An old thread but wanted to post my experience with using a mirror to increase projector throw distance.
My room is (14' wide X about 11' deep) with the screen on the long wall.
Due to the limited throw distance the largest screen I could use was about 88" wide.
I read about using a mirror but thought it would be too difficult to setup with my cheap DLP because of the limited zoom range and lack of lens shift.
In reality it was quite easy.
I first used a larger cheap mirror, then when everything was working took measurements, added an inch or so for hangers then ordered an optical grade glass first surface mirror from Firstsurfacemirros.com. There may be other sources but I didn't find any Google searching.
The difference between a first surface and a regular mirror is that the first surface has the reflective coating on the front of the glass, not behind. A regular mirror may give you a second, very dim ghost reflection from the glass itself. In practice I didn't notice this, but what I did notice was that mirrors are not as flat as one might think. I tried three different regular mirrors (trying to save the money of ordering the first surface) and each of them produced very slight banding in bright areas of the picture (like sky scenes).
The mirrors from the site listed are optical grade, with extremely tight tolerances. They are also very reflective so any drop in light output from the projector is insignificant.
I gained over 3' of throw and was able to go with a 110" wide screen reflecting off a 24"X12" mirror.
The attached drawing should be self explanatory. I drew it up for another member's room but it is very similar to mine. Each square = 6 inches.Update Nov 11
: I bought the best optical glass FS mirror sold by the link above. Unfortunately I can still see very, very slight banding in panned bright scenes. As with most imperfections casual viewers won't notice but once you see it, it drives you crazy. I don't blame the vendour as likely a polished glass surface worth 10X what I paid would be required. Oddly enough I don't think the plastic FS mirros found in rear projection tv's would suffer from the waves. I think that imperfection is a byproduct of float glass manufacturing. I wouldn't go the mirror route again and am in fact contemplating going back to a smaller screen with a straight shot. Live, experiment and learn.Edited by DavidK442 - 11/11/12 at 7:26pm