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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 | |
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 | |
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:
Not seen in home units
| 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).
| |
Used in all home units today
| 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.
 Photo courtesy Texas Instruments MEMS projector using three DMD chips |
Recent innovations in reflective technology have been in the the following areas:
| Microelectromechanical systems (MEMS)
Liquid crystal on silicon (LCOS) | |
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Finishing Touches
In addition to the projection technology, there are a few more components needed complete the projection TV system.Screen
For the most part, the screens used in projection TV systems are similar to
those used in movie theaters (see How
Movie Screens Work for details). However, the choice of screens varies
depending upon whether the system uses front or rear projection. Rear projection
TVs with built-in components do not generally give you a choice of screens. For
information on choosing a screen, see the links
at the end of this article.
Control Panels
For any system, one unit must have the various inputs (VCR,
DVD player) and
outputs (audio, video, S-video) with controls. For a projection TV in a cabinet,
this circuitry is often built-in and operated by remote control. If you are
assembling a home
theater system, it is likely that an audio/video receiver will be the
central unit. If you are equipping a conference room or large lecture hall with
a projection TV, the central piece can be an audio/video
receiver, a computer
or a laptop computer.
I have seen some lecture halls that have a high-quality sound system, LCD
projector and VCR all controlled by a PC equipped with a DVD
drive. The type of control system used is dependent upon the intended use of the
projection TV.
Speakers
All projection TV systems need some type of sound system. If you buy a rear
projection or CRT-based system that is stored all in one cabinet, you will
probably have stereo speakers.
If you are using your projection TV as part of a home theater system, or setting
up for conferencing in a large room, you will most likely bypass the stereo
speakers in the unit and use a more advanced sound system:
| Dolby Surround Sound
| Dolby Pro Logic
| Dolby Digital
| Dolby Digital EX
| Digital Theater Sound
| Home THX | |
The Future of Projection TV
With the advances in MEM and LCD technologies, projectors will become smaller, lighter and have better resolutions. Projection TVs may even allow applications beyond lectures, conferences and home theaters. One such application is for a type of virtual reality that allows you to get into the projected images.
 |
 |
In this system, the LCD projector has a wide-angle lens that projects the image on to a hemispherical screen. The viewer gets the feeling of being inside the projected image. Typically, a computer drives the images on the projector, and can be used for engineering or simulation purposes as well.
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