Direct-view televisions are the sets that we're all familiar with. They have a cathode ray tube (CRT), with a scanning electron gun that paints the picture on a phosphor-coated screen and a tuner that picks up broadcast signals. Good direct-view televisions deliver an excellent picture, but because of the tube technology, they are limited in size. The biggest direct-view television screen you can get these days measures 40 inches across (diagonally).

             This is a pretty big picture, of course, and will work well in a basic home theater setup. You might even be content with a 27-inch model. The general rule for television size is that you want a screen that measures about one-third your distance from the screen (if you sit 9 feet from the screen, a 36-inch television screen would be perfect). These are the guidelines for standard televisions, because if your screen is bigger, or you sit closer, the scan lines that make up the picture will seem fairly large, which translates to a lower resolution. This is inherent in the standard television signal -- it has a set number of vertical lines of resolution -- the number of horizontal lines in one screen -- no matter how big your screen is. High-definition television (HDTV) has more vertical lines of resolution, so you'll be able to sit closer and still see a clear picture when watching HDTV-formatted video.

             When looking for direct-view televisions, pay attention to image contrast. A television with a darker screen will give you a better picture because there will be a stronger contrast between light and dark -- black will actually appear black, rather than gray. You should also look for a television with a flatter screen. If the tube is more curved, the picture will be more distorted and you'll see more glare from other light sources. A perfectly flat screen will usually give you the best picture.

             If a very large screen size is important to you, look into rear-projection televisions. These sets don't have the same size constraints as direct-view televisions because they don't use the cathode ray tube for the display. Instead, they use a projection screen. Projection televisions actually use three different cathode ray tubes that split up the video signal into three different colors -- red, green and blue. The different combinations of these colors of light can produce the entire visual spectrum. Inside the television, the three CRTs project onto a mirror, which bounces the full-color image up to a screen.

             The advantage of these televisions is that you can get a very large picture for a relatively low price. For example, you can get a 45-inch screen for less than $1,500. At first introduced, there were some major drawbacks with rear projection TVs; the picture had a fairly low resolution, and it wasn't nearly as bright as a direct-view set. Recent rear-projection models have overcome these shortcomings, and are approaching the picture quality of direct-view televisions.

             Some rear-projection sets may have a smaller viewing angle that direct view sets. No matter where you sit in front of a direct-view television, the screen maintains the same picture quality. If you look at a rear-projection screen from an extreme angle, the picture may be much darker and you won't be able to see what's happening on the screen. Newer projection sets use high-quality screens that work well from most angles, but older sets may have a fairly narrow viewing area.

             If you're looking to buy a rear-projection television, the main things to compare are cathode-ray-tube size and design and screen quality. Larger CRTs will project a better picture, and glass lenses work better than plastic ones. Darker screens are better because they present an image with better light-and-dark contrast. You should also look for a screen made of glare-resistant material.

             Standard front-projection televisions work in pretty much the same way as rear-projection televisions, but the system is not contained in a television case. They are set up more like a film projector -- the three CRT projectors are combined in one unit, and you center the television image on a separate fabric screen.

             The main advantage of a front-projection television is very large screen size. Since the components don't have to be packaged together, screen size is limited mainly by the room space. Screens as wide as 200 inches are not uncommon.

             One drawback of front-projection televisions is that they are difficult to install, and they may require extensive maintenance. You have to mount them to the floor or ceiling at the right distance from the screen. For standard CRT models, you also have to align the different CRTs so that the different color pictures are lined up exactly. Otherwise, the images in your picture will have a color fringe around the edges. Usually, you need a professional to install the unit properly.

             Another drawback is they only work properly in a darkened room, so they are really only suitable for a separate home theater space, rather than a family room or ordinary den. Since they are designed for watching movies, front projectors don't usually have a built-in television tuner: They don't receive television signals themselves, so they must be hooked up to a separate tuner (such as the tuner in a VCR).

             In recent years, several new front-projection technologies have hit the market. Liquid crystal display systems have only one projector and lens, so you don't have to bother with configuring the different colors. These units generate the video picture on an LCD screen, and then project this image. This makes it much easier to set up your system.

             Another system, the light-valve projector, combines CRTs and LCDs. These projectors use three CRTs and three LCD screens. They project the three LCD pictures on the screen, just like a CRT projector. By combining the two technologies, these units project a clearer picture than either CRT or LCD models. The drawback is that these units are much more expensive -- you can get a CRT or LCD projector for less than $2,000, but you can expect to pay $6,000 or more for a light-valve model.

             The other major option is Texas Instruments' Digital Light Processing (DLP) technology. DLP projectors generate pictures by manipulating a panel of thousands of tiny, sensitive mirrors on a semiconductor chip (called the Digital Micromirror Device or DMD chip). A high-power lamp shines light through a rotating color wheel, which rapidly changes the color from red to green to blue. The colored beam hits a semiconductor chip covered in more than a million hinged mirrors.

             Based on the information encoded in the video signal, the semiconductor chip tilts some of the tiny mirrors to reflect the colored light out of the projector, towards the screen, and tilts some mirrors so they don't reflect any light.

             Most individual mirrors are actually flipped "on" (reflecting light) and "off" (not reflecting light) several thousand times per second. A mirror that is flipped on a greater proportion of the time will reflect more light and so will form a brighter pixel than a mirror that is not flipped on for as long.

             DLP projectors boast the clearest, brightest pictures of the front projector technologies. Like light-valve projectors, they are relatively expensive. But prices are dropping rapidly. These days, you can pick one up for as low as $1,600.

             The most recent addition to the world of televisions is plasma flat-screen technology. These televisions don't have CRTs or projector devices, so they have extremely thin designs. The typical plasma screen is less than 6 inches deep. These televisions are also very light, so it's fairly easy to mount one on your wall. If you plan to set up a home theater in a smaller room, this is a definite plus -- you don't have to worry about hauling a giant direct-view or rear-projection model in, and you don't need to figure out where to position a projector.

             Plasma televisions create pictures with an array of cells that receive a constant flow of low-pressure neon and xenon gas. The cells are arranged in a matrix between sheets of thin glass and are covered with electrodes. When an electrode applies a charge to a particular cell, the voltage ignites the gas, changing it to plasma, which emits ultraviolet (UV) light. The UV light activates colored phosphors on another layer, and the phosphors emit visible colored light (this is the same basic process that occurs in a fluorescent lamp).

             Each cell is dedicated to a particular color -- red, green or blue. Each pixel, the individual dots that make up a television image, has three different cells, one for each color.

             Plasma displays offer great picture quality, but not always the best. They may take the lead as the preferred future technology, but at this point their performance advantages may not justify the price, which is upwards of $6,000. The real benefit of a flat plasma screen is its compact size, and if you have a small theater space, this may be reason enough to shell out the extra money.