Difficult one to word in the subject, but basically I want my pyro sim to cast light onto my object and then render that object illumination out as a separate pass I can comp onto my render.

The render is done in another software, so really I only want a pass to show that the object lighting is changing with the effect of the pyro. I guess ideally I'd have it render out on black with just the light flicker on top.

I'm guessing I have to switch on GI to get the illumination working right, then then what shader is it to just catch this light?

Any ideas?

Hope I've explained this right.

Much appreciated!