<a href=showthread.php?s=&postid=13600221#post13600221 target=_blank>Originally posted</a> by beerguy
I've never used one but I have a friend who's sponsored by them. He swears by them.
For folks who don't know, it's a cap that fits over the lens. You point it at the light source and set a custom white balance.
You can do a close approximation by holding a standard coffee filter over the lens.