in construction,
due to the use of a small chip, rather than three CRT tubes,
are still much more complex to produce than traditional CRT
projection televisions. This is due to the higher cost of
manufacturing the LCD chips themselves. Thus, LCD-based rear-projection
televisions are usually several hundred dollars more than
their CRT-based counterparts (with size and features being
equal).
3. Since an LCD chip
is made up of a panel of individual pixels, if one pixel burns
out it displays an annoying black or white dot on the projected
image. Individual pixels cannot be repaired, if one or more
pixels burn out, the entire chip has to be replaced.
4. Since LCD chips
have a finite number of pixels, signal inputs that have higher
resolutions must be scaled to fit the pixel field count of
the particular LCD chip. For example, a typical HDTV input
format of 1080i needs a native display of 1920x1080 pixels
for a one-to-one display of the HDTV image. However, if your
LCD chip only has a pixel field of 1024x768, the original
HDTV signal must be scaled to fit the 1024x768 pixel count
on the LCD chip (in addition the image will also have to be
letterboxed to reproduce the correct widescreen aspect ratio).
This is where CRT-based rear-projection sets can excel over
an LCD rear-projection set. Since CRTs are not limited by
a fixed pixel field, they are more flexible at displaying
various resolutions, due to being able to variably scan the
image onto the projection tube surface.
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Variants
of LCD
Other variants of LCD video projection technology in use are:
LCOS (Liquid Crystal on Silicon), D-ILA
(Digital Imaging Light Amplification), and SXRD
(Silicon Crystal Reflective Display).
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DLP
Projection Technology Overview - The DLP (Digital Light Processing)
Projection System
The most promising heir to the CRT-based rear-projection television
in use today is the DLP (Digital Light Processing) rear-projection
television. Like LCD, the actual image is displayed on a chip;
however, the chip used in a DLP projection television is different.
The chip in a DLP projection television is referred to as
a DMD (Digital Micromirror Device). In essence, every pixel
on a DMD chip is a reflective mirror.
The video image is displayed on the
DMD chip. The micromirrors on the chip (remember: each micromirror
represents one pixel) then tilt very rapidly as the image
changes. This process produces the grayscale foundation for
the image. Then, color is added as light passes through a
high-speed color wheel and is reflected off of the micromirrors
on the DLP chip as they rapidly tilt towards or away from
the light source.
The degree of tilt of each micromirror
coupled with the rapidly spinning color wheel determines the
color structure of the projected image. As the amplified light
bounces off the micromirrors, it is sent through the lens,
reflected off a large single mirror, and onto the screen.
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Advantages
Of DLP Video Projection
1. DLP projection technology is suitable
for not only for projection televisions, designed for home
use, but DLP technology is also in use in some movie theaters
for feature film projection. Essentially, the films are digitally
converted and stored to either to a hard drive or optical
disc (similar to DVD - only in High Definition), then fed
into the DLP projector and projected onto the movie screen.
The high resolution DLP chips made for this application render
an image that is almost as good as 35 or 70mm film, without
all those film scratches!
2. Other advantages
of the DLP projection technology include excellent color accuracy,
no "screen door" effect (as with LCD), due to its
micro-mirror construction, compactness, low power consumption,
and high contrast and brightness (although typically not as
bright as LCD types but much "smoother" looking).
Also, DLP technology enables a very thin depth cabinet profile,
just like LCD rear-projection sets.
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Limitations
Of DLP Video Projection
1. Just as with LCD, each DLP chip has a
finite number of pixels.
2. Although a DLP
rear-projection television doesn't exhibit the "screen
door" effect of many LCD-based units, a DLP rear-projection
television can exhibit what is referred to as "the rainbow
effect". Basically, the "rainbow effect" is
exhibited by a brief flash of colors (like a small rainbow)
when the viewer rapidly looks from side to side on the screen
or looks rapidly from the screen to side of the room. Fortunately,
this does not occur frequently and many people do not have
sensitivity to this effect at all.
Although DLP isn't perfect, DLP projection has become a real
favorite amongst home theater enthusiasts
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Additional
Technologies Used In Rear Projection Televisions
The previous pages outlined the first basic element of rear-projection
televisions; the projection technology used. The remaining
four elements in rear-projection television construction are
outlined on this page.
Projection Lenses
The second key element in rear-projection television, are
the type of lenses used to magnify the projected image. There
are usually several lens elements employed. It is usual for
a rear-projection television to employ four or five lens elements;
some sets employ more. The reason for this is that several
successive lenses are needed to maintain shape and brightness
integrity of the image. It is very easy for a single lens
to make things look good in the center of the image, but as
the light and image content is dispersed across the area needed
for the size of screen employed, additional lens elements
are needed to maintain the overall image shape so that objects
in the image look the same on the sides and corners as they
do in the center of the screen.
In terms of the physical construction
of the lens assemblies themselves, all-glass assemblies are
the best, especially in terms of withstanding high temperature
variations from both internal and external factors. However,
many manufacturers use a combination of glass and acrylic-based
lens construction. This may not be bad, but there may be differences
noticeable not only in the final image, but overall image
performance in the long term.
