Digital micromirror device

Digital Light Processing (DLP) is a trademark owned by Texas Instruments, representing a technology used in projectors and video projectors. It was originally developed in 1987 by Dr. Larry Hornbeck of Texas Instruments.

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 or more pixels in the projected image. The number of mirrors corresponds to the resolution of the projected image (often half as many mirrors as the advertised resolution due to wobulation). 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).

Rapidly toggling the mirror between these two orientations (essentially on and off) produces grayscales, controlled by the ratio of on time to off time.

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. A third method, sequential illumination by three colored light emitting diodes, is being developed.

Single-chip projectors

DLP chip

In a projector with a single DMD chip, colors are either produced by placing a spinning color wheel between the lamp and the DMD or by using individual light sources to produce the primary colors, LEDs for example. 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). The color wheel technique was used in the early 1950s by the original CBS color television system before the standardization of NTSC color.

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 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.

The DLP "rainbow effect" in single-chip systems

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 are 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. The effect is likely rooted in the concept of the flicker fusion threshold. In some viewers the effect can lead to eye strain, headaches, or migraines after as little as a few minutes of viewing. New LED based DLPs can produce the alternating images fast enough so that most people will not be affected by the rainbow effect.

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. The spiral wheel can greatly reduce these effects.

Later LED-based DLPs are equipped with red, blue and green LEDs, providing both the primary colors and illumination in one device. The LEDs, which turn on and off almost instantly, are activated in sequence to display each color channel. The use of LEDs eliminates the color wheel mechanism and metal halide lamps required by traditional DLP designs.

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, there won't be any flicker or rainbow effect like with the single chip solution. Like three-tube CRT projectors, the optics for some three-chip DLP projectors must be carefully aligned.  (Continue)