The Great Technology War: LCD vs. DLP
July 11, 2005,
A Potential Problem with LCD: Long Term Image Degradation
In 2003 Texas Instruments sponsored and published the results of a lab test which highlighted a failure mode in LCD technology that does not exist with DLP. The test indicated that given enough time LCD panels, primarily those in the blue channel, will degrade, causing shifts in color balance and a reduction of overall contrast. The test included five LCD projectors that were run constantly in 24/7 operation for several months. Thus while the test revealed a failure mode in LCD technology, it did not include a large sample of test units. Nor were the projectors run in conditions approximating real life usage. Therefore it was difficult to draw any conclusions about anticipated rates of degradation under normal operating conditions.
We believe that at the current commercial state of the art, LCD panels will eventually degrade over time. However, the degree to which they will degrade is dependent upon a variety of factors. Some of those factors are related to the projector's light engine design and cooling system, the presence of internal UV filters, and so on. In essence, some LCD projectors may have more of a tendency to degrade than others based on their design. On the other hand, some factors may be related to usage. If air filters are not cleaned when they need to be, the internal operating temperature will rise. Usage in a chronically warm environment may have an impact. We would not be surprised to discover that projectors used at higher elevations could be more susceptible to LCD degradation due to higher operating temperatures in thinner atmosphere. However, this is all speculation. There is no hard data on the subject of LCD failure rates under various operating conditions.
The bottom line is that there exists the possibility that those who invest in an LCD projector may find that the LCD panel and polarizer in the blue channel may eventually need replacement. This is not much of a problem if the unit is under warranty. But if it isn't, the replacement of an LCD panel will represent an unpleasant incremental investment in your projector that you were not anticipating.
A Potential Problem with DLP: The Rainbow Effect
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. Not only can some see the colors break out, but the rapid sequencing of color is thought to be the culprit in reported cases of eyestrain and headaches. Since LCD projectors always deliver a constant red, green, and blue image simultaneously, viewers of LCD projectors do not report these problems.
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 render the picture literally unwatchable. 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 them.
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.
How big of a problem is the rainbow issue for you?
If you've seen earlier generation DLP machines and detected no rainbow artifacts, you won't see them on the newer machines either. The majority of 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 user are among those that may be bothered by ill-effects from the spinning color wheel without sitting down and viewing a DLP projector for a while.
Therefore, if you think you've identified a DLP projector that is just right for your needs but you are not sure whether this will be a problem, there is an easy solution. Find alternative products that are either LCD- or LCOS-based that would be your second choice if you find that DLP won't work for you. Then find a customer service oriented dealer who will allow you to switch the DLP product for the alternative after testing it out. There are a number of service-oriented Internet dealers who will be happy to make such arrangements, and there are plenty who will not. But if you choose a dealer who is more interested in your satisfaction than in closing a quick deal (and they are definitely out there), you will end up with a good solution in the end.
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