Mirrors
The third element necessary in the
design of a rear-projection television is the mirror. Since
the projected image has to project on a large surface within
a relatively small space (in comparison to a standard video
projector and screen) a mirror is placed in the path of the
projection element, magnifying lenses, and the screen. This
is enables the projection elements and magnifying lenses to
be placed at the bottom of the projection television cabinet.
This gives the projected image a longer path to the screen
itself so that a larger image can be presented as well redirecting
the path of the projected image so that it will be facing
the viewer correctly.
The Screen
The fourth element of a rear-projection
television is the actual screen upon which the image is seen
by viewer. The type of screen used in projection televisions
is much different than those used for front video projection
or film. A rear-projection television screen is made up of
two primary elements. First, there is the inner Fresnel surface,
which further magnifies and disperses the brightness of the
image across the screen. Second, there is an outer Lenticular
surface, which helps in the final shape integrity of the image,
as well as contributing to contrast and the wideness of the
televisions viewing angle. If you were to touch a rear-projection
television screen (which you should never do under normal
circumstances) you would notice that its surface is not smooth,
as is a standard film or video projection screen. This is
because you are actually touching the Lenticular surface etched
into the screen itself.
The Box
Of course, the fifth key element of
the rear-projection television itself is the box in which
all the elements are sealed in. The size of the box is directly
related to the size of the screen surface itself. Rear projection
televisions can be of any size, but, practically speaking,
they come in sizes housing 41-inch to 76-inch screens. The
size of the box that is right for you is determined by your
room size and/or you actual viewing distance from the television
screen.
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Other
Factors When Considering A Rear-Projection Television
On the previous pages, outlined
are the five basic elements of a rear-projection television,
however, there are some additional factors to be aware of.
Additional
Considerations
When making a rear-projection television
purchase, make sure you include the following factors: What
you want to watch (and where), Brightness, Contrast Ratio,
Pixel Density, Color Reproduction, Inputs, Viewing Angle,
and Price.
Brightness: Without
sufficient brightness your image will look muddy and soft,
even in a dark room. Check the ANSI Lumens rating. Relatively
speaking, projection assemblies producing 1,000 ANSI Lumens
is plenty for a rear-projection television. However, since
the light is reflected onto a screen, the light intensity
emanating from the screen, into the viewing room itself, is
more important.
The measure of brightness that comes
from the screen surface in any particular direction is calculated
in cd/m2 (Candela/Square Meters), also called 'nits'. Brightness
measurements on rear-projection televisions can vary widely,
anywhere from 400 to 800 cd/m2. This specification should
be labeled in the specifications sheet for the television;
it may also be listed as Foot Lamberts (fL), where 1 fL is
approximately 3 cd/m2.
In the final analysis, don't get bogged
down in the math; regardless of what the numbers say, make
sure you are comfortable with the brightness of the image
that is on the screen.
Contrast Ratio: Contrast
ratio complements brightness. High contrast ratios deliver
whiter whites and blacker blacks. A rear-projection television
may have a great Lumens and Foot Lamberts rating, but if the
contrast ratio is low, your image will look washed out. A
Contrast ratio of at least 1,000:1 or higher is considered
excellent.
Pixel Density: Pixel
Density is important, with regard for LCD/DLP units. As stated
earlier, LCD and DLP-based video projectors have a fixed number
of pixels on their display chips. If most of your viewing
is HDTV, get as high a native pixel count as possible. For
instance, 720p HDTV signals require a 1280x720 pixel count
to give you a one-for-one representation of a 720p signal
while, as mentioned earlier, a 1080i HDTV input signal needs
a native pixel count of 1920x1080 for a one-for-one representation
of the 1080i signal. If a projection set's pixel count is
less and it accepts HDTV input signals, the signal is scaled
to fit the number of pixels on the chip.
On the other hand, a native pixel
count of 1024x768 is sufficient for DVD. In addition, some
projection sets also upscale a lower resolution image to match
a higher pixel count on the chip. Scaling can work both ways.
Check specifications for this capability.
Color Reproduction:
Check for natural flesh tones and color depth. Check how colors
look in the brightest and darkest areas of the image. Check
the degree of color stability from input to input. Everyone
has a slight difference in color perception and what looks
pleasing. Look carefully.
Viewing Angle: All
projection televisions have a problem with side viewing. Although
viewing angles for rear projection televisions have improved
greatly in recent years, with some offering 130 degrees or
more, optimum viewing is still best from the center of the
screen out to a 45 degree angle, with acceptable viewing possibly
out to 90 degrees. In other words, all the viewers sitting
on a large couch shouldn't have a problem, but someone sitting
in a chair off to the side will not get an optimum view of
the screen.
Inputs: Make sure
the projector has the inputs you need, such composite and
S-video for analog sources, component inputs for DVD, and
DVI or HDMI inputs for HDTV. Some rear-projection televisions
now have VGA or similar inputs that enable it to be used with
a computer for video presentations or game play.
Price: Rear-projection
televisions are perfect for the bargain hunter looking for
an inexpensive way to get that big screen HDTV experience.
Prices continue to come down for all types and sizes of rear-projection
televisions.
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