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Projection Television

Projectors | Plasma | L.C.D. | D.L.P.

How Projection Television Works

If you're thinking about assembling a home theater system, you may be looking at large screen televisions as the heart of your system. Projection TV could give you the size that you want -- CRT screens generally top out at 40" (101 cm) or so, and at that size, they are huge and heavy. Plasma screens can be bigger than that and still manageable, but they can be extremely expensive. Projection TV technology can create large screen sizes at a reasonable price.

Or maybe you need to equip a room, like a classroom or conference room, for multimedia presentations with a large audience. A projection TV gives you a lot of flexibility and is usually much better than the standard combination of a 35mm slide projector, overhead projector and TV/VCR.

Here you will find an explanation of the technology of projection TVs and portable projectors, and find out how they work to display such large, clear images.

Projection vs. Conventional TV

A conventional TV uses a device called a cathode ray tube (CRT) to display its image.


The negatively-charged cathode makes a beam of electrons that is drawn toward the screen by the positively charged anode. The beam strikes at the front of the CRT, which is coated with special chemicals called phosphors. When the beam strikes the phosphor, it lights up that area of the tube (a pixel). For a color TV, there are three electron beams and three types of phosphors on the tube for each pixel -- red, blue and green -- which, when excited, can make any color. The electron beam moves along the tube, either horizontally or vertically, using magnetic coils alongside the tube.

Because the CRT is made of glass, there are limits to its maximum size. Today's CRT TVs usually measure less than 40 inches (101 cm) diagonally. This size is not practical for a home theater or large auditorium.

In contrast to conventional TVs, projection TVs form a small image on a device inside the projector -- either a CRT or LCD -- and then shine that image onto a large screen located elsewhere. In one type of projection TV, the screen is located within the TV box itself. This type of projection TV is called a rear or reflective projection. In this type, light reflects off the projection display panel and is then projected onto the screen.


Rear or reflective projection display system

In another type of projection TV, the screen is located across the room. In this type of projection TV, called a front or transmissive projection, light passes through the image-forming display panel and is then projected onto a screen.


Rear or reflective projection display systemFront
or transmissive projection display systems

Now let's see how these projection systems work.

Creating Projection TV

A projection TV has four basic parts:

 Projector
 Screen - separate or built-in
 Control panel - separate or built-in
 Sound system - separate or built-in

The arrangement of these parts varies with the different types of projection TV. Most rear projection TVs for home theater systems tend to look like very large-screen conventional televisions. Each one is basically a large box that contains all of the above parts. Rear projection TVs for conference rooms can have a separate projection room behind the screen.

In contrast, front projection TVs are spread out. The projector is at one end of the room, the screen is at the other end, the control panel may be in the middle (on a table or as a mobile device such as a laptop computer) and the speakers may be located in different parts of the room.

Each of these components will be examined in detail in the following sections.


Projectors

The projector is the heart of the projection TV system -- this is where the technological advancements have centered. The projectors used in these systems rely on two general approaches:

 Transmittive projectors - Shine light through the
 image-forming element (CRT tube, LCD panel)  Reflective projectors - Bounce light off the
 image-forming element

In both types of projectors, a lens collects the image from the image-forming element, magnifies the image and focuses it onto a screen. Also, it is important not to confuse reflective projectors with rear projection. The terms "transmittive" and "reflective" refer to the optoelectronics inside the projector, not to how the projector is arranged within the projection TV system.

Some of the most progressive technologies use the reflective approach, but the transmittive approach has been around longer and appears in many of the small portable projectors on the market today. We'll look at transmittive technologies here and then look in depth at several different reflective technologies.

Transmittive Projectors
Transmittive projectors use two basic image forming elements:

 CRTs
 LCDs
Both types are discussed below.

CRT
Like conventional TVs, some projectors have smaller CRT tubes built into them. These tubes are small (perhaps 9-inch diagonal), expensive and extremely bright. In the basic layout, you have one or more CRT tubes that form the images. A lens in front of the CRT magnifies the image and projects it onto the screen. There are three CRT configurations used in CRT projectors:

 One color CRT tube (red, blue, green phosphors) displays an image with one projection lens.

 One black-and-white CRT with a rapidly rotating color filter wheel (red, green, blue filters) is placed between the CRT tube and the projection lens. The rapid succession of color images projected onto the screen forms an apparently single color image (the images are projected too quickly for your brain to distinguish between them).

 Three CRT tubes (red, green, blue) with three lenses project the images. The lenses are aligned so that a single color image appears on the screen.

One of the problems with CRT projectors is that, with anywhere from one to three tubes and accompanying lenses and/or a filter wheel built in, the projectors can be quite heavy and large. Also, CRT devices do not have the fine resolution that LCD devices do, especially when projected.

LCD
To make projectors lighter and increase their resolution, newer LCD technologies have been developed (see How LCDs Work for details on LCD panels). Transmitted LCD projectors use a bright light to illuminate the LCD panel, and a lens projects the image formed by the LCD onto a screen. There is not a huge difference between the LCD panels used in projectors and those found in something like a PDA, except that the LCD is smaller and backlit by a very bright halogen lamp. The LCD acts very much like a color slide in a slide projector. The advantage of this approach is that the projector can be very small.

The most exciting advances in projector technology can be found in reflective projectors.

Reflective Projectors
In reflective projectors, the image is formed on a small, reflective chip. When light shines on the chip, the image is reflected off it and through a projection lens to the screen.


MEMS projector using three DMD chips

Recent innovations in reflective technology have been in the the following areas:

 Microelectromechanical systems (MEMS)
    Digital micromirror device (DMD, DLP)
    Grating light valve (GLV)
 Liquid crystal on silicon (LCOS)

Projectors | Plasma | L.C.D. | D.L.P.