This video tutorial explains the use of a raw light render element when editing your rendered images in photoshop. The key steps in this video can be found below:
00:00 - Introduction
00:54 - Adding a 'Raw Light' render element
01:19 - Rendering the scene
01:45 - Saving all render elements simultaneously
02:25 - Loading the render elements into a photoshop file
03:25 - Using the raw light to brighten the lighting in the scene
04:10 - Using the raw light to darken the lighting in the scene
04:40 - Using in combination with the 'Material ID' element
06:14 - Comparison with the original render