In a projector with a single DLP chip, colors are produced either by placing a color wheel between a white lamp and the DLP chip or by using individual light sources to produce the primary colors, LEDs or lasers for example. The color wheel is divided into multiple sectors: the primary additive colors: red, green, and blue, and in many cases white (clear). Newer systems substitute the primary subtractive colors cyan, magenta, and yellow for white. The use of the subtractive colors is part of the newer color performance system called BrilliantColor which processes the additive colors along with the subtractive colors to create a broader spectrum of possible color combinations on the screen.
The DLP 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, blue and other sections. The colors 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. Now, most systems operate at up to 10× the frame rate.
The black level of a single-chip DLP depends on how unused light is being disposed. If the unused light is scattered to reflect and dissipate on the rough interior walls of the DMD / lens chamber, this scattered light will be visible as a dim gray on the projection screen, when the image is fully dark. Deeper blacks and higher contrast ratios are possible by directing unused HID light away from the DMD / lens chamber into a separate area for dissipation, and shielding the light path from unwanted internal secondary reflections.
The color wheel “rainbow effect”
DLP projectors utilizing a mechanical spinning color wheel may exhibit an anomaly known as the “rainbow effect”. This is best described as brief flashes of perceived red, blue, and green “shadows” observed most often when the projected content features high contrast areas of moving bright or white objects on a mostly dark or black background. Common examples are the scrolling end credits of many movies, and also animations with moving objects surrounded by a thick black outline. Brief visible separation of the colors can also be apparent when the viewer moves their eyes quickly across the projected image. Some people perceive these rainbow artifacts frequently, while others may never see them at all.
This effect is caused by the way the eye follows a moving object on the projection. When an object on the screen moves, the eye follows the object with a constant motion, but the projector displays each alternating color of the frame at the same location for the duration of the whole frame. So, while the eye is moving, it sees a frame of a specific color (red, for example). Then, when the next color is displayed (green, for example), although it gets displayed at the same location overlapping the previous color, the eye has moved toward the object’s next frame target. Thus, the eye sees that specific frame color slightly shifted. Then, the third color gets displayed (blue, for example), and the eye sees that frame’s color slightly shifted again. This effect is not perceived only for the moving object, but the whole picture. Multi-color LED-based and laser-based single-chip projectors are able to eliminate the spinning wheel and minimize the rainbow effect, since the pulse rates of LEDs and lasers are not limited by physical motion. “Three-chip DLP projectors have no color wheels, and thus do not manifest this [rainbow] artifact.”