I’m trying to light my scene physically accurately and I’d like to know if emissive materials have a unit or if they are completely artist determined?
I see in the docs
If you want to be really neurotic about it, you’ll need to use static lighting. Frankly, it’s easier to eyeball it 
Yes, as shown by the screenshot above. You can measure the luminance with the HDR (Eye Adaptation) view mode, but because there’s such a huge range in luminance values with real sources and they can vary drastically across a single surface, you’re not going to find too many helpful reference values to compare against.
@ClockworkOcean Thanks for that, I missed that page. Maybe it’s a stupid idea - I’m not really trying to light anything with the emissive, but trying to accurately set the brightness of stars in the sky relative to my sunlight. I currently have a PP material which additively projects stars into the sky. Caveat of the project is that the sun can be any star (thus be much brighter or dimmer than our sun) and the directional light brightness will be set accurately. Having a physically accurate star light should make it appear correctly in every lighting scenario.
Will post the results when I have them 
Sounds interesting 