I've been reading some interesting articles that advocate the use of LAB mode and with it the Lightness channel for use when applying sharpening - USM. I know from other threads it's a topic of frequent debate so i thought I'd share my findings.
The benefit seems to be that rather than sharpening the colour channels (RGB) and risk introducing colour artifacts, you apply the sharpening to the Lightness channel, which is black & white, and then convert back to rgb.
It would be interesting to hear from anyone who has experience of this method of sharpening and their findings as its benefits seem to be excellent over and above the 'normal' rgb method.
For those who want to try it you change the image mode to LAB, apply a duplicate layer, select channels palette and then lightness, then apply your usm at whatever levels normally used. Once complete convert back to rgb mode. If your familar with layers and can get around photoshop it's easy enough to do.