The 3 types of Projector explained: LCD, DLP, CRT

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The 3 types of Projector explained: LCD, DLP, CRT

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Oriphus

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1. March 2004 @ 11:20

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Opening Note: When buying a home cinema projector, it is better to go for a true 16:9 widescreen projector rather than a 4:3. This is because a 4:3 screen has a lot of pixel wastage projecting black lines top and bottom. If you want an LCD home cinema, buy a Panasonic PTAE500. If you want a DLP one, get an Optoma H56 (or H76 if you can afford it).

Four Main things to look for:
1). The type of projector - LCD, DLP, CRT
2). The Lumens output (brightness) of lamp - 800-1000 is good for LCD, DLP at 1000 is good. Dont get it to high or colours will look poor, less than 1500 is really needed for good colour reproduction.
3). The Resolution. For optimal picture make sure to get XGA or WXGA. A cheaper method is VGA/SVGA, but if you can get XGA or higher you wont regret the moeny spent.
4). A very key point is contrast ration. 300:1 and less is poor meaning blacks look dark grey. 700:1 and higher is good giving darker blacks and whiter whites (contrast is difference between these two). 1000:1 + is excellent.

DLP Projection

DLP technology is based on an optical semiconductor called a DMD chip (Digital Micromirror Device), which was invented in 1987 by Texas Instruments (cant remember the guys name). The way it works is that a DMD chip is made up of millions of tiny tiny mirrors, that can rotate at a 10 degrees angle (12 degrees on better models). These mirrors are literally capable of switching on and off thousands of times per second and are used to direct light towards and away from a dedicated pixel space. The lenght of time a pixel is on/off determines the level of gray seen in the pixel. Completely on is lightest (white) and off is darkest (black), although neither are ever pure white or black. The current DMD chips can produce 1024 shades of grey. However, colour is still to be added.
DLP Projection: (note its an old image thus only 3 colour segments)

On a DMD chip, colour is added using a colour wheel. This is a wheel that spins round anything from 150x per second to 250x and more. Most of the cheaper models will be made up of a 4 segment colour wheel. This will be Red/Green/Blue and usually a clear segment. As light is passed through a point on the spinning colour wheel the mirrors switch in accordance to the light. 4 segment colour wheels can produce an effect that is known as rainbow. This is when the projector, in effect, struggles to change between all the colours quick enough, and along a line, the viewer may see a rainbow affect (See fig 1):

6 Segment colour wheels are not effected in this way as badly as 4 segment colour wheels. Note that most viewers will not notice any rainbow affects, but some may. 6 Segment Colour wheels tend to spin faster and are made up of 2 sets of Red/Green/Blue segments thus enabling faster changes between the colours. Some projectors like the Marantz system (Absolutely amazing projector) have 7 segment wheels with an extra dark green colour which can give even greater contrast rations (3000:1+). This is achieved with the Texas Instruments DMD HD2 chip, which is found in some more expensive projectors. A new feature due out in 2004 sometime is the xHD3 technology. It will take the single chip technology to even greater heights by adding a new rear coating to the mirrors and eliminating more of the latent brightness when in the off position. Another upcoming technology is called Sequential Color Recapture (SCR) where DLP systems will replace the traditional color wheel with essentially, a Spiral of "Archimedes" RGB color pattern. This new technology has been mathematically projected to rival the current quality of 3-modulator DLP Cinema systems.

DLP is the future of Home Cinema Quality projection. However, due to the high cost of manufacturing the chips, LCD and CRt are more viable options on the cost front. The potential drawback of this single-chip DLP technology is that in any given instant, the picture on the screen is not the total image, but is instead rapidly alternating between images consisting of the individual red, green, and blue colors. Thus the eye and the brain play the last critical role in making single chip DLP projectors work, by combining or averaging or integrating the picture, so that the viewer perceives the desired image and not the rapidly flashing momentary components of the image.

A problem with DLP can also be light leakage around the mirrors ending up with a halo effect on some scenes. This has been addressed with the new Dark chip (DDR) which has a dark coating underneath the mirrors.

LCD Projection

LCD Projection is by far the cheaper method of getting great home cinema. I have a Panasonic PTAE500 myself which is great and highly recommended. LCD (liquid crystal display) projectors contain three separate LCD glass panels, one for red, green, and blue components of the image signal being transfered to the projector. As the light passes through the LCD panels, individual pixels can be opened to allow light to pass or closed to block the light. This activity modulates the light and produces the image that is projected onto the screen.
Below is a diagram showing how it works:

LCD is generally more 'light efficient' than DLP meaning that a 850 Lumen LCD projector (like my Panasonic) will produce a brighter image than an 850 Lumen DLP projector. LCD tends to produce more saturated colours. However, the colours being more saturated makes people perceive the projector to be brighter overall, even though the DLP white may be brighter. LCD tends to produce a sharper image. This can actually be a bit of a disadvantage for video, where it makes the pixellation more obvious. However, on a good XGA model you wont really notice it unless up close. To explain this more look at the following image. LCD's have a low fill factor whereas DLP has a high fill factor:

This can sometimes cause what is known as a chicken wire effect. Again this is really not noticeable to most people unless viewing the screen up close. This is sometimes referred to as Screen Door Effect (SDE). An example of this can be seen below: Note the image is magnified a lot.

DLP projector technology is generally preferred for home cinema and portablity. LCD projectors are generally better where colour reproduction is critical. To be honest it is a personal choice and a financial constraint that helps people make up there mind.

CRT Projection

CRT- Cathode Ray Tube
This is the largest and most experienced player. Many of you have probably seen this kind of projector at a bar or in an auditorium at a school. CRT utilizes three tubes, sometimes called "guns." The three colors combine or converge to make your image. CRTs do not have a fixed number of pixels, making them versatile machines capable of clear images from higher or lower resolution sources. CRTs also require periodic 'calibration' by a trained professional, which can mean additional expense for the end user down the road. These projectors are generally used in fixed installations because of their size. CRTs are typically not as bright as the other technologies, but the refined picture quality of a properly calibrated CRT is tough to beat. Its worth noting that to replace the Guns, it costs quite a bit of money (on average £500 per gun).

Conclusion

If you want the best money can buy, a three chip DLP projection is the best you can get. At around £20,000 for a Marantz system its expensive. Next best is something like Marantz 7 Segment Colour DLP projector at around £7,000. Down from that you have the Optoma H76 and H56 DLP projector which in my opinion are as good as anyone would want.

The slightly cheaper method would be the Pansonic LCD projector i have already mentioned. There are a vast number of projectors out there. Be sure to remember the four key points i made at the top of this post when looking for your projector.

Hope this helps everyone
Thanks
Chris McCann
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