What Are Rear-Projection TVs?

Rear-projection TVs are TV sets that cast their video pictures by projecting them from behind the screen and then reflecting it, as opposed to projecting it from in front of the screen, like films in movie theaters do. Rear-projection TVs use technology very similar to old-fashioned cathode ray tube (CRT) TVs, and many even use their own version of CRTs. While flat-panel TVs are growing more popular, rear-projection high-definition (HD) TVs have a niche among consumers, mainly due to their combination of higher picture quality and lower cost, especially with the advent of digital light processing (DLP) TVs. CRT and DLP are the two most common types of rear-projection TVs.

Function

A rear projector TV uses a type of projector to transmit its video image. This image then passes through lenses that magnify the image. The magnified image must then be reflected off mirrors so that it displays the picture backwards on the inner side of a glass screen. The viewer sees the image on the screen's outer side, which is inverted, so it appears correctly. All of this is contained within an airtight box, so no light can escape from within and all light is used to project the image.

CRT TVs

Cathode ray tube sets use the same method used by older CRTs, where an electron beam in a vacuum tube projects the image onto the mirror and screen. The biggest difference from its predecessors is that instead of one CRT, the rear-projection model uses three small CRTs---one for each of the three main colors, red, green and blue.

DLP TVs

DLP TVs use a digital micromirror device (DMD), which is a chip containing millions of tiny aluminum mirrors. The image is displayed on the DMD, and light from the TV's internal lamp is shone onto the chip with the help of lenses and a rotating color wheel. The light reflects off the mirrors and then passes through a projection lens, where it then reflects off one more larger mirror and onto the screen.

Comparisons

CRT-based TVs are often less expensive to produce, since they use older technology, and are usually priced much lower. They also have much more flexibility in picture resolution because they don't use a fixed pixel field like their DLP and LCD counterparts. However, they are also much larger (deeper) than other TVs and have a greater risk of screen burn-in. DLP sets produce high-contrast, smoother pictures and consume less power, but can sometimes produce a brief flash of colors called a "rainbow effect." Also, its lamp bulb must eventually be replaced.

LCD Rear-Projection TVs

There are a few models of liquid crystal display (LCD) rear-projection TVs. These use technology that is essentially a combination of CRT and DLP, sending a strong light through a transparent chip of pixels, which then travels through magnifying lenses and a large mirror to the screen. LCD rear projectors can produce the same high contrast at low power as the DLP models. However, LCD models are usually much more expensive, and their pixels can often be seen on a big screen. Generally, flat-panel LCD TVs are in higher demand than their rear-projection counterparts.