How Projection Television Works


Projection TVs can have front- or rear-projection.

Enormous televisions and home theaters used to be a real luxury. But in the last few years, many people have started to view large screens with great pictures as necessary for watching TV and movies at home. Although old-fashioned cathode ray tube (CRT) TVs can provide a great picture, they can't support the screen size that people look for today. Projection TVs can provide a much bigger picture than CRT sets can, and front- and rear-projection models can suit a range of rooms and budgets.

In this article, we'll discuss the differences between front- and rear-projection TVs. We'll also explain the differences between the various types of projection TV technology and what to look for when you go shopping.

A good way to understand how a projection TV works is to compare it to a standard TV. A conventional television uses a cathode ray tube (CRT) to create a picture.

A CRT fires a beam of electrons at a phosphor-coated screen. Every time an electron comes into contact with the screen, that point, called a pixel, glows. Color CRT televisions use three electron beams and separate phosphors for red, green and blue. When you watch, you're looking directly at the surface that the TV uses to create the picture. That's why traditional CRT sets are called direct-view displays.

­ CRTs are very reliable and have good picture quality. But they do have one big drawback -- since the screen is made of glass, size is limited. The largest CRT screens measure about 40 inches diagonally. A CRT TV with a screen that size is deep, heavy and unwieldy.

That's the main reason for projection TVs. Even though some models can't rival the quality of a direct-view CRT set, they can be much bigger. Instead of using a direct-view setup, a projection TV creates a small picture and then uses a beam of light to display that picture at a much larger size. Next, learn about projection TV configuration.

Projection TV Configurations

A transmissive, front-projection display system
A transmissive, front-projection display system
Photo courtesy Philips Research

Projection TVs are available in two main configurations -- front projection and rear projection.

A reflective, rear-projection display system A reflective, rear-projection display system
A reflective, rear-projection display system
Photo courtesy Philips Research

A front-projection system uses a projector and a separate screen, and it projects images onto the front of that screen. This setup looks most like what you'd find in a movie theater -- the projection unit is completely separate from the screen. The projector can be placed on a table or mounted to the ceiling. The picture looks best when displayed on a high-quality screen, but a specially painted, flat wall will work as well.

Rear-projection systems look more like traditional televisions. They display images on the back of a screen rather than the front, and the projector is completely contained within the television itself. You can also set up a rear-projection system with a projector and a special screen, but the term is most often used to describe self-contained TV sets.

Both configurations use tiny devices capable of making very detailed pictures. These devices can be reflective, meaning that light picks up the picture by bouncing off the device. Or, they can be transmissive, meaning that light picks up the picture by traveling through the device. Once the light picks up the picture, a lens magnifies the picture and projects it onto the screen.

Next, we'll take a look at the major types of television technologies and how they compare to one another.

Television Technologies

A DMD is small enough to fit in a person's hand.
A DMD is small enough to fit in a person's hand.
Photo courtesy Texas Instruments

The transmissive projection types include CRTs and liquid crystal displays (LCDs). Here's a brief overview of how they work:

CRT: A CRT projector uses much smaller tubes than a standard television. These tubes create the picture the same way a standard TV does -- by firing electrons at a phosphor-coated screen. A CRT projector can include:

  • One color CRT with red, green and blue phosphors
  • One black-and-white CRT and a spinning color wheel that adds the color
  • Three CRTs, one each for red, green and blue

Liquid crystal display (LCD): Electrical currents can cause liquid crystals to change their shape. This allows them to act as light valves -- different amounts of current allow different amounts of light to pass through the crystal. This lets the LCD device create a greyscale image. To add color, most projectors use a series of mirrors that split the light into red, green and blue beams. Each beam passes through a separate LCD, and a lens collects the three beams and projects the image on the screen. LCDs can also be used to create flat-panel televisions. See How LCDs Work to learn more.

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Reflective displays types include digital light processing (DLP) and liquid crystal on silicon (LCoS):

  • DLP: A DLP projector uses a digital micromirror device (DMD) - a small, rectangular device made of microscopic mirrors -- to make a picture. The mirrors point toward or away from a projection lens, depending on whether the corresponding pixel needs to be light or dark. Most DLP rear-projection sets have one DMD and a spinning, multicolored wheel that adds color. Some front-projection units have separate DMDs for red, green and blue. You can get all the specifics on DLP televisions at How DLP Sets Work.
  • LCoS: LCoS is simultaneously reflective and transmissive, and it's like a combination of DLP and LCD technology. In an LCoS set, light passes through a liquid crystal layer, then bounces off a reflective surface. When the light passes back through the liquid crystals, the crystals act as light valves to create the light and dark areas of the picture. Most LCoS projectors use separate devices for red, green and blue, and a lens combines the three colors. To learn more, read How LCoS Works.

