Here's an interesting link to what Kodak says:

Note Prolonged exposure to light and heat adversely affects most photographic dyes. Do not subject valuable original slides to prolonged or repeated projection, e.g., in a commercial display.

The same holds true for transparencies. Although the level of illumination is usually lower in light boxes, prolonged exposure to light is harmful, especially if direct sunlight (which contains a large amount of ultraviolet radiation) strikes the transparency. Use an ultraviolet absorber for fluorescent lighting displays. Rosco Laboratories, Incorporated, 36 Bush Avenue, Port Chester, NY 10573, produces such an ultraviolet absorber. Use duplicates of valuable originals for displays. Store the originals under the best possible storage conditions, as described in the preceding sectio