This script contains a few simple tools related to luminance.
It installs three separate scripts under Filters/Luminance. These are:
Extract - this extracts a layer's luminance to a separate layer, and leaves only the chroma information to the other layer. You can get the original layer back by merging the layers.
Remove - this removes luminance information from a layer, ie. sets the luminance to 50% on the whole layer. Basically it makes the layer into a chroma layer.