First, lets take a look at what I *think* the whole point of the condenser lens is. For simplicity, I will be using a spherical reflector for all examples, with the light source located at the focal point. The blue line represents a fresnel lens and the thick green line represents the LCD panel.

In this example, no condenser lens is used. Note how little of the "light sphere" that is utilized. Probably only 30 percent. You could put the LCD panel closer to the light source and use a fresnel lens with a shorter focal length, but the closer the panel is to the light source, the more heat problems you will have.
In this example, a condenser lens is used. A lot more of the "light sphere" is utilized. Probably 60 percent. The fresnel lens would need to have a much greater focal length.
If you had a condenser lens large enough to encompass the entire LCD panel, you wouldn't need a fresnel lens. This might be possible with very small LCD panels, but as LCD panels get larger, the size of the condenser lens required would become prohibitively large.

Now lets take a look at how things change when a parabolic reflector is used. A parabolic reflector has the potential for capturing more light, but has potential problems with uneven lighting (just try getting an even light spread on a wall using a flash light, for an exaggerated example).

Here is the classic diagram of how a parabolic reflector will focus the light when the bulb is at the focal point. The light beams are parallel.
But the last example is only looking at the light from the back of the bulb that is reflected by the reflector. In this example, we see the light that is emitted from the front of the bulb.
Here the two light components are combined. The two separate beam patterns can present some problems when trying to focus the light into a desired stream. This was not a problem with the spherical reflector.
When using a parabolic reflector, the main light used is the reflected light, which comprises maybe 75 percent of the total light. Here we look at the remaning 25 percent or so of light coming from the front of the bulb. The beams in grey are essentially lost light. Don't really care about that. The blue beam will add to the reflected beam, as it is pretty much parallel to the reflected beam. This is nice, as the reflected beam that would have filled this space was actually partially blocked by the bulb (look back at the first example and notice that the beam directly behind the bulb will reflect and be at least partially blocked due to the bulb being in it's path). So far all is well. But the red beams in this example are potential cause for concern. Some of the beams in this range will be close enough to parallel to make it through all of the lenses in the projector, yet they are not entirely parallel. This could lead to some potential blurring of the projection.

Now lets take a look at some possible projector ideas I have in mind. In general, I'm looking to keep fans and exhaust vents on the back of the unit. I'm also keeping elecronics and plugs away from hot areas as much as possible, with the exception of the ballast, which may generate heat of it's own, and can probably tolerate high temperatures.

Here is a look at a simple straight-line projector. This is as simple as the design can get, unless you think you can do away with the condenser lens and/or heat screen. In the example, the heat screen is the gray line, and would be something like tempered glass. Air vents would likely be between the heat screen and the LCD panel, with room for air to move around the edges of the heat screen and out the back of the projector. This would be the easiest, but may look a bit odd with it's long and narrow shape. The plugs (power and video connectors) are in a less than optimal spot. I would go with a spherical reflector in this design, which may reduce bulb life, but keeps things simple.
Here is my first idea for an advanced projector. This type of design has potentially smaller size, better shape, and better plug placement at the back. Potentially the most interesting design aspect is the first mirror. It is a "cold mirror". Such mirrors are generally designed to reflect at 45 degree angles +/- a few degrees. The reflect most all visible light, but allow light responsible for the majority of the heat to pass through. This provides for a convenient corner chamber that can be vented to the rear. This should handle much of the heat produced by the light, but there is still concern for ambient heating from things like the reflector, blub, and various other components being hot. In this design there would be extra vents around the bulb for some passive cooling. The second fan at the back is an optional idea which may help eleviate ambient heat issues if passive cooling is insufficient. In this first advanced design, the bulb is NOT at the focal point of the reflector, but placed such that an optimal bean is produced for the fresnel lens. Not sure how feasible this is. I'm also not sure what kind of reflector that would require (round, parabolic, or other).
This design simplifies the previous slightly by using a parabolic reflector with the bulb at the focal point. This should produce fully parallel beams, that can then be spread using a proper lens, which I assume would be some sort of concave lens. Such a lens may be more difficult find. The condenser lens used in others so far can likely be salvaged from an old overhead projector; not so easy with the lens required for this setup.
In this version, we go back to using a simple condenser lens with a round reflector. The reason it was not used previously in the advanced designs is that it may very well block much of the heat that is supposed to be projected onto the cold mirror. This would result in concentrated heat near the bulb. I honestly have no idea how much heat would transfer through a condenser lens. It would take some experimentation to find out. Surely at least some of the heat would be blocked though. It may be possible to compensate for this with a fan near the ballast that blows past the ballast and eventually exhausts out the side of the enclosure. The fan is placed on the far side of the ballast to keep the fan from blowing air directly onto the bulb assembly, which would be undesireable. It will also help cool the ballast if the ballast generates heat. If the condenser lens blocks most of the heat, rather than just some, then the exhaust chamber at the back could be eliminated, the cold mirror replaced with a regular mirror, and more agressive cooling of the front light and ballast chambers would likely be required. This design has the additional pitfall that now heat is being exhausted out the side rather than the back as desired.

Last Updated 12/14/2005 by Scott Arnold