Bob Atkins , May 15, 2003; 11:58 a.m.
I think the answer is that it's not just heat and UV. If you hit a slide with enough visisble light for long enough, it will fade.
Projectors do use filters, both to cut down on heat and UV and all have forced air cooling to keep slide temperature down.
My guess is that the major problem with a cold light source (essentially a fluorescent bulb) is that you can't get small, high intensity, cold light sources with a continuous spectrum thoughout the visible. Fluorescents also tend to put out UV so you'd need the same UV filtering as for hot lamps, and of course high intensity visisble light will still cause problems.