One of the biggest technology stories of the new century is the development of flat-matrix electronic displays for front and rear projection of super-size, high-brightness video and computer graphics. As recently as 1990, these products were still on the drawing board. Today they're a prominent part of stage sets; mounted in enclosures, stacked up on scaffolding, or flown from rigging.
The term flat matrix may be unfamiliar to you, but chances are you've heard of Digital Light Processing™ (DLP™), liquid-crystal displays (LCDs), or even plasma display panels (PDPs). All of these are members of the flat-matrix family, as are other technologies such as liquid-crystal-on-silicon (LCoS).
Flat-matrix displays have revolutionized the entertainment industry. Now, it's possible to show electronic images with sufficient brightness to keep pace with bright stage wash levels (and often exceed them). More importantly, those images can have a much higher resolution than standard video. Best of all, it's now an easy task to seamlessly mix a wide range of video and computer sources and show them on flat-matrix displays.
The Basics
Although DLP, LCD, and LCoS do their jobs differently, they have one thing in common: All three are considered light-shuttering systems. That is, they modulate the light from a high-power projection lamp to change the brightness and contrast of an image from fully off to fully on, and any level in between. The projection lamp always runs at the same power level.
In contrast, your everyday television set creates different levels of image brightness by varying the intensity of the scanning electron beam. And there's a finite level of brightness that can be generated by that beam before it simply burns out.
Here's another difference: The resolution of that same electron beam, its ability to draw images with fine detail, is directly dependent on the intensity of the beam. Crank up the brightness, and the picture becomes fuzzier. Turning down the brightness will make the image sharper, but ambient lighting levels must also be reduced to discern the image.
Not so with flat-matrix projectors. Their ability to resolve images with lots of detail is completely unaffected by the brightness of those images. In fact, the resolution of a flat-matrix projector is the same whether the projector is turned on or off. That resolution is a function of the picture-forming elements used in the projector, and the pixels in those elements can be counted in vertical and horizontal rows.
You can actually see the pixels in a DLP chip or LCD panel by using a powerful magnifying glass. Imagine an office building with hundreds of windows, and you have some idea of what an LCD panel looks like. As for a DLP chip, think of those rows of spectators who hold up cards to form large signs and pictures at football games, and you've got some idea of how DLP works.
Let's take a closer look at each of the flat-matrix systems and see how they differ:
Liquid-crystal displays (LCDs) have been around a long time. The principles of LCD imaging were first discovered in the 1880s, not long after the telephone was invented. Further developments took place after World War II in LCD imaging, and the first commercially available LCD products (watches and calculators) appeared in the early 1970s.
The liquid-crystal display panel has thousands of tiny pixels, each of which contains a liquid-crystal suspension. These tiny liquid crystals are scattered randomly within the pixel until a voltage is applied, at which point they will align precisely. Light passing through the pixel is polarized into two beams (a principle known as birefringence), so the amount of light can be controlled by changing the alignment of the liquid crystals.
This process can happen quickly enough and smoothly enough to show full-motion video. If the LCD panels are large enough — such as those used in your notebook computer — color filters can be applied directly to each pixel to achieve full-color images. If the panels are smaller (like those used in high-brightness projectors), then three panels must be used with dichroic mirrors and combining prisms to achieve full-color imaging. The majority of transmissive LCD projectors use panels manufactured by Seiko Epson and Sony.
Most LCD projectors use this process of transmissive light shuttering. Calculators, personal digital assistants, cell phones, and other small displays also use transmissive LCD panels. They are available in sizes as small as .7" (diagonal) and with resolutions as high as 1,365 × 1,024 pixels (1,365 horizontally by 1,024 vertically). Ultra-portable projectors typically employ .7" and .9" LCD panels, while larger models can use 1.3", 1.5", and even 1.8" panels.
