Digital Light Processing (DLP) is a technology used in projectors and video projectors. It was originally developed at Texas Instruments, in 1987 by Dr. Larry Hornbeck.
In DLP projectors, the image is created by microscopically small mirrors laid out in a matrix on a semiconductor chip, known as a Digital Micromirror Device (DMD). Each mirror represents one pixel in the projected image. The number of mirrors corresponds to the resolution of the projected image. 800x600, 1024x768, 1280x720, and 1920x1080 (HDTV) matrices are some common DMD sizes. These mirrors can be repositioned rapidly to reflect light either through the lens or on to a heatsink (called a light dump in Barco terminology).
The rapid repositioning of the mirrors (essentially switching between 'on' and 'off') allows the DMD to vary the intensity of the light being reflected out through the lens, creating shades of grey in addition to white (mirror in 'on' position) and black (mirror in 'off' position).
Digital micromirror device
A Digital Micromirror Device, or DMD is an optical semiconductor that is the core of DLP projection technology, and was invented by Dr. Larry Hornbeck and Dr. William E. "Ed" Nelson of Texas Instruments (TI) in 1987.
The DMD project began as the Deformable Mirror Device in 1977, using micromechanical, analog light modulators. The first analog DMD product was the TI DMD2000 airline ticket printer that used a DMD instead of a laser scanner.
A DMD chip has on its surface several hundred thousand microscopic mirrors arranged in a rectangular array which correspond to the pixels in the image to be displayed. The mirrors can be individually rotated 10-12, to an on or off state. In the on state, light from the projector bulb is reflected into the lens making the pixel appear bright on the screen. In the off state, the light is directed elsewhere (usually onto a heatsink), making the pixel appear dark.
To produce greyscales, the mirror is toggled on and off very quickly, and the ratio of on time to off time determines the shade produced (binary pulse-width modulation). Contemporary DMD chips can produce up to 1024 shades of gray. See DLP for discussion of how color images are produced in DMD-based systems.
The mirrors themselves are made out of aluminium and are around 16 micrometres across. Each one is mounted on a yoke which in turn is connected to two support posts by compliant torsion hinges. In this type of hinge, the axle is fixed at both ends and literally twists in the middle. Because of the small scale, hinge fatigue is not a problem and tests have shown that even 1 trillion (1012) operations do not cause noticeable damage. Tests have also shown that the hinges cannot be damaged by normal shock and vibration, since it is absorbed by the DMD superstructure.
Two pairs of electrodes control the position of the mirror by electrostatic attraction. Each pair has one electrode on each side of the hinge, with one of the pairs positioned to act on the yoke and the other acting directly on the mirror. The majority of the time, equal bias charges are applied to both sides simultaneously. Instead of flipping to a central position as one might expect, this actually holds the mirror in its current position. This is because attraction force on the side the mirror is already tilted towards is greater, since that side is closer to the electrodes.
To move the mirror, the required state is first loaded into an SRAM cell located beneath the pixel, which is also connected to the electrodes. The bias voltage is then removed, allowing the charges from the SRAM cell to prevail, moving the mirror. When the bias is restored, the mirror is once again held in position, and the next required movement can be loaded into the memory cell.
The bias system is used because it reduces the voltage levels required to address the pixels such that they can be driven directly from the SRAM cell, and also because the bias voltage can be removed at the same time for the whole chip, meaning every mirror moves at the same instant. The advantages of the latter are more accurate timing and a more filmic moving image.
Color in DLP projection
There are two primary methods by which DLP projection systems create a color image, those utilized by single-chip DLP projectors, and those used by three-chip projectors.
Single-chip projectors
In a projector with a single DMD chip, colors are produced by placing a color wheel between the lamp and the DMD where it is reflected out through the optics. The color wheel is usually divided into four sectors: the primary colors: red, green, and blue, and an additional clear section to boost brightness. Since the clear sector reduces color saturation, in some models it may be effectively disabled, and in others it is omitted altogether. Some projectors may use additional colors (for example, yellow).
A single-chip projector alternates between colors and produces separate red, green, and blue images when displaying a moving image, or in this case, illuminating a moving hand.
The DMD chip is synchronized with the rotating motion of the color wheel so that the green component is displayed on the DMD when the green section of the color wheel is in front of the lamp. The same is true for the red and blue sections. The red, green, and blue images are thus displayed sequentially at a sufficiently high rate that the observer sees a composite "full color" image. In early models, this was one rotation per frame. Later models spin the wheel at two or three times the frame rate, and some also repeat the color pattern twice around the wheel, meaning the sequence may be repeated up to six times per frame.
In some recent high-end models, the spinning color wheel and the white bulb have been replaced with a package containing super-bright LEDs in the three primary colors (red, green, and blue). Since LEDs can be switched on and off very quickly, this design allows even higher rates of sequential single-color image projection. Bulb life is also much longer (and light intensity more consistent over the life of the bulb) with the LED pack than with earlier lighting technologies.
