First off, I hope you have a mask for the object so you can adjust it independent of the exterior background. With the object selected, you can try playing around with the Curves, the Levels, or (as someone else has pointed out) the Hue/Saturation (where you will also find a setting for Lightness). Or... you can try this: with just the object showing, hit Ctrl-Alt-Shift-~ (the tilde character, left of the 1 key) [or Command-Option-Shift-~ if you're on a Mac]. This will select the light/luminous areas of the displayed image. If you then hit Ctrl-J or Command-J, those light areas will be copied onto its own layer, and you can set that layer at Screen. Not sure if that will solve your problem exactly, but you could probably modify or add to those steps somewhat and get what you need in the end. To be honest, I have absolutely no idea what you mean by natural and spot luminosity. Would be nice if you could post an image similar to the one you're working on so you can protect your client's privacy while at the same time providing us with a visual aid. :)