I just made a few checks indoors using a red LED with my 350D DSLR, 18mm lens with a Star Analyser 200 on the front. There was a slight reduction in dispersion close in but once beyond 5m the change was very small
Relative to the dispersion at 10m, the dispersion reduced by
5m =0.9% 1m = 3.5% 0.5m = 5.7%
Cheers
Robin