Liquid crystal on silicon is another form of LCD that creates images by reflecting the light off a mirrored surface and through individual pixels. While it may seem fantastic to do this in the same light path, remember that the light from the projection lamp is polarized. This makes it possible for two beams to travel in the same space, through a collection of mirrors and prisms that often resemble air-conditioning ducts.
One version of LCoS is the Digital Image Light Amplifier™, or D-ILA™. This technology was developed by JVC, and is now available in a series of desktop and large-venue projectors. The actual device is quite small — .9" diagonally — and three are used to create full-color images. Light from the projection lamp (typically xenon) enters the front surface of the D-ILA after being polarized. The mirrored backplane then changes the angle of the light as it is reflected back out, and the shutter action of the individual pixels determines exactly how much light is reflected.
Is there an advantage to reflective LCD imaging? Possibly. The tiny transistors that control the light-shuttering action of the pixels are mounted on the back of the panel, out of the light path. In contrast, the light in transmissive LCD systems must pass through not only the pixels, but the tiny control transistors that are mounted on each pixel as well.
On the other hand, the light path in a transmissive LCD projector is quite simple, and resembles that of a slide projector. The light path in a D-ILA projector is far more complex, due to the two-way “highway” required.
D-ILA projectors also provide high-resolution imaging. Current models feature 1,365 × 1,024 pixels, enough to show SXGA and XGA graphics plus HDTV. Later this year, JVC plans to release panels with 2,048 × 1,536 pixels, sufficient to show the highest resolution HDTV images with no compression.
Digital Light Processing™ is a proprietary technology developed by Texas Instruments. Although it is quickly gaining acceptance in projectors and even consumer rear-projection TV sets, DLP was originally developed for laser printer imaging. The heart of a DLP system is the Digital Micromirror Device™, or DMD™, a small chip that has thousands of tiny moving mirrors formed on it.
Each mirror has two positions, On and Off, determined by a movement of about 10°. There are no in-between steps with a DMD, so image brightness, contrast, and grayscale are determined by an all-digital process known as pulse-width modulation. This PWM system creates levels of brightness by a ratio of On cycles to Off cycles in a given time interval.
If there are more On cycles than Off, the image will appear brighter. If there are more Off cycles than On, the image will appear darker. The tiny mirrors in the DMD are quite busy, cycling back and forth hundreds of times each second (no, you can't hear them) so the projected images appear without any noticeable flicker.
As with LCD technology, DMDs are basically black-and-white devices, so three DMDs are used with dichroic mirrors and a combining prism to achieve full-color images in large-venue projectors. Several companies, including Digital Projection, Barco, Christie Digital, NEC Technologies, Panasonic, and Sony manufacture these projectors. Currently, DMDs are available with resolutions as high as 1,280 × 1,024, and in sizes as small as .7" (1,024 × 768 pixels).
The Same, But Different
All three of these flat-matrix technologies act as light shutters. Think of Venetian blinds, and you've got transmissive and reflective LCD. Think of those flash cards from the football game, and you've got DLP. One important distinction between the three is that only DLP is a purely digital imaging technology, while LCD imaging does use some analog steps.
The three-panel/chip color process is also similar, although the optical paths are not. Light from a projection lamp (typically xenon, metal-halide, or small-arc mercury vapor) passes through a series of special dichroic mirrors, which act as pass/reject filters. In this way, the red, green, and blue components of the lamp from the projector are separated, and directed toward three individual LCD panels or DMD chips.
The same black-and-white electronic images appear on all three LCD or DLP devices. Red light passes through or is reflected by one device, while green light passes through/reflects from the second, and blue light is similarly processed by the third. These three monochromatic images are then recombined in a special prism to produce the final full-color image. As you might guess, the three devices must be precisely aligned for this process to work correctly.
LCD and DLP imaging devices can withstand a fair amount of heat, so they are suitable for use in high-brightness projectors. Infrared light is often “detoured” away from the optical prism to a device known as a heat dump in DLP projectors, which helps to keep the imaging surfaces cooler. Right now, the brightest projectors (10,000 to 12,000 lumens) all employ DLP technology and some sort of heat dump in the optical path.
