AE Expression to tie image brightness to 3d light intensity value

[Policar]

I'm compositing some stuff in After Effects and I want to light up some fog (generated in Particular) with a 3d light that flickers at the same rate as a practical in the scene. The practical flickers from very dim to very bright and is static in terms of position in a static frame... is there a way to tie the brightness of a sampled pixel value to values for opacity, light intensity, etc.? Some expression?

Any good After Effects training online, btw? Beyond the free stuff like video copilot. My game has stagnated and I either have to learn it a lot better or move up to Nuke (unlikely).