How Projection Television Works

Buying a Front Projection TV

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

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.