Has anyone ever come up with the idea of building a box out of fresnel lenses with the center of the box being the focal point for all the lenses? You get 100% collimated light out of this setup, it's just not all collimated in the right direction.

For simplicity let's suggest a typical fresnel in front, and two additional fresnels on each side. You've got good collimated light coming out of three faces. And conveniently enough professional PJs do just that, pulling it all back together with the x-cube prism. Yes, we would need a "prism" the size of a hat box, but maybe someone else can add information about partially silvered mirrors to the conversation???

Yes, there are obvious enhancements, such as a spherical reflector positioned such that light only really exited the front, side, and bottom of the cube, but why worry about the little things this early.

I estimate the typical system with a rear reflector only makes use of about 20% of the light that comes off the bulb, most of it being wasted in the left/right/up/down directions. The problem of recombining three separate light paths has obviously been overcome, but can we figure out how to do it on a scale and cost effectiveness that puts this in reach of amateur builders with 15" screens? How about with 7" screens? Just my $0.02

Well the obvious problem is that the fresnels will probably melt.

Even with a single fresnel 220mm away from the light , without proper cooling it will melt.

A light box full of fresnels would most likely become a gooey melted mess.

I know this idea will probably never fly, easier to put brighter bulbs in than to build more efficiently, but its an interesting experiment.

I put the top/bottom fresnel on the bottom to avoid heat. Other than that, the three fresnels are the same distance from the light as every other PJ, just more of them. And the most important argument against meltdown is that the bulb would be one fifth as powerful to put out the same energy. Surely you could contain a 100 watt bulb or smaller 220mm from a fresnel behind glass (or a glass fresnle).

And actually the use of parabolic reflectors is a much simpler gain anyway, one that I'm plogging in another thread.