Transmissive LCD projectors aren't quite as bright, but there are several models available from Barco, Sanyo, Sony, and Sharp that can produce over 4,000 lumens. As for reflective LCDs, JVC recently showed a high-brightness D-ILA projector that is rated at 5,000 lumens and will be available later this year. It's a simple matter to stack projectors to attain brighter images, and most large-venue LCD and DLP projectors have built-in stacking “detents” for that purpose.
It bears mentioning again that all flat-matrix projectors have a predetermined resolution, whether they are powered on or switched off. This means they'll look their best when the images they show precisely match the projector's native resolution. That's not always practical, particularly if you are showing lower-resolution interlaced video.
All of the LCD and DLP projectors offered for sale accept a wide range of signal formats, from interlaced composite video to progressive-scan component computer and HDTV signals. However, the quality of their signal processing circuits varies from manufacturer to manufacturer.
Plasma Display Panels
Plasma displays have really caught on as a “hip” way to show video. Because of its thin profile and comparatively light weight, it can be mounted anywhere, or even flown. At least one company (NEC Technologies) offers a four-way stacking configuration for its plasma panels, and they have been built into large videowalls and even a huge dance floor under the Pioneer booth at InfoComm 2000.
Plasma panels may seem fairly exotic, but they have a closely related cousin — the common fluorescent lamp. Each PDP is made up of thousands of tiny red, green, and blue pixels, and each pixel is filled with a rare gas mixture, such as xenon and neon or argon and neon. When a voltage is discharged through the pixel, the gas mixture changes into a plasma state and conducts electricity while emitting ultraviolet light.
This ultraviolet light then strikes the red, green, and blue phosphors, causing them to glow. The duration of the charge/sustain/discharge cycle determines how brightly each pixel will glow, and thus creates full-color images of varying brightness and contrast. As a result, plasma panels are considered emissive displays, much like a conventional television monitor with a cathode-ray tube (CRT).
Since the viewer is looking directly at the source of the light, the resulting images are very bright and hold up well even in high ambient light levels. They're being used in retail stores, theme parks, at trade shows, and in public concourses such as airports. Plasma display panels are available in a number of sizes, from as small as 25" to as large as 50" (with 60" models coming soon).
PDPs are “full members” of the flat-matrix family and have a fixed pixel structure. For the most common panels (42" widescreen), the pixel count is either 852 × 480 or 853 × 480. Smaller panels are available with 1,280 × 1,024 resolution, and there's even a 24" widescreen panel with 1,920 × 1,024 pixels. 50" widescreen panels typically have 1,280 × 768 or 1,366 × 768 pixels, as will the new 60" models.
The list of plasma retailers is long, but most plasma glass panels are manufactured by a few companies including NEC, Fujitsu, Hitachi, Samsung, LG, Matsushita, and Pioneer. The raw glass is also incorporated into panels sold by Sony, JVC, Philips, Zenith, and Sharp, to name a few.
Pick a Display, Any Display
The sky's the limit when you want to incorporate any of these electronic imaging technologies into a production. In general, front- and rear-projected images look best when ambient light is kept off screens (that holds true whether you use a 1,000- or 10,000-lumen projector), but it's safe to say that if you can tolerate higher black levels, the brighter projectors will be able to keep up with intense stage washes.
Plasma panels are quite tolerant of high ambient lighting levels, but their imaging area is more limited than a projected image. Both PDPs and LCD/DLP projectors can be mixed within a production, as can self-contained projection cubes. These cubes, which use single-panel LCD or single-chip DLP imaging systems for simplicity, are often stacked into walls (square, rectangular, and triangular), or even arranged in arcs.
Whichever way you choose to go, all of these flat-matrix imaging systems offer high brightness, good contrast ratios, and high resolution. The choice of video and computer signal sources is yours, along with how you'll want to work LCD, DLP, and/or plasma into your next production.