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Rear-Projection Television - What You Need To Know

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 article outlines the technology and basic elements of a rear-projection television. Also, on the side bar of this article, there are link resources to related topics, such as HDTV, Widescreen vs Standard screen, and more.

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.

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.

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, for the purposes of this article, I have chosen to present an overview of the three major video projection technologies.

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, a CRTs can produce excellent high resolution images.

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).

3. CRT-based rear-projection televisions, since they are based on technology that has been around for decades, is less expensive to manufacture. Thus, prices of CRT-based rear-projection televisions are several hundred dollars less than their DLP and LCD rear-projection counterparts (screen size and features being equal).

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.

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.

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.

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.

Variants of LCD

Other variants of LCD video projection technology in use are: LCOS (Liquid Crystal on Silicon), D-ILA (Digital Imaging Light Amplification - developed and used by JVC), and SXRD (Silicon Crystal Reflective Display - developed and used by Sony).

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. For further technical explanations, click on the DLP resource link on the upper right sidebar of this page.

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.

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.

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 lens 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. Before making a rear-projection television purchase, it may be wise to check the manufacturer's website for information on the type of lenses used in their projection assemblies.

Mirrors

The third element necessary in the design of a rear-projection television is the mirror. Since the projected image has to projected 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. For a good calculator to determine optimum television screen viewing distance,

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 brighest 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. Atlhough 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. For some examples, check out some of my projection television Top Picks selections listed in the sidebar of this article.

The HD or Analog Decision

In addition, you need to decide if you want a standard analog rear-projection television, an HD-upgradable unit, or an Integrated HDTV set. Also, there is also the widescreen vs standard screen dilemma. For more assistance in this matter, check out my article: Widescreen Television - The 16x9 Factor

Final Words

I hope that this article has given insight on some the "secrets" of rear-projection television technology that will enable consumers to make their buying decisions in a more informed manner.