A Lesson on Projector Types

by Alan Staples. Last Updated March 14, 2006

Our website is dedicated to people who want to build their own video projector, or more specifically, a "DIY LCD projector". But what does that really mean? Here is an article for those of you who ask that very question. To someone who has been a member in our forums for a long time, or who has done a substantial amount of reading, this article is probably review.

Firstly, let's list the various types of projectors - there are more than you realize once you actually sit down and think about it. Firstly, we have LCD, the technology you are probably most familiar with, then we have CRT and CRT - two other projector technologies that are incredibly common, but perhaps not as well-understood. While we're mentioning acronymic projector types, we had better mention DIY, OHP, and finally OTC or OTS.

That's a lot of acronyms to be sure, but with a short bit of reading, you will be well on your way to mastering the domain of video projector technology.

CRT

Where it all began. CRT, or Cathode Ray Tube, is the technology that your first television set used. These are typically large, non-cylindrical glass tubes with a phosphorus coating on the inside of the image area. This coating glows when an electron beam strikes the surface, creating the image that you view. In a CRT projector, the beam is intensified so that you get an extremely bright image. Then a lens, typically placed directly over the entire front surface of the CRT, focuses the image onto a viewing screen at some distance in front of the unit. Because CRT tubes aren't very bright relative to the size of image that is demanded of them, most manufacturers use three tubes instead of one. Instead of having each tube display the same exact image, each tube displays only a certain color of the final image - one tube each for red, green, and blue (RGB).

LCD

This is an aged technology that has only recently become one of the most popular display technologies for the end consumer. LCD's (or Liquid Crystal Displays) are what your flat computer screen or flat screen television are probably made from, though there are other technologies that can achieve similar results. LCD's, unlike CRT technology, don't produce their own light. Think of them as a transparency sheet that can dynamically and continuously re-draw the image. How we see that image is by placing a backlight behind the LCD, in the case of computer monitors, or to place a very bright point light source behind the LCD, in the case of LCD projectors. Similarly to a CRT projector, three different LCD panels (RGB) are typically used in a projector to achieve greater light output.

DLP

Digital Light Processing projectors are an incredibly smart invention thanks to Texas Instruments. This projector technology requires more space for explanation than this article allows. The basic concept is to use millions of tiny mirrors in front of the light source to reflect the light towards the projection lens whenever a certain part of the image needs to be illuminated. Whenever that same location needs to be dark (i.e. black), the mirror reflects the light away from the lens. This is advantages because it allows for near-perfectly black pixels to be displayed, which isn't as easy with LCD projectors. The only downside to DLP projectors is that instead of drawing the red, green, and blue images simultaneously, each is drawn after the other quickly so that your eye cannot distinguish any one color from the other - this process works okay, but some people can actually see the individual colors so that the image appears to flicker red, green, and blue at times. Entry-level models of this type of projector tend to cause eye fatigue for this reason, though progressive model revisions are improving this effect.

DIY

The theme and soul of this website - Do It Yourself. It is indeed possible to build your own video projector. This notion was started a while back when some people used small television sets (CRTs) and built relatively inexpensive CRT projectors with them. The results weren't fantastic, but at the time the alternative was to purchase a $10,000 CRT projector. More recently, we have been able to build relatively inexpensive LCD projectors thanks to the boom that market has experienced. To learn more about DIY LCD projectors, visit nearly any page in this website - it's full of DIY projector knowledge and how-to documents!

OHP

Overhead Projectors are very common in classrooms and presentation halls everywhere. Recently, they have started to be replaced by relatively inexpensive LCD projectors instead. This has brought many surplus OHP's to market, especially at auctions (e.g. eBay.com). An OHP can be used to build an inexpensive DIY LCD projector since it has most of the necessary components built-in. The disadvantage is that the lower cost of an OHP comes at the expensive of quality - OHP's tend to produce a yellow light that is undesirable for LCD projectors. Also, OHP lamps typically fail every 50-100 hours, which can eventually be expensive as well as frustrating when the lamp life of a DIY projector is considered (DIY projector lamps typically last for 10,000 to 20,000 hours depending on lamp model).

OTC, OTS, and OTB

These are become more common terminology for the consumer market, and don't apply only to projectors. OTC (Over The Counter) or OTS (Off The Shelf) projectors refer very generally to and video projector that is mass produced and available ready for plug-and-play out of the box (OTB). These used to be so expensive to purchase that they were impractical for nearly any application. Recently they have become very much more affordable, but there is a caveat that people should consider when purchasing one. Even though an inexpensive OTS projector may cost you less initially, its lamp fails much more frequently than a DIY projector lamp does, and replacements cost anywhere from $250 to $700+, whereas DIY projector lamps cost about $50. This effectively means that you can spend $250-700 or more per year to enjoy your OTS projector, or spend about $50 every 3-7 years to enjoy your DIY projector. Can you guess what our investment advice is here?

Regardless of what projector technology you use, we feel that it is very important to understand the differences before buying. This can save you a significant amount of money in terms of lamp replacements, or the frustration of immediately wanting to upgrade in the case of a cheap DLP projector that causes eye fatigue. Either way you go, a video projector is an excellent way to bring the Silver Screen experience into your own home theater.

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