Projector Technologies Explained
Currently there are three main types of digital video projector technology that are in use today: CRT, LCD, and DLP. There are also variations within these three three types such as LCOS and further variation with LCoS such as JVC’s D-ILA and Sony’s SXRD branded projectors.
Below is a brief overview of each of the three major projector types.
CRT
CRT (Cathode Ray Tube) projectors were developed from the tube television technology which has been around since the 1950’s. You might have ran across one of these bulky units hanging from the ceiling of an older airplanes coach compartment or in a companies auditorium.
A CRT projector is a video projector that uses three small, high brightness CRT (or picture tubes for each primary color) as the image generating element. The image is then focused and enlarged onto a screen using the proper video processing circuity, CRT size, and lens combination in front of the CRT face. A CRT projector is capable of producing excellent high resolution images.
Advantages
- Long service life, CRT tubes maintain good brightness to 10,000 hours.
- High end CRT projectors can precisely display images up to 1920 x 1200 with accurate color reproduction. A few projectors can scan at even higher resolutions up to 3200 x 2560, although their ability to resolve fine detail at this resolution is greatly reduced.
- Superior black level compared to LCD and DLP based projectors.
- As with CRT monitors, the image resolution and the refresh rate are not fixed but variable within some limits. Interlaced material can be played directly, without need for imperfect deinterlacing mechanisms.
- CRT projectors do not show a Rainbow effect seen with single chip DLP projectors.
Disadvantages
- CRT projectors are both considerably larger and heavier than comparable LCD and DLP projectors so installation into a home setting can be more of a challenge.
- CRT projectors require far more time to set up and adjust than LCD and DLP based projectors.
- Absolute ANSI brightness achievable with CRT projectors is lower than with comparable LCD/DLP projectors.
- Low-end or poorly tuned CRT projectors may suffer from color divergence.
As of this writing (April 2008), CRT projectors still provide one of the best quality image for a home theatre use. The overall sharpness, focus, lifelike fleshtones and three dimensionality of the image still cannot be duplicated by any other type of video projector. With the recent release of HDMI and DVI modules, all data grade CRT projectors can now be connected to any current digital device, including Blu-ray players. CRT obsolete? I think NOT!
Although the CRT video projector option is no longer available for general consumer use and are quite expensive and generally reserved for high-end, custom installations. There is a striving surplus market with most of these units coming from the US government and large cooperations like Disney, Boeing, and IBM who are liquidating these large 100-250 lbs CRT projectors for the smaller 10lb DLP and LCD Models. Most of these surplus CRT projectors date from 1988 - 1999 it is still used in some commercial applications and there are still many older units style in use.
LCD
The next video projector technology to discuss is the LCD (Liquid Crystal Display) projector. Unlike a CRT projector, the LCD projector is not based on the traditional projection tube. To display an image a LCD (liquid crystal display) projector typically sends light from a Metal halide lamp through a prism that separates light to three poly silicone panels — one each for the red, green, and blue components of the video signal. As polarized light passes through the panels (combination of polarizer, LCD panel and analyzer), individual pixels can be opened to allow light to pass or closed to block the light. The combination of open and closed pixels can produce a wide range of colors and shades in the projected image.
Advantages
- One of the first key advantages of digital LCD projectors is how it deals with color saturation. The technology behind an LCD projector allows it to generate better color saturation than a DLP designed projector. This is because DLP projectors aim too boost brightness at the expense of color saturation. Therefore, digital LCD projectors will provide an image that is more rich and have a noteworthy advantage in this area.
- LCD projectors also produce sharper images than DLP designs. This isn’t really a concern when you’re using the projector for videos, but you will notice it if you need to display detailed data on the screen. Although the DLP variety don’t produce fuzzy images, digital LCD projectors still produce sharper images in comparison.
- Digital LCD projectors also have an edge in light efficiency. DLP designs with a lamp of the same wattage will not generate higher lumen outputs when compared to LCD designs. Therefore, it’s best to use digital LCD projectors when you need the brightest image possible.
- Another noted advantage is the rainbow effect is not a concern for LCD projectors.
Disadvantage
- One of the first disadvantages of LCD projectors is pixelation or what is commonly referred to as screen door effect. These projectors produce images using individual pixels which are static, and contain wasted space, and this can become visible to the viewer. Because an LCD projector contains a low fill factor (pixels are farther apart than that of a DLP) this will give the appearance of looking through a screened door when using a digital LCD projector to project onto a large screen. This is normally a problem when watching video instead of looking at data.
