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The Great Technology War: LCD vs. DLP

Evan Powell, December 7, 2005

A Potential Problem with DLP: Rainbow Artifacts

If there is one single issue that people point to as a weakness in DLP, it is that the use of a spinning color wheel to modulate the image has the potential to produce a unique visible artifact on the screen commonly referred to as the "rainbow effect." This is simply due to colors separating out in distinct red, green, and blue because of the sequential color updating from the wheel. (Three-chip DLP projectors have no color wheels, and thus do not manifest this artifact). Basically, as the color wheel spins the image on the screen is either red, or green, or blue at any given instant in time, and the technology relies upon your eyes not being able to detect the rapid changes from one to the other. Unfortunately some people can see it. Since LCD projectors always deliver a constant red, green, and blue image simultaneously, users of LCD projectors do not have this problem.

How big of a deal is this? Well, it is different for different people. Most people cannot detect color separation artifacts at all. However, for those who are most sensitive to rainbows, they are so distracting that they can render the picture literally unwatchable, and a couple of folks on staff here at ProjectorCentral get headaches from it. Many others fall between these two extremes--they report being able to see rainbow artifacts on occasion, but find that they are not particularly annoying and do not inhibit the enjoyment of the viewing experience.

Texas Instruments and the vendors who build DLP-based projectors have made strides in addressing this problem. The first generation DLP projectors incorporated a color wheel that rotated sixty times per second, which can be designated as 60Hz, or 3600 RPM. So with one red, green, and blue panel in the wheel, updates on each color happened 60 times per second. This baseline 60Hz rotation speed in the first generation products is known as a "1x" rotation speed.

Upon release of the first generation machines, it became apparent that quite a few people were seeing rainbow artifacts. So in the second-generation DLP products the color wheel rotation speed was doubled to 2x, or 120Hz, or 7200 RPM. The doubling of the color refresh rate reduced the time between color updates, and so reduced or eliminated the visibility of color separation artifacts for most people.

Today, as noted above, many DLP projectors being built for the home theater market incorporate a six-segment color wheel which has two sets of red, green, and blue filters. This wheel still spins at 120Hz or 7200 RPM, but because red, green, and blue are refreshed twice in every rotation rather than once, the industry refers to this as a 4x rotation speed. This further doubling of the refresh rate has substantially reduced the number of people who can detect the rainbows.

For the large majority of users the six-segment, 4x speed wheels have solved the problem for home theater or video products. Meanwhile, due to the higher lumen output requirements for business presentation use, most commercial DLP units still use the four-segment, 2x speed wheels. However, rainbow artifacts tend to be less of a problem in commercial presentation for two reasons. First, viewers tend to sit back farther from the screen, and there is a smaller angle of view from edge to edge. That means less eye movement is required to see the entire image, and it is eye movement that causes you to see the colors separate. Second, data presentations are static; there are no moving images that cause the eye to pan or rapidly refocus on different areas of the image in a continuous fashion. Since the eye typically does not move as much or as rapidly when viewing data images from a distance as it does when viewing widescreen video or games from relatively close up, rainbow artifacts tend to be less of a concern with commercial projectors.

If you've seen earlier generation DLP machines and detected no rainbow artifacts, you won't see them on the newer machines either. Many people can't see them at all on any of the current machines. However there is no way for you to know if you or another regular viewer in your household are among those that may be distracted by the spinning color wheel without sitting down and viewing a DLP projector for a while. So if you are uncertain, it would be wise to arrange an audition of a DLP projector you might be interested in prior to committing to it for your home theater.

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