They're the cheapest path to big-screen heaven these days, until projector prices come down. But watch out for the pitfalls.

For a truly big-screen home-theater experience, a front projector is the only way to go. As the name implies, a front projector sits in front of a separate screen much like the setup of a film projector in a movie theater. And similar to the cinema experience, you'll need to turn the lights way down and block the windows to see the picture in all its glory. There are basically three types of front-projection technologies available today: CRT, LCD, and DLP; and each one has its limitations. The oldest and still the reigning champ in picture quality is the cathode-ray-tube-based (CRT) projection TV. Though CRT-based front projectors produce the best picture, especially in terms of the resolution, the black level, and the contrast ratio, the drawback is that the pictures these projectors offer are rarely bright enough. What you may not realise, though, is that screen size becomes an important factor when dealing with CRT-based front-projection TVs.

Blown up and blown out

Most projectors are capable of producing an image on a 15-foot-wide screen, but you don't really want to make your system do that. Once you exceed the optimum screen size of your projector, you have to drive the machine harder just to light up the screen. This has two nasty side effects: first, the tubes will burn out much faster than they would with more reasonably sized screens, and secondly, the picture quality will begin to suffer.

When you crank the contrast control and drive the projector too hard, you introduce geometric distortion (where straight lines begin to curve), and the colour fidelity goes down the drain. The scan lines that make up the picture also become more visible on a very large screen, especially if you're sitting close to it. Properly set up on a reasonably sized screen, CRTs can last up to about 10,000 hours. However, if you severely overdrive the projector to accommodate a larger screen, the tube life will drop by 60 percent or more. For optimum picture quality on CRT-based front projectors, follow these basic guidelines:

• For 7-inch CRT front projectors, the optimum screen size is a 72-inch-wide, 16:9 aspect-ratio screen.

• For 9-inch CRT projectors, the optimum screen size is an 84-inch-wide, 16:9 aspect-ratio screen.

Fixed-pixel pitfalls

If you're not willing to install a large, heavy CRT projector, you'll find two other common types of projectors: liquid crystal display (LCD) and digital light processing (DLP). Neither of these suffers from the same problems as CRT-based projectors, but the newer technologies do bring some new problems. Both LCD- and DLP-based projectors have plenty of light output to drive big screens. In fact, in the case of DLP projectors, you'll have light output to burn, so to speak.

The image above approximates the screen door effect, which occurs when you shine an LCD or DLP projector at too large a screen or sit too close to the screen.

LCD and DLP projectors are fixed-pixel displays. Rather than creating a video image by scanning an electron beam from left to right (the lines you see when you put your face too close to your TV), with these systems, individual pixels make up the structure of the picture. When you blow up an image on a giant screen then, these pixels become increasingly visible. Very high-resolution LCD and DLP projectors can, of course, project a larger image without the pixel elements becoming visible. But even the highest-resolution LCD and DLP projectors will show the structure of the picture when projecting too large an image.

Calculate the distance between your couch and the screen by multiplying the height of the screen by 3 or 4.

The most common resolutions of both LCD and DLP projectors are 800x600 pixels and 1,024x768 pixels. For high-end, high-resolution projectors, a good rule of thumb would be to use a screen that's no wider than 84 inches. For best results with other 4:3 LCD and DLP projectors, stick with an 84-inch-diagonal, 4:3 aspect-ratio screen. If you start to go beyond these recommendations, you'll get what is commonly referred to as the screen door effect. It's literally like looking through a screen door. You'll see the little square pixels or mirrors that make up the structure of the picture, which is very distracting. The severity of this effect depends upon how far you're sitting from the screen.

Going the distance

The last thing to take into consideration when deciding on the size of the screen you want for any projector is how far you'll be sitting from the screen. There are some basic guidelines to consider here: Assuming there will be video processing (line doubling, tripling, scaling, and so forth), you would multiply the height of the screen by three or four to determine the correct distance between your chair and the screen. Grab a calculator, and follow along with this example: With a 7-inch or 8-inch CRT projector using the ideal screen width of 72 inches, you would multiply 40.5 inches (the height of a 72-inch-wide, 16:9 screen) by three. You'll find that, ideally, you should be seated about 121.5 to 162 inches--or 10 to 13.5 feet--from the screen. Provided that you have a high-quality projector with good video processing, this arrangement should yield excellent results. To further minimise the screen door effect with DLP and LCD projectors, multiply the height of the screen by four rather than three.

So do yourself--and your projector--a favor and resist the temptation to go with a monster-sized screen just because it's big. Take into serious consideration the type of projector you will be using, the room constraints, and the seating distance from the screen. Once all that is out of the way, you can start planning where to put the ticket office.

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