- Another disadvantage of digital LCD projectors is that the pixels on the chip can burn out. When one pixel burns out, you won’t be able to repair it. This will cause digital LCD projectors to show a white or black dot on the screen where the pixel has burned out, and if the pixels near it are opposite this burned out pixel will become very noticeable. . In order to fix the problem, you will have to replace the whole chip.
- Another potential problem is stuck pixels. A pixel may become stuck in the on or off position. This results in a light or dark dot remaining on the screen at all times. Often the dot may go unnoticed until a scene with a large number of dots of the opposite extreme surround the affected pixel.
- The LCD light source (bulb) in an LCD projector must be replaced periodically, depending on the projector and application, about every 1,000 to 2,000 hours, at a cost of several hundred dollars. On the positive side of this, a new lamp basically gives you a new LCD projector, as your original brightness and contrast are restored, and most bulbs can be self-installed by the consumer.
DLP
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.
DLP-based projectors come in all prices, resolutions, and picture qualities, so it’s much more difficult to speak in generalities. That won’t stop me from saying, in general, that DLP projectors can produce a brighter image than their LCD counterparts, and their black-level performance is also usually slightly better. DLPs suffer from the same rainbow effect as their rear-projection brethren, although very expensive three-chip models do not ($20,000 and up units).
It should also be noted that DLP based projector technology is the fastest growing segment within the projector industry in terms of sales.
Advantages
- Smoother video
- Smaller box
- Pixels far less visible, (although normally not an issue for business use)
- More “filmlike” on DVD and HDTV
- Generate “blacker” blacks
- Higher contrast
- good pixel fill;
Disadvantages
- Rainbow effect bothers some
- More moving parts (color filter wheel)
- Color filter wheel often produces soft but audible whine
- Poorer reds and yellows at full power
- Color saturation
- Need more lumens than LCD, for rich colors, when dealing with ambient light
LCOS and D-ILA
The LCOS technology is the newest one on the block. It uses the simplicity and cheapness of a standard LCD, and adds elements of the reflective technology perfected by D-ILA to produce an innovative solution.
The 3-Chip version of the system beams pre-coloured red, green and blue light into three separate LCOS chips. The light is simply run through a polarizing filter and aimed at the chip. At the chip, the light pases through the LCD layer, reflects off the back surface and passes back through the LCD layer. The light then exits the chip and heads towards a second polarizing filter. The three separate coloured images are combined into one image using a prism, and then projected through stand lens optics onto a screen.
A single chip LCOS device uses a colour wheel (in the same way as the DLP single chip option) to create the full colour image.
It is very likely that LCOS will quickly move to completely replace conventional LCD, largely because of the cost effectiveness of production and two major optical advantages. The first of the two optical advantages are that the pixel pitch (space between pixels) is at least as good as and possibly better than D-ILA. Secondly, the quality of blacks can be enhanced due to the fact the light has to pass through the LCD layer twice before leaving the projector.
LCOS is used by many different manufacturers and labeled as their own. For example, D-ILA is essentially JVC’s take on LCOS. Sony is also utilizing LCOS technology in it line of “SXRD-branded” products.
Advantages
- Excellent color reproduction
- Excellent contrast ratios
- Nearly seamless images
- Reduction of visible pixelation on the projection image. The gap between the pixels is so small that even when the image is projected onto a 3m screen, it is not possible to see the edges of the pixels. Say good bye to screen door.
Disadvantages
- Still expensive despite more acceptance
- Currently geared towards high-end home theater and commercial uses
Summary
LCD and DLP are very close in terms of performance, and even LCD’s ability to produce good blacks has improved quite a bit. DLP still enjoys a slight edge in contrast ratio, however, mainly because its projectors can get a bit brighter than those of LCDs. The screen-door effect, which appears as a fine grid of pixels overlaying the screen, is also more noticeable on most LCD projectors.
We want to add that 720p LCD projectors still enjoy more features than their budget 720p DLP competition, including highly desirable items such as horizontal and vertical lens shift.
The general consensus is that DLP is a better technology for home theatre than LCD . Most people would say there’s a great deal of similarity between the sets. It seems there will always be competition in LCD versus DLP. Choose your poison.