Demystifying Projector Technologies

By Amir Majidimehr

DLP Imaging Element

If you take a look at the leading suppliers of projectors for the home theater market, you see that there is no consensus as to choice of the core component of the projector: the imager. This is the element which transforms the video signal/image pixels into light. Competing technologies are DLP, LCD, and D-ILA/SXRD. There are variations even within the same theme with some using a single imager, but others three for each primary color (red, green and blue).

There are no shortages of articles online about this topic but despite it all, the overall message is still not clear as to benefits and applications of each. I hope to shed some light on the topic, helping to steer you in the right direction in your projector purchase decision.

DLP

DLP technology was invented by Texas Instrument or TI for short. The chip is a marvel of engineering, combining mechanical movement of tiny mirrors which reflect light inside a silicon chip. The mirrors tilt to create a darker shade, proportional to the value of the image pixel (dot). See the picture above which shows a single mirror under extreme amplification using an electron microscope.

Here is an overview promotional video from TI that details how the chip works:

The DLP mirror by definition is a black and white device since it reflects all colors equally. This has both a benefit and a drawback. The benefit is that all colors are treated equally. This is important as that is the ideal definition of an ideal projector – one that has flat response for the entire gamut of the colors we see. The drawback is that we have to somehow turn these devices into color. There are three solutions here:

Use a color wheel which changes the lamp color as it reflects onto the DLP mirrors. If we then synchronize what color pixels we send to the DLP (i.e. send it the blue pixels when the light is filtered to be blue), then we get our colors out, one at a time. That is, in one instant we will have blue, in another red, and so forth.

You might be wondering how that gets us all the colors at the same time. That function is performed by your eye. You probably know that video and film is shot one frame at a time but by showing the frames quickly in sequence, it appears to be moving images. By the same token if we flash the colors in sequence very rapidly, the eye thinks they are all occurring at the same time and superimposes the colors.

That’s the theory at least and works for most people. But a few are sensitive enough to detect the flashing of the individual colors and can occasionally see a rainbow of colors. The effect is one that can be learned so I am hesitant to explain it to you, lest you learn to see it now where you could not before! So if you have not seen it, do not worry about it.

Note that some projectors are better than others and recent advance in turning the wheel faster has helped considerably with this issue.

Bulb choices as mentioned are UHP and Xenon. UHP is used in projectors that require light at low to medium levels. For very bright/large screen applications, Xenon is the used since it has no upper limit far as power.
LED light source. Traditional projectors use UHP or Xenon bulbs (similar technologies to what is used in your car headlights). But now there are now projectors using LEDs as the light source. A trio of LEDs (red, green and blue) is used to shine the appropriate light onto the DLP. Since LEDs have very fast response, they can be flashed much faster than the color wheel and with it, sharply reduce the rainbow/strobing effect of bulb based projectors. I am sensitive to this effect but in all the LED projectors I have looked at, I have only seen it once that was in in a low cost unit.

Luminus is the leading supplier of these high power LEDs used in projectors. This is a good introduction to their capabilities:

As the video describes, another advantage of LED is very long bulb life. Traditional projectors require replacement of the bulb on a regular basis (roughly 1000 hours depending on the projector). LED light sources have 10 to times longer life and last probably as long as the projector. They also turn on much faster and do not need the warm up time of UHP/Xenon. Another bonus is that hold their calibration/color fidelity unlike UHP/Xenon which may require recalibration (if you have a critical eye) as the bulb ages.

Elimination of the color wheel means there is one less thing to cause reflections inside the projector resulting in increased contrast.

Another benefit of LEDs is that they have a much wider color gamut as compared to bulbs and color wheels. They can easily exceed the range needed for consumer video applications. Note that exceeding the specifications is not a good thing necessarily as that creates images with incorrect colors. Proper projector calibration assures that this cannot happen, putting the extra dynamic range of LEDs to good use in bringing the colors into compliance and provide a very rich and sharp image.

