So I have been doing a bit of experimenting with light tunnels used in DLP projectors.
By having a light source converge into a rectangular mirror tunnel matching the aspect ratio of the LCD you are have you can achieve 90% or more light capture with near 100% light distribution.
I actually found a patent on it when doing research. https://www.google.com/patents/US5902033
When the light exits the tunnel it creates a number of duplicate rectangular projections which scatter. By placing a lens after the light tunnel, it overlaps all the rectangular projections over each other, exactly the same way fly eye lenses work in LCD projectors. By placing two lenses after the light tunnel with short focus lengths you can reduce the amount light deflecting out away from the tunnel and creating a smaller emitting area while having better control of the projection size.
There are a few issues.
1. Long focus points. based on my experiments you looking at minimum 500/600mm fresnel for the back, so folding mirrors will have to be used. This can be countered by using a larger or shorter light tunnel but mean larger lenses and larger emitting area. However the benefit of a longer focus point is less deviation of the light meaning better detail. 4K anyone?
2. The smaller the light tunnel the more heat. Front surface mirrors are only about 97/98% efficient, meaning a small amount of light is absorbed. Any where from 15000 to 20000 lumens and you looking at 1 watt. Not much but can become a lot the more when it bounces multiple times within the light tunnel.
3. Getting the light into the tunnel. Getting a 15x15mm 128W LED array into a 10x6.25mm light pipe (1.6 aspect ratio) can be tricky. By projecting the LED onto a round Fresnel, then using a number of smaller fresnels like a fly eye lens to break the light into smaller pieces. You then have a second fresnel focus all those smaller fresnels over each other onto a point getting down to a 5mm focus point. The more smaller fresnel/lenses the smaller the emitting area.
The alternative is to use an array of smaller LED's like the Cree XHP70 rated at 4000 lumens. With a 7mm emitting area this would be perfect.
4. Light pipe size. The average sizes I have found online are about 5mm wide internally which is what I've been experimenting with. I'm looking for a about 10x6.25mm +-. so when the holidays are over I'm going to try get a front surface mirror water jet cut to my required sizes. The finish and if they can cut the small sizes are my concern.
I looked at making a tapered light tunnel, the issue I encounted was as the tunnel got smaller the light interacted more with the joints between the mirrors when bouncing more between the horizontal and vertical mirrors causing more light loss into the mirror and defects in the light. But will try again with the water jet cutting.
Im awaiting some new fresnels and short focus lenses and will post a test rig Im building. Struggling to get anything larger that 6" with a 600mm fresnel, but hoping the short focus lenses will get that up to 10".