The DLP "Rainbow Effect"
This visual artifact is best described as brief flashes of perceived red, blue, and green "shadows" observed most often when the projected content features bright/white objects on a mostly dark/black background (the scrolling end credits of many movies being a common example). Some people perceive these rainbow artifacts all of the time, while others say they only see them when they let their eyes pan across the image. Yet others do not notice the artifact at all. The effect is likely rooted in the concept of the flicker fusion threshold.
The "Rainbow Effect" is unique to single-chip DLP projectors. As described above, only one color is actually displayed at any given moment. As the eye moves across the projected image, these separate colors become visible, resulting in a perceived "rainbow". The manufacturers of single-chip DLP projection systems have used color wheels rotating at higher speeds, or with more color segments, in order to minimize the appearance of the artifacts. These are referred to as 2x, 3x or 4x wheels. For example, a six segment wheel(RGBRGB) rotating at two revolutions per frame would be a 4x wheel.
Another way to reduce the rainbow effect is to replace a segmented wheel with a wheel whose colors are in an Archimedean spiral. This forms bands of color that move down (or up) the screen. With segmented wheels, the DMD must "go black" while the wheel transitions from one color to another. Not only can this interfere with persistence of vision and thus accentuate the rainbow effect, it means that the more segments there are, the darker the display will be, all else being equal. With a spiral wheel, the mirrors can display more than one color at a time, each moving down (or up) as the wheel turns.
The LED light packs now being introduced in DLP projectors may eliminate rainbow effect for all but a few very sensitive viewers thanks to their high switching frequency and a complete lack of "black" segments as described above. Additionally, the LED pack can display any color of light at any intensity, a capability which, if exploited, provides the potential for increased color gamut and improved contrast compared to displays employing color wheels with fixed-color segments.
Three-chip projectors
A three-chip DLP projector uses a prism to split light from the lamp, and each primary color of light is then routed to its own DMD chip, then recombined and routed out through the lens. Three-chip DLP projectors can resolve finer gradations of shade and color than one-chip projectors, because each color has a longer time available to be modulated within each video frame; furthermore they have a reduced potential for flicker and rainbow effect.
Manufacturers and market place
Texas Instruments remains the primary manufacturer of DLP technology, which is used by many licensees who market products based on T.I.'s chipsets. The Fraunhofer Institute of Dresden, Germany, also manufactures Digital Light Processors, termed Spatial Light Modulators, for use in specialized applications. For example, Micronic Laser Systems of Sweden utilizes Fraunhofer's SLMs to generate deep-ultraviolet imaging in its Sigma line of silicon mask lithography writers.
DLP is rapidly becoming a major player in the rear-projection TV market, having sold two million systems and achieved a 10% market share. Over 50 manufacturers offered models during the 2004 holiday season, up from 18 the previous year. DLP chips currently constitute 5% of Texas Instruments' total sales. Small standalone projection units (also called front projectors) using DLP technology have become very popular for office presentation and home theater duties.
Pros
Smooth (at 1080p resolution), jitter-free images.
Perfect geometry and excellent grayscale linearity achievable.
Usually great ANSI contrast.
No possibility of phosphor burn-in.
Less "screen door effect" than with LCD projectors.
DLP rear projection TVs are smaller, thinner, and lighter than CRT projectors.
The use of a replaceable light source means a potentially longer life than CRTs and plasma displays.
The light source is more-easily replaceable than the backlights used with LCDs, and is often user-replaceable.
Cons
In single-chip designs, some viewers are bothered by the "rainbow effect," explained above.
Not as thin as LCD or plasma displays (although approximately comparable in weight).
Fan noise.
"Screen door effect" (SDE) may be visible at close distance and/or with lower resolution models (720p resolution and lower). SDE can also be perceived as artificially sharp looking (due to dark gaps between mirrors/pixels which are high frequency content, not part of the image displayed) and not film-like.
Dithering noise may be visible, especially in dark image areas. Newer chip generations have less noise than older ones.
Error-diffusion artifacts caused by averaging a shade over different pixels, since one pixel cannot render the shade exactly.
Mediocre on-off contrast compared to CRT reference.
Response time in video games may be affected by upscaling lag. While all HDTVs have some lag when upscaling lower resolution input to their native resolution, DLPs are commonly reported to have noticeably longer delays. Newer consoles such as the Xbox 360 and Playstation 3 do not have this problem as long as they are connected with HD-capable cables. [1] (Samsung's newer TV's have a "game mode" which is supposed to reduce the lag by not doing as much processing.)
Color rendition can be off, especially the bright reds and yellows when at maximum brightness.
More mechanical than traditional CRT, LCD, plasma, and LCoS displays.
Replacement lamps can be expensive (USD $200).
DLP and LCoS
The most similar competing system to DLP is known as LCoS (Liquid Crystal on Silicon), which creates images using a stationary mirror mounted on the surface of a chip, and uses a liquid crystal matrix (similar to a Liquid Crystal Display) to control how much light is reflected.
Consumer Electronics Technicians Group
Information and pictures gathered from the Wikipedia and other sources.