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.
Photo courtesy Philips Research
Projection TV display in the lab showing a bright, contrast-rich, high resolution picture on a 64-inch screen
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In this article, we will look at 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 (see How Television Works for details).
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.
Photo courtesy Philips Research
Rear or reflective projection display system
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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.
Photo courtesy Philips Research
Front or transmissive projection display systems
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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:
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.
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)
We'll discuss the new technology of MEMS next.
Microelectromechanical Systems
MEMS have a movable or deformable reflective surface on top of a semiconductor chip.
The chip generates voltages in response to digital information. The
voltages change the shape of the reflective surface rapidly and in a
controlled way to produce the image that was encoded by the digital
information. The projected light bounces off the reflective surface and
gets collected by the projector lens.
Digital Micromirror Devices (DMD)
DMDs, also called digital light processing (DLP), were developed by Texas Instruments. The DMD is a chip that has anywhere from 800 to more than 1 million tiny mirrors on it, depending upon the size of the array.
Photo courtesy Texas Instruments
Micrographic photo of ant leg on the DMD surface: Each mirror is 16 µm2, with 1 µm separation between pixels.
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Each mirror rests on support hinges and electrodes.
Photo courtesy Texas Instruments
The top left view shows nine mirrors. In the top
right, one central mirror was removed to expose the underlying
hidden-hinge structure. The bottom right shows a close-up view of the
mirror substructure. The mirror post, which connects to the mirror,
sits directly on the center of this underlying surface. The bottom left
shows several pixels with the mirror removed.
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Find out more about DMDs in the next section, or skip ahead to learn about the technology of grating light valves.
Digital Micromirror Devices
The DMD is a chip that has anywhere from 800 to more than 1 million tiny mirrors on it, depending on the size of the array. Each 16-µm2 mirror (µm = millionth of a meter) on a DMD consists of three physical layers and two "airgap" layers. The airgap layers separate the three physical layers and allow the mirror to tilt +10 or -10 degrees.
When a voltage is applied to either of the address electrodes, the
mirrors can tilt +10 degrees or -10 degrees, representing "on" or "off"
in a digital signal.
Photo courtesy Texas Instruments
A blow-up of two mirrors on the DMD, one "on" and one "off"
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In a projector, light shines on the DMD. Light hitting the "on"
mirror will reflect through the projection lens to the screen. Light
hitting the "off" mirror will reflect to a light absorber. Each mirror is individually controlled and is totally independent of all the other mirrors.
Photo courtesy Texas Instruments
Incoming light hits the three mirror pixels. The two
outer mirrors ("on") reflect the light through the projection lens and
onto the screen, producing square, white pixel images. The central
mirror is tilted ("off") and reflects light away from the projection
lens to a light absorber -- no light reaches the screen at that
particular pixel, so a square, dark pixel image is produced. In the
same way, the remaining mirror pixels reflect light either to the
screen or away from it.
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Each frame of a movie is separated into its red, blue, and green
components and digitized into 1,310,000 samples for each color. Each
mirror in the system is controlled by one of these samples. By using a color filter wheel
between the light and the DMD, and by varying the amount of time each
individual DMD mirror pixel is on, a full-color, digital picture is
projected onto the screen.
Photo courtesyTexas Instruments
White light is forced down onto a color wheel
filter. This wheel spins in sequence with the red, green and blue video
signals being sent to the DMD. Mirrors are turned "on" depending on
where and how much of each color is needed for each TV field. The human
visual system integrates the sequential color and sees a full-color
image.
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Photo courtesy Texas Instruments
This digitized photograph of a parrot demonstrates the seamless, film-like DMD picture.
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Because the micromirrors are only 1 µm apart, an image takes up a
larger percentage (89 percent) of space on the DMD chip's reflective
surface, as compared to a typical LCD (12 to 50 percent). This reduces
the pixelation of the image.
