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?