Anamorphic Lenses - What they do & Why they're cool:
An anamorphic lens will stretch the picture either by compressing the height (vertical compression, VC), or by extending the picture width (horizontal expansion, HE). For the type we're concerned with, you need two triangular prisms of sufficient size that are placed in front of the triplet. The same two prisms can be configured for HE or VC depending on how they're set up in relation to the projector. The prismasonic page does a fantastic job of explaining how the lenses work for different types of panels.
The following is a short and dirty version of the explanation from the link - if you don't feel like checking it out (but you really should).
This is a 4:3 panel, with a widescreen image being displayed. The black bars take up 33% of the panel, and represent wasted resolution and brightness, as they aren't contributing to the output of the projector.
The image is then stretched, easy to do via computer, and now the full panel is being used - you regain the lost brightness and resolution. But now everyone is too tall- the geometry of the picture is not correct.
The anamorphic lens then modifies the picture to restore the correct geometry. This can be done either by vertically compressing the picture with the lens, or by expanding it horizontally. If you use VC, the picture width stays the same. If you use HE you get a larger picture than you would without the lens.
The net result is an increase in available resolution for watching widescreen material, and also an increase in brightness. These lenses are useful for those with widescreen projectors as well - check out the above link. And who doesn't want a higher resolution and brighter projection with the addition of only two external lenses and a software tweak or two?
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I have been looking into a solution to use the extra resolution available on my LCD for widescreen movies. The idea is that you use the full panel for the movie- stretching the image vertically (so everyone looks tall). The lenses then compress the output of the triplet back down to the correct image height. The benefit is that the resolution and brightness which would otherwise be wasted displaying black bars are now used to make the image.
Here is a sketchup model demonstrating this. The yellow is the lightpath from the full panel, and the red is the same light compressed by the prisms. (not to scale- for illustration purposes only
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DIY anamorphic lenses are not exactly a new idea, but applying them to the DIY projector is a little harder than on a commercial unit - which is what has been done. Due to the size of the triplets that we use, the prisms to compress the image must also be quite large. A huge thread can be found here, detailing some of the progress others have made here. I used some info I found on that page to construct two prisms. They found that two 10 degree prisms are sufficient to compress the image. The prisms need to be filled with some clear material - they had used water or mineral oil. As there seemed no discernible difference in quality, I went with water, as it is much cheaper (free). I made my prisms from picture frame glass, and a cutting board (well, 2 actually). The people from that thread are now using optic crystal wedges- unfortunately, the wedges are much too small to be used on our projectors.
The glass might be sourced from a glass shop or building supply for cheaper, but Micheals had a 35% off sale, so the price was right. Plus all my glass was exactly the same size.
The cutting board was the only material I could think of that was pretty easily machinable, thick enough to cut grooves into (~5/8"), and plastic. Plus, at $7 they're not too pricey. You can see the groves cut, I used a dremel with a collar attachment to get an even height. I cut the cutting board with a hand held wood saw very easily.
)- so I cut out the silicone and re-applied. 24 hours from now I'll refill it and hopefully have better luck. 