Photo courtesy Texas Instruments
Close-up photo of the parrot's eye projected through
(A) a three-panel poly-silicon VGA resolution LCD projector and (B) a
one-chip VGA resolution DMD projector. Both the LCD and DMD photos were
taken under the same conditions, with each projector optimized for
focus, brightness and color.
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If you think this technology is cool, check out the GLV technology in the next section!
Grating Light Valves
Another MEM device is the grating light valve (GLV). GLV
technology, licensed to Sony, was developed by Professor David Bloom at
Stanford University, and is now produced by Silicon Light Machines in
Sunnyvale, California.
The GLV chip consists of tiny reflective ribbons mounted over a silicon chip.
The ribbons are suspended over the chip with a small airgap in between.
When a voltage is applied to the chip below a ribbon, the ribbon moves
toward the chip by a fraction of the wavelength of the illuminating light. The deformed ribbons form a diffraction grating,
and the various orders of light can be combined to form the pixel of an
image. The shape of the ribbons, and therefore the image information,
can be changed in as little as 20 billionths of a second.
To make a projector, the GLV pixels are arranged in a vertical line that is 1,080 pixels long. Light from three lasers,
one red, one green and one blue, shines on the GLV and is rapidly
scanned across the display screen at 60 frames per second to form the
image.
Photo courtesy Silicon Light Machines
Diagram showing the scanned architecture of GLV
technology: As the scan moves horizontally, the GLV pixels change to
represent columns of video data, thereby forming one two-dimensional
image per scan. The scanning rate is 60 frames per second.
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A major advantage of GLV technology is that GLV chips can make
high-resolution images at a relatively low cost. For example, because a
1,920x1,080 pixel image can be achieved by scanning a 1,080-pixel
linear array, a GLV chip can be manufactured to achieve this resolution
with only 1,080 pixels, instead of the 2 million needed for other
technology, such as DMD. Also, because the ribbons are aligned
vertically, there are no horizontal gaps in the image -- there is a
very high fill space on the chip.
In addition to MEMS, a special liquid crystal technology has
been developed for reflective projectors. See the next section for
details.
Liquid Crystal on Silicon
LCOS is a relatively new LCD technology. In contrast to nematic twisted LCDs,
in which the crystals and electrodes are sandwiched between polarized
glass plates, LCOS devices have the crystals coated over the surface of
a silicon chip. The electronic circuits that drive the formation of the image are etched into the chip, which is coated with a reflective (aluminized) surface. The polarizers are located in the light path both before and after the light bounces off the chip. The advantages of this setup are:
- The LCOS devices are easier to manufacture than conventional LCD displays.
- They have higher resolution because several million pixels can be etched onto one chip.
- They can be much smaller than conventional LCD displays.
While LCOS devices have been made for projection TVs (such as those made by Philips), they can also be used for microdisplays used in near-eye applications like wearable computers and heads-up displays.
Photo courtesy Colorado MicroDisplay, Inc.
Example of LCOS device used for a microdisplay
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For an LCOS projector, the following steps are involved:
- A digital signal causes voltages on the chip to arrange in a given configuration to form the image.
- The light (red, green, blue) from the lamp goes through a polarizer.
- The light bounces off the surface of the LCOS chip.
- The reflected light goes through a second polarizer.
- The lens collects the light that went through the second polarizer.
- The lens magnifies and focuses the image onto the screen.
There are several possible configurations when using LCOS: A projector
might shine three separate sources of light -- red, green and blue --
onto different LCOS chips; it might use one chip and one source with a
filter wheel; or it might utilize some combination of these two
options.
Now that we have discussed the latest advances in projector
technology, let's take a look at the remaining parts of a projection TV
system.
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
For a detailed discussion of these various sound systems, see How Surround Sound Works, How Movie Sound Works and How THX Works.
For a look at some future developments in projection TV, see the next section.
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.
The ELumens VisionStation projection TV system
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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.
For lots more information on projection TV and related topics, check out the links on the next page.
Lots More Information
Related HowStuffWorks Articles
More Great Links
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