how to render light only

[sfillat]

rendering with Arnold 5 & May 2019. I'd like to emit light from a cellphone type screen onto the walls of my set but only render the light itself so that I can composite that onto my photographic background. I'm using shadow matte material on my set to catch shadows from my device, but shadow mattes don't reflect light. I tried render layers but could find the right attribute to override to just have the light visible.

tldr: how do I get a render pass with just light and no RGB?

[lee_griggs]

Would Emission work better for this?

[madsd · Accepted solution]

Im using 3ds max,, but you would do the exact same thing in Maya.

A quick solution is to utilize Emission like Lee suggests.
take a look at how to handle the entire proccess of seperating light and composit to get something you can tweak like you want in post.

Set the light layer to Linear burn as a start and go from there.