Flat-panel models are another option for people who are looking for a larger TV. Plasma and SED-TV sets can provide a very large picture in a set that's only a few inches thick. Flat-panel LCD sets are another option, but they're limited in size to about 40 inches diagonally.

All three flat-panel set types tend to be more expensive than most projection models. In addition, plasma screens are susceptible to burn-in -- if the same image stays on the screen for a long time, it could become permanently seared into the plasma. Check out How Plasma Displays Work, How LCDs Work and How SED-TV Works to learn more.

There are a few other lesser-known TV technologies, like grating light valves. But if you're shopping for a projection TV, the ones you're most likely to see will use CRT, LCD, DLP or LCoS to create a picture.

When you start shopping for a projection TV, your first major decision will be whether to buy a front-projection or rear-projection model. Both types use the same technology -- CRT, LCD, DLP or LCoS -- to create the picture. However, front and rear projection have some substantial differences. Here's what you should consider when making your decision:

  • The size of the room and the screen: Front-projection setups don't take up a lot of floor space like a rear-projection TV can. Front projection can also provide the biggest screen size. But if you want a huge picture, you'll need to be able to sit far from the screen. A good rule of thumb is that your distance from the screen should be 1 ½ times its diagonal measurement. In other words, if you want a 100-inch (250 centimeter) front-projection screen, you'll need to sit about 12 feet (3.7 meters) away.
  • How you plan to use your set: Front-projection setups work best in dark rooms. Even models with very high luminance, or light output, can't really overcome the light in a sunny room. They may not be the best choice for daily TV viewing or for rooms with lots of ambient light.
  • How much money you have to spend: Some projectors cost about as much money as a high-end rear-projection set. But a projector on its own isn't very useful - you'll also need a screen and speakers. If you want to watch television shows on your set, you'll need a TV tuner, too. The total price for all these components add up to substantially more than a rear-projection set.

Next, we'll discuss what to look for if you're shopping for a front projector.

Buying a Front Projection TV

Standard vs. high-definition aspect ratio
Standard vs. high-definition aspect ratio
HowStuffWorks

If you decide to buy a front-projection system, here are the things you should keep in mind:

