Recently in my work for a different graphic designer I needed to remove from my B&W render with lights only (Vray Raw Lightning channel) all black color leaving only white light with a transparent background. I couldn’t render it but after some search I have found a way how to do it in Photoshop. This is how to:
1.Copy your image to a new layer. Optionally, place empty layer below.
2.Double-click the new layer thumbnail to open the Blending Options dialog.
3.In the “Blend If” section, drag the “This Layer” black slider to the right to remove the completely black background, then hold down Alt (Option on Mac) and drag the right half of that slider further to the right until you see the effect you’re looking for. (See the attached image)
4.Select All (Ctl/Cmd-A) and Copy Merged (Ctl/Cmd-Shift-C) then paste wherever you need the new layer with transparent background. (NOTE: If you have other layers below this layer, turn them off before copying so that all you retain the is transparency.) Optionally, Merge Down with empty layer.
5. At the end with Levels (Ctr-L) shift all grays to white.
I did it for shadows also (Vray Raw Shadows channel) I hope my readers will find application for this trick.