How projection TV works

By George Garza
Info Guru,

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Large screen TV
Even though projection TV models don't have the quality of a direct-view CRT set, the pictures can be much bigger without having very large and bulky TV set taking up space in your living room
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Here's what makes projection TVs a great option.

The cathode ray tube (CRT), the technology basis of TV's, is very reliable and has good picture quality. But it is limited by screen size because the screen is made of glass. The largest CRT screens measure about 40 inches diagonally. A CRT TV with a screen that size is deep, heavy and unwieldy.


Enter the Projection TV


Even though projection TV models don't have the quality of a direct-view CRT set, the pictures 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.


The projector and the traditional image source can be either a CRT or a liquid crystal display (LCD) and then it shines that image onto a large screen located elsewhere. Liquid crystal displays are common and found in clocks and laptop computers.


How it Works


Projection TV systems consist of the projector unit, a projection screen and the relevant control electronics. In the projection unit there are two basic design deployments: the front and rear projection.


Front Projection


In this system, the image path starts from the projection onto a screen situated elsewhere in the room. The projected image size is normally limited only by the size of the room itself. For optimum performance a front projection set-up calls for a room where you have control over the room environment. In particular you need to have all lights switched off completely.


Rear Projection


In this system the image path starts from the projection and goes to a mirror within the projection box itself. The mirror then bounces the image on a screen located at the front of the box which houses the various components of the video projection system.


The advantage is that this makes a rear projection TV similar in to a large-screen conventional television. A rear projection system is better since room lighting does not interfere with the light path between the projection unit and the projection screen on the front of the TV. 



The Control Electronics


Detailed pictures are possible from both configurations. They can use small reflective or transmissive devices. The reflective devices pick up the picture by bouncing off the device. The transmissive devices pick 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.


Transmissive-Type Video Projectors


Transmissive-type projectors use either CRT tubes or an LCD type display to generate the image.


CRT Type


Typically this technology uses small 9" tubes, which are similar to those used in conventional TV sets except that these are extremely bright and more expensive.


LCD Type


These make use of a small color LCD display panel to generate the image. A bright light source is used to backlight the LCD and an optical lens arrangement is used to project the image formed by the LCD onto a screen.


Reflective-Type Video Projectors


There are two types of technologies for the reflective type of video projectors: the liquid crystal on silicon (LCoS) and digital micromirror device (DMD). With LCoS, light passes through a liquid crystal layer and it bounces off a reflective surface. When the light returns back through the liquid crystals, the crystals act as light valves to create the light and dark areas of the picture.


With DMD a small, rectangular device uses microscopic mirrors to make a picture. The mirrors point toward a projection lens or away from it.


Both Options are Good


A rear projection TV gives you an immediate affordable solution for a thoroughly big screen TV. All you have to do is unpack the product and there you have a big screen TV in front of you.


A front projector TV, on the other hand, resembles the movie theater experience. If you have the room space, this is the way to go. Home theater projectors are more affordable than ever and can support video projections up to 300-inch diagonal.


With these technologies, you have an idea how projection TV works. The best part is that you can get very large pictures without having very large and bulky TV sets.

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