The drawback of LED technology as of this writing is cost and limit on light output/screen size. The cost is partially compensated if you consider the fact that you do not need to replace the bulb. The light output level means screens above say, 10 foot wide (138 inches diagonal), are not recommended.
Three DLP imagers. By using three separate imagers, independently filtered red, green and blue colors can be shined on each, and the resulting image combined optically before it comes out of the projector. The result as with LED light source is increased contrast and elimination of rainbow/strobing effect.

Unfortunately the imagers are expensive so tripling them sharply increases the cost. As a result, three chip DLP projectors only exist in the high-end of the market. The other drawback is panel alignment. All three imagers have to be in the same alignment or their images do not overlap. Resulting output will be a slightly softer image with some color bleeding around sharp edges over bright backgrounds. Perceptually, most people are not sensitive to such artifacts but expectations of perfect alignment should not be there (until you get into very expensive projectors where in the field alignments are allowed).

Being targeted toward the high-end of the market, the range of products is tremendous with light output that can easily fill a screen that is tens of feet wide. Xenon projectors especially, can sale up to 50 feet or even higher. Recent advancements in UHP technology such as used in Sim2 Cinema projectors also provides for very high light output giving Xenon a run for its money especially considering that UHP bulbs age much more gracefully.

One of the other benefits of DLP is its ultra-fast response time. The mirrors can move very fast allowing the video frames to change nearly instantly from one to the other. This means that when the scene is changing rapidly, the image maintains its full resolution. Competing solutions can’t switch so fast so the content of one frame runs into another, causing some amount of blurring and loss of resolution.

The above becomes even more important when displaying 3-D video. There, each frame of video represents a frame for one eye versus another. If done perfectly, an illusion of 3-D video is created. But if the video frames blur into each other, the images are no longer independent and create a type of distortion called crosstalk. This causes ghosting of 3-D perspective and eye strain.

See this article on more explanation of 3-D technologies: 3D video explained.

D-ILA/SXRD/LCOS

Next category of projectors is D-ILA, pioneered by JVC courtesy of US government funding for video simulations of aircrafts and such. This same technology was adopted by Sony and named SXRD. The technology is similar to LCDs in that based on an electrical signal a polarization layer can reduce what amount comes back and thereby controlling the intensity of the light. Each imager is black and white and therefore it takes three to create a color image.

D-ILA/SXRD uses a reflective back on the device which differentiates it from the 3LCD technology described below. In general, the technology has been perfected to have some of the best contrast ratios available. This means that there is a higher difference between black and white, creating the impression of a snappier picture.

The drawback is similar to 3-chip DLPs in that panel misalignment can reduce effective image sharpness. And differing response to color in each panel causing non-uniform frequency response with respect to each color. Subjectively, single-chip DLP projectors tend to have sharper images, but losing in contrast to D-ILA/SXRD.

3LCD

3LCD is the brand name of an LCD technology pioneered by Epson but now used by many other manufacturers. The technology has a market share great than 50% due to use in very low cost units used for data presentations and the lower price point of 3LCD home theater projectors.

As with D-ILA, three chips are used for each primary color. Unlike D-ILA however, the light goes completely through the chip and then combined into a single image. In D-ILA, the chip has a reflective back so it sends the light back out the front.

Original LCD units lacked contrast, showing somewhat washed out images. But incredible improvements have been made in the last few years, putting LCD units near the performance of DILA/SXRD but at lower cost.

So if what you are looking for is price/performance, 3LCD units rule the world. You can get an image that three times bigger than your flat panel TV but at roughly the same price! The price of entry in home theater has never been lower.

Summary

All of the projection technologies have advanced to produce incredibly good images. They give you images far bigger than any flat panel, making you feel like you are part of the movie experience. Still, different approaches to imaging device and lighting can result in differing performance and price/capability range. LCDs provide the best price performance, followed by DILA/SXRD (with some overlap). DLP covers the mid to high-end.

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