Separate Luminance

FX-Foundry/Color/Separate Luminance...

Creates two new layers: one containing the Luminance, and the other the Chroma information in the form of a Grain Merge. Optionally, it will color-enhance (maximize saturation in) the Chroma layer while still keeping it luminance-neutral.

This allows you to do noise-reduction and sharpening on the two layers separately.

Update (23-Jul-2009):
Noticed that highly-saturated colors exceed the ability of the grain-extract/grain-merge to handle without clipping the color, so I have made the excess color get folded into the luminance layer. Most of the time, it will be un-noticeable, and even when it is, it shouldn't make any real difference. If you want to be sure your Lum layer is pure luminosity, you can check Excess color on separate layer. When merging, always merge onto the Lum layer rather than merging the two color layers together.