Projection Theory

Light Sources

Every LCD projector needs a light source. A good light source is one that has high output, but low temperature and has a high color temperature and is relatively small. Light output or brightness is measured in lumens. A good lumens rating is 15,000+ for a DIY projector light source. In actuality you will get a fraction of this many lumens from your system because of inefficiencies in design and mainly because the LCD panel filters all but about 10% of incoming light. Color temperature determines what part of the color spectrum a lamp produces the most. For example, an orange parking lot lamp produces color in the 2000-3000k range, but a higher number of 6,500k or 10,000k would be closer the color of xenon head lamps in many new cars. 

There are many different technologies available for lamps. They break down into two basic categories: incandescent and gas discharge. Incandescent lamps include the light bulbs that are used in your home, in most appliances, and in most flashlights. They are not ideal for use in projectors because they tend to have a low color temperature and don't produce enough lumens per watt of energy consumed. This brings us to gas discharge lamps. These include fluorescents, xenon, metal halide, sodium and other gas vapor lamps. You are probably familiar with these lamps, which as commonly used in grocery stores, warehouses, and outdoor lighting (i.e. parking lots and highways). These lamps have color temperatures that vary over the entire spectrum, but they are very appealing because they typically are very efficient. That is, they produce a high number of lumens per watt of energy. This is ideal for LCD projectors since higher wattage lamps produce more heat, and too much heat will harm LCD panels.

Interesting Information
If you own a luxmeter, you can measure the brightness of your projected image by using the following formula:

ANSI Lumens = Illumination (Lux) x Screen Surface (square meters)

Best Choice: Metal Halide
Metal halide lamps are a type of gas discharge lamp that produce all of their light from a region that is usually about one inch in length. These lamps produce an amazingly high number of lumens per watt of energy consumed. This means that it is possible to get a very bright light while producing a relatively low amount of heat. One great benefit to metal halides is that they tend to produce light that is of a high color temperature. Two of the most commonly used metal halide setups are a 400 watt mogul setup and the 250 watt HQI setup.

Both types of setups will give good results in a DIY projector. More watts means you get more light output, but only if you are comparing lamps of a similar technology. For example, a 400w metal halide is brighter than a 250w metal halide, but a 400w incandescent lamp is not brighter than a 250w metal halide. Our 250w and 400w metal halide kits run for 10,000-20,000 hours between lamp changes, which means that you can run your projector about 5-10 years (depending on particular usage) before needing to change lamps.

The type of ballast, or lighting electronics, that a metal halide uses is what separates cheap, bulky units from our higher-end, lightweight ballasts. A parking lot light will use a cheap ballast that uses a magnet and capacitor to keep the light going. These ballasts weigh 20-50 pounds, generate magnetic interference, and make a "buzzing" noise. Our ballasts weigh 3-5 pounds, generate mild magnetic interference (but at a higher frequency so that it is less detectable) and make no sound at all. The difference in price will be $50+ depending on whether you buy used, new, etc., but it is well-worth the extra money.

Not Recommended: Halogen and Florescent
Halogen lamps are most commonly used in automobile headlamps and most overhead projectors. They are bright, but tend to be yellow in color and require replacing relatively often to other types of lamps. Fluorescent lamps are very efficient, but their spectrum is not always ideal for projection, and they are typically too large to be useful in DIY projectors, which require a "point source". A point source means that all of the light generated by a particular light source comes from a very small region. For example, a compact florescent lamp produces light from a region that is about 6 inches long (not a point source), whereas a typical camera flash bulb produces all of its light from a region that is less than 1 inch long (a point source). If a light source is not a point source, then it can most typically not be applied to a DIY projector.

There are some people who claim to get good results with Florescent light sources. This would require that the lamp be used as a backlight, placed directly behind the LCD panel. That means that the light source would most likely need to cover the entire back surface of the LCD. Unfortunately that is very difficult to do while maintaining an evenly lit image. Also, most florescent lamps don't have a color temperature that is ideal for video projection. It is, however, still possible to use florescent lamps as your light source if you so choose.

Alternative Light Source: LED
LED is an acronym for "Light Emitting Diode". These are most commonly used in indicator lights on computers, backlights for telephone keypads, and in small flashlights. The appeal to using LED's as a light source is due to their compact design, very low energy consumption (<0.05 watts per LED), and they produce a very true white light. All of this adds up to an ultra compact, ultra-white light source. The problem is that they cannot be used as a point source light because one LED by itself doesn't produce an appreciable amount of light output. Instead, LED's can be clustered together in large arrays and then used as a backlight, placed nearly in contact with the back of the LCD panel. Our tests have concluded that even dense arrays of super bright ~40,000 mcd LED's are still not quite bright enough to work as a light source for an LCD projector, though there are some exceptions. This doesn't rule out LED's as a possible light source though since this technology is constantly improving, but as of May 2006 LED arrays are still not able to compete with metal halide light sources.

Comparison of Lamp Sizes

© 2006 DIY For Life Inc