If you are new to the world of digital projectors, you won't have to shop around very long before discovering that the terms "LCD" and "DLP" refer to two different kinds of projectors. You might not even know what LCD and DLP are before asking the obvious question "which one is better?"
The answer is simple. Sort of. LCD and DLP each have unique advantages over the other. Neither one is perfect. So it is important to understand what each one gives you. Then you can make a good decision about which will be better for you.
By the way, there is a third significant light engine technology called LCOS (liquid crystal on silicon). It is developed and marketed by several vendors, most notably JVC, Sony, and Hitachi. Several outstanding home theater projectors have been manufactured with this technology. However the discussion of LCOS technology is beyond the scope of this article.
The Technical Differences between LCD and DLP
LCD (liquid crystal display) projectors usually contain three separate LCD glass panels, one each for the red, green, and blue components of the video signal being fed into the projector. As light passes through the LCD panels, individual pixels ("picture elements") can be opened to allow light to pass or closed to block the light, as if each little pixel were fitted with a Venetian blind. This activity modulates the light and produces the image that is projected onto the screen.
DLP ("Digital Light Processing") is a proprietary technology developed by Texas Instruments. It works quite differently than LCD. Instead of having glass panels through which light is passed, the DLP chip is a reflective surface made up of thousands of tiny mirrors. Each mirror represents a single pixel.
In a DLP projector, light from the projector's lamp is directed onto the surface of the DLP chip. The mirrors wobble back and forth, directing light either into the lens path to turn the pixel on, or away from the lens path to turn it off.
In the more expensive DLP projectors, there are three separate DLP chips, one each for the red, green, and blue channels. However, in most DLP projectors under $10,000 there is only one chip. In order to define color, there is a color wheel that consists (at minimum) of red, green, blue, and sometimes white (clear) filters. This wheel spins in the light path between the lamp and the DLP chip and alternates the color of the light hitting the chip from red to green to blue. The mirrors tilt away from or into the lens path based upon how much of each color is required for each pixel at any given moment in time. This activity modulates the light and produces the image that is projected onto the screen. (In addition to red, green, blue, and white segments, some color wheels use dark green, cyan, magenta, or yellow segments as well.)
Performance Advantages: LCD vs. DLP
Both of these technologies have evolved rapidly over the last ten years. Both of them are much better than they used to be, and the radical differences we used to see between them have narrowed quite a bit. There are still some noteworthy differences, but the story is a classic example of how open market competition drives improvements in technology.
Historically speaking, one traditional benefit of LCD was that it delivered better color saturation than was possible from a DLP projector. That was because in most single-chip DLP projectors built for the commercial presentation market, a clear (white) panel is included in the color wheel along with red, green, and blue in order to boost light output. Though the image is brighter than it would otherwise be, this tends to reduce color saturation, making the DLP picture appear not quite as rich and vibrant. This is not an issue with data display as colors are plenty vibrant with a data signal. But it is something to consider if you plan to use the projector for high quality video display.
To compensate for the lack of color saturation and to improve color accuracy, many of the DLP-based products made for home theater or other high quality video applications now have six-segment color wheels that feature two sets of red, green, and blue filters. Meanwhile the white segment is eliminated. (Some wheels actually have seven or eight segments to include a dark green in addition to the red, green and blue.) These wheels boost color accuracy and color saturation while sacrificing light output. This is a good trade-off because good video performance depends on high image contrast on the screen. Therefore video material is best viewed in a dark environment. In a dark room, high lumen output is not required. Thus the home theater-oriented DLP projectors have closed the gap with LCD in the area of color saturation, whereas the DLP products built for high-brightness commercial presentation use still tend to have a weakness in this area.
