(NOTE: This is the sixth update of an article originally posted four years ago. The technologies continue to evolve quickly, hence the periodic updates.)

Introduction

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" somehow 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 being developed 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. 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 very expensive DLP projectors, there are three separate DLP chips, one each for the red, green, and blue channels. However, in most DLP projectors under $15,000 there is only one chip. In order to define color, there is a color wheel that consists of (at minimum) red, green, blue, and sometimes white (clear) filters. This wheel spins in the light path between the lamp and the DLP chip and the filters determine the color of the light hitting the chip. 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 now use dark green or yellow segments as well. Furthermore, Texas Instruments has just announced plans for future color wheel designs to incorporate six colors in the rotation-red, green, blue, cyan, magenta, and yellow. This enhancment, known as "BrilliantColor(TM)", will boost color performance on single chip DLP projectors to new levels, and should begin to appear on new models by the end of the year.

Performance Advantages: LCD vs. DLP

Both of these technologies have been evolving rapidly over the last five 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 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 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 for video applications because high quality front projection video depends on high image contrast and color saturation, not lumen output. Front projection systems are always best viewed in a dark environment where high lumen output is not required, and can actually be detrimental. 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 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 detailed computer data, like 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. However, brighter DLP models are beginning to encroach upon LCD here as well. Last September, there were about 60 projector models in the database rated between 3000 and 6000 ANSI lumens at retail prices under $10,000. All of them were LCD projectors with the exception of one DLP model. At this writing, there are 90 models rated between 3000 and 6000 lumens under $10,000. Seventy-five of them are LCD and fifteen are DLP. Still, the brightest of the DLPs in this group is only 4000 lumens. So LCD continues to maintain a significant competitive edge in light output per dollar spent.

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, LCD has made strides in these areas. 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 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, nine months ago there were 38 LCD projectors rated at 1000:1 contrast or higher in our database. Today there are 63. And the addition of dynamic aperture control is able to boost contrast on LCD projectors to unheard of heights. Sony's VPL-HS51 has a contrast rating of 6000:1, and the Panasonic AE700 is rated at 2000:1. With contrast performance in this ballpark, LCD products continues to remain competitive with DLP, although most DLP models will still outperform comparably priced LCD products in contrast.

The consumer should bear in mind that while high contrast is critically important for high quality video presentation in a darkened environment, it is entirely irrelevant in commercial data presentation in a fully or partially lit room. Once you have lights on in the room, black levels get hammered, and the real contrast on the screen usually drops to well below 20:1 no matter what the theoretical contrast rating on the projector is. For "lights on" data presentations, adequate lumen output is essential, and contrast is meaningless. Accordingly, many commercial LCD projectors continue to to be sold despite contrast ratings of 400:1 or lower. The reason is that they produce a substantial amount of light and razor sharp data images for extremely competitive prices. They are designed for commercial presentation environments in which the contrast rating is a non-issue.

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 52 DLP projectors currently on the market weighing 4.0 pounds or less. Meanwhile, there are only seven LCD models in this weight category, six of which are from Epson. However, all six Epson models are near the top of the list when all of the sub-4 lb models are ranked by lumen output. So LCD's advantage in light efficiency manifests itself across the entire spectrum of projection products.

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.

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.

DLP continues to be exceptionally strong in video and home theater. TI's introduction of the 854x480 resolution DLP chip created a host of inexpensive but amazingly high performance entry level home theater projectors, now selling well below $1500. Practically speaking, LCD has no answer to this, and DLP is dominating this particular market niche.

DLP is also dominant in the higher price, higher resolution home theater niche as well. Projectors featuring the 1280x720 resolution DLP chips have dropped significantly in price, and consumer demand for these units is high. In this category however, LCD does have an answer--impressive widescreen 1280x720 LCD models are selling at prices that are half to two-thirds the prices of DLP models of the same resolution. While they don't have quite the contrast and black level of their DLP counterparts, color is excellent and price/performance is outstanding. Given LCD's 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, LCD remains competitive in this market segment.

When it comes to commercial applications, we noted previously that LCD has a latent competitive advantage where high lumen output is required. Most of the 6000 lumen light cannons retailing for under $20,000 are LCD products. And Epson's array of extremely bright sub-4-lb portables establish a significant presence for LCD in a mobile presentation niche where DLP otherwise holds a commanding lead.

Nevertheless, DLP has emerged as a favored technology in the large volume commercial presentation market, offering a wide variety of models in any given price and performance range. For example, at this writing there are 20 XGA-resolution projectors on the market that are at least 2000 lumens, retailing for under $2,000. Of those, 14 are DLP and only six are LCD.

Across all price and performance categories, we currently have 349 DLP projectors listed as in production, as compared to 330 LCD models. This is highly significant--for the first time in the history of the projector industry, DLP has just surpassed LCD in the total number of products on the market.

Both LCD and DLP are evolving rapidly to the benefit of the consumer, and they will continue to do so. ProjectorCentral continues to recommend both LCD and DLP projectors for a variety of applications. 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 both technologies are capable of delivering much higher quality video for home theater than ever before.

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