  • Aspect ratio and maximum picture size: Standard TVs usually have a 4:3 aspect ratio, or relationship between the width and height of the TV screen. High-definition TV (HDTV) and many movies on DVD use a wider 16:9 aspect ratio. Home theater projectors can generally support a 16:9 aspect ratio, and you can make the picture bigger or smaller by moving the projector or adjusting its settings. However, most projectors have a maximum picture size and maximum distance from the screen. If you try to make the picture bigger or move the projector farther away than the maximum, your picture quality will deteriorate.
  • Resolution: A projector's resolution is measured in pixels. The higher the resolution, the clearer and sharper the picture will be. Standard TVs have a minimum resolution of 640 x 480 pixels. HDTV (hdtv.htm) uses resolutions up to 1920 x 1080 pixels. If you plan to use your projector to watch high-definition broadcasts and shows, make sure it supports high-definition resolutions.
  • Frame rate: A set's frame rate is measured in frames per second and includes whether the picture is interlaced or progressive. Interlaced frame rates create half of the picture with every other frame. Progressive frame rates create the entire picture with every frame. The fastest frame rate is 60 progressive frames per second, or 60p. The 18 Primary DTV Standards
  • Black level: A projection TV's black level is its ability to produce the color black. A good black level is important for rendering detail and for making dimly lit scenes look good. DLP projectors usually have the best black levels, while CRT and LCD projectors can have trouble producing true black. Some LCoS projectors include an iris that adjusts to allow different amounts of light to pass, which can improve black level. In addition, some screens are specially designed to help improve black level.
  • Contrast ratio: The difference between the darkest and lightest color a set can produce is its contrast ratio. A contrast ratio of 1000:1 means that white is a thousand times brighter than black. A set with a high contrast ratio can produce a wider range of colors and more detail than one with a low ratio. However, contrast ratios are a little like clothing sizes - there's no standard way to measure them. Take a DVD with you when you go shopping, and check out how projectors display colors firsthand.
  • Visual artifacts and burn-in: TV technologies have a few quirks when it comes to visual artifacts. DLP projectors that use only one DMD are prone to the rainbow effect, in which people see a brief rainbow of colors when they move their eyes over the screen. Newer sets have color wheels with more segments, which reduces the issue. LCD and older DLP sets are sometimes prone to the screen door effect -- the image appears with a sort of black grid over it, much like when you look through a screen door -- due to spaces between the pixels. Finally, like plasma screens, CRT projectors are prone to burn-in.
  • Screen quality: Even the best projector can't overcome an inferior screen surface. When shopping for a screen, pay particular attention to the gain (reflectivity of the screen) and viewing angle. A higher gain measurement means a brighter image. A gain of '1' is comparable to a matte-finish, flat white surface. A gain measurement of '2' means the resulting image will be twice as bright. Typical measurements run between "1.2" and "2," but gains as high as "4" are possible. However, keep in mind that the higher the gain, the more limited the viewing angle. Also, decide whether you want your screen to be fixed or portable, or whether you'd rather paint your wall with a simulated screen.
  • Candlepower: A projector's candlepower, measured in lumens, is its light output. High-candlepower projectors can display images in rooms with more ambient light. However, more isn't always better. In a very dark room, a projector with too much light output will create a picture that's too bright. Projectors designed for conference rooms have a higher candlepower than those for use at home, so make sure you're looking at home theater projectors when you shop. Unlike many other TV measurements, the American National Standards Institute (ANSI) has set standards for measuring lumens. Photo courtesy HowStuffWorks Shopper (1) RCA connections, (2) a cable connection, and (3) an HDVI connection
  • Connections: Your projector won't be of much use to you if you can't connect anything to it. If you plan to use a DVD player, game console or other device, make sure the projector has the connections to support it. If your set doesn't have the right connections, you'll need adapters to use your existing equipment. The most common types are: DVI and HDMI for digital devices like DVD players, cable for cable and antenna connections, composite video and component video, which use RCA connections, and S-video.
  • Lamp life and replacement cost: Projector lamps last about half as long as rear-projection TV lamps. Depending on how much you use your projector, you may have to replace your lamp as often as every 18 months.

Next, we'll take a look at what to keep in mind if you're shopping for a rear-projection set.

Buying a Rear-projection TV

Front- and rear-projection TVs have some substantial differences. However, you'll still need to keep most of the same things in mind if you're shopping for either one. If you're in the market for a rear-projection set, here's what you should watch for:

  • Aspect ratio, screen size, resolution and frame rate: Most projection TVs have a 16:9 aspect ratio, and most, but not all, can display high-definition broadcasts. Make sure you get an HDTV or HDTV-ready set if you plan to watch programs in high definition. Also, unlike with a projector, the size of your set is fixed, so make sure to measure your room and figure out what size screen you want before you shop. Photo courtesy Panasonic and Matsushita Electric Corporation of America Rear-projection TVs take up more space than front-projection models, but they're more economical.
  • Color, black level and contrast ratio: DLP sets have a very good black level, and LCoS sets can as well, depending on the model.
  • Visual artifacts and burn-in: Rear-projection TVs have the same visual artifacts as front-projection TVs. Viewers experience the rainbow effect with some DLP sets, and LCD sets sometime have a visible screen door effect. CRT rear-projection sets are also prone to burn-in. In addition, the picture quality of a CRT set can degrade if the individual tubes move out of alignment. You'll need to calibrate the tubes in your set periodically to get the best picture quality. Most new sets will do this automatically.
  • Viewing angle and glare: CRT sets in particular tend to have a poor viewing angle. In addition, some projection TV screens have a noticeable amount of glare. You should check out both of these in person as you're shopping rather than relying on manufacturers' claims.
  • Connections: Rear-projection TVs have the same connection needs as projectors do. Make sure you'll be able to use your existing equipment with your TV. If you plan to upgrade to Blu-ray or HD-DVD, make sure your set has an HDMI connection for digital input.

Check out the links on the next page to learn more about projection TVs and different TV technologies.

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