Another area where these two technologies have differed is in image sharpness with data applications. LCD can usually deliver a slightly sharper image than DLP at any given resolution. The difference between the two is noticeable in the display of material like detailed financial spreadsheets. However, there is no visible difference in sharpness with video. This is not to say that DLP will project a fuzzy spreadsheet--it doesn't. When you look at a spreadsheet projected by a DLP projector it looks sharp enough. It's just that when a DLP unit is placed side-by-side with an LCD of the same resolution, the LCD typically looks a bit sharper in comparison.
A third traditional advantage of LCD is that it is more light-efficient. LCD projectors usually produce significantly higher ANSI lumen outputs than do DLPs with the same wattage lamp. Thus LCD products dominate the bright end of the performance range. As of this writing, there are over 60 projector models in the database rated between 3000 and 6000 ANSI lumens at retail prices under $10,000. All of them are LCD projectors with the exception of one DLP model. And the lone DLP projector in the group is rated at 3200 lumens, or the low end of the brightness range for the group. Therefore, LCD competes extremely well when high light output is required.
LCD projectors have historically had three weaknesses, all of which are more relevant to video than they are to data applications. The first is visible pixelation--the ability to see the discrete pixels on the screen. The second is commonly referred to as the "screendoor effect" because it looks like you are viewing the image through a screendoor. The screendoor effect is caused by the space between the pixels, known as the inter-pixel gap. The third weakness is not-so-impressive black levels and contrast, which are vitally important elements in a good video image. LCD technology has in the past had a hard time being taken seriously among some home theater enthusiasts (understandably) because of these flaws in the image.
However, in most of today's LCD projectors these problems aren't nearly what they used to be. The inter-pixel gaps on LCD panels have been reduced and physical resolution--the number of pixels on the screen--has been increased. In the early days of the digital projector industry resolutions were low, generally at VGA (640x480) or lower. The industry then stepped up to SVGA (800x600), and then to XGA resolution (1,024x768) and higher. Many of today's widescreen format projectors use either 1280x720 or 1366x768 resolution displays). With each step up in the number of pixels that produce the image, visible pixelation and the screendoor effect have been reduced. At the new pixel densities, visible pixelation is usually eliminated at normal viewing distances.
Second, the inter-pixel gaps on all LCD machines, no matter what resolution, have been reduced compared to what they use to be. So even today's inexpensive SVGA low-resolution LCD projectors have less screendoor effect than did earlier models.
Since DLP technology creates a pixel by reflecting light from a tiny mirror, its edge definition is softer and less well-defined than LCD (this is what accounts for the slightly softer image in detailed spreadsheet presentation, but also DLP's traditionally smoother image in video). So for the most part, at any given resolution, DLP still holds an advantage over LCD in visible pixelation. However due to the recent advances in LCD technology you need to stand closer to the screen to see the differences than you used to.
Now when it comes to contrast, LCD still lags behind DLP by a considerable margin. But both have made significant strides forward. Just three years ago typical LCD projectors were rated at 400:1 contrast or lower while comparable DLP models were at 600:1 or 800:1. But major improvements in both technologies have boosted contrast ratings to amazing new levels. Many commercial DLP projectors are rated at 2000:1 these days, and models built specifically for home theater carry ratings of up to 5000:1.
Meanwhile, there are at this writing 38 models of LCD projectors rated at 1000:1 contrast or higher. And the addition of dynamic aperture control appears to be boosting effective contrast on LCD projectors to unheard of heights. Sony just announced the VPL-HS51 with a contrast rating of 6000:1, and the Panasonic AE700 was just announced at 2000:1. With contrast performance in this ballpark, LCD has essentially closed the gap with DLP on this important performance feature. Any projector with a rating of 2000:1 or greater is capable of producing a sparkling image with excellent shadow detail. So once again, the performance differences between LCD and DLP in this regard are not as significant as they used to be.
Finally, one of the key advantages of DLP over LCD is small package size, a feature most relevant in the mobile presenter market. Since the DLP light engine consists of a single chip rather than three LCD panels, DLP projectors tend to be more compact. There are over 50 projectors currently on the market weighing 4.0 pounds or less, and only three of them are LCD; the rest are all DLP products. Most LCD projectors are five pounds and up.
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 some who can see the rainbow effect, it is so distracting that it renders the picture literally unwatchable. Others report being able to see the 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 again 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. However, there are now commercial products being marketed for presentation use by BenQ and others with 3x speed wheels in order to reduce the chance that users will encounter the problem. [Correction: As of 12/8/04, BenQ has informed us that the 3x rotation speed specifications on several of their models were incorrect. Their business presentation models have 2x rotation speeds. EP]
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 either by distracting rainbows, or possibly eyestrain and headaches, 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 an alternative product that is 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 sells both models, and who will allow you to switch the DLP product for the alternative after testing it out for a few days. 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.
A Potential Problem with LCD: Long Term Image Degradation
Last year 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.
However it is possible that those who invest in an LCD projector may find that eventually the LCD panel and polarizer in the blue channel may 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.
The Current State of the Art
The largest developers and manufacturers of LCD technology are Sony and Epson. These companies have no interest in standing by and letting Texas Instruments sweep the digital projector market with its competing DLP technology. So competition has driven both the LCD makers and Texas Instruments to improve their respective products in the ongoing battle for market share.
LCD has made significant improvements in visible pixelation and contrast, which were the two big deficiencies in LCD that caused many people to prefer DLP for video use. Based on the latest product announcements from Sony and Panasonic, it appears that the use of a dynamically reconfiguring aperture may be the key to effectively mitigating the contrast advantage that DLP has always enjoyed. If this is the case, then the battle between the technologies will shift to considerations of cost and resolution.
When it comes to cost there is a peculiar dichotomy in the marketplace. Commercial DLP products tend to compete well against LCD at any given resolution. The two least expensive SVGA resolution projectors on the market at the moment are DLPs. Of eleven SVGA products currently retailing under $1100, seven are DLP and four are LCD. Similarly, of fourteen native XGA resolution products currently retailing for under $2,000, ten are DLP and four are LCD. Thus when one considers standard 4:3 aspect ratio commercial projectors, DLP appears to have a latent cost advantage.
The same does not hold true for widescreen video projectors. Of the eleven HDTV resolution 1280x720 projectors currently shipping at a retail price under $5,000, all eleven are LCD products. Sharp has recently announced the Z2000, a DLP-based 1280x720 unit that will carry an estimated street price of $3,999. This is the first of the 1280x720 DLP products to drop below $5,000 retail. Yet this is still double the price of the least expensive LCD products of the same resolution. Since there is no similar cost disparity in commercial products, it appears that a premium is being charged for DLP-based video products due to the perceived incremental value of its superior contrast and video performance. If this is the case, once the new high-contrast LCD products begin to ship we will likely see a tumble in widescreen DLP product pricing.
Both LCD and DLP are evolving rapidly to the benefit of the consumer. The race for miniaturization has produced smaller yet more powerful projectors than we might have even imagined possible just a couple of years ago. Light output per pound has increased dramatically. And video quality on the best LCD and DLP projectors now surpasses that available in a commercial movie theater.
ProjectorCentral continues to recommend both LCD and DLP projectors for a variety of applications. For mobile presentation it is hard to beat the current group of 3-pound DLPs on the market. However LCD products like the Epson 740c at 3.8 lbs make it clear that LCD is still a very strong contender in the mobile presentation market. And for larger conference rooms that require higher light output, LCD technology holds a commanding lead.
When it comes to home theater, DLP has continued to make competitive advances in color, contrast, and image stability that have served to make it a technology preferred by many for home theater use. But the gap has closed and DLP and LCD are both capable of delivering much higher quality video for home theater than they ever were before.
Which technology is the best? Well, it depends. Both technologies have advantages, and both have weaknesses. Neither one is perfect for everything. So the technology war continues. The only clear winner in sight continues to be you, the consumer.