I’m simulating a box made of translucent fabric, light a photo light box… I haven’t figured out how to get this to work. I can -see- the light where it’s hitting the material, but it doesn’t seem to be actually distributing any inside the box. I’ve tried flipping normals.
In this model, there’s a single spotlight above. No ambient light, world color is black.
However… the outside is lighting up 
Any suggestions?
Don’t - you will be lighting your scene with caustics, which is very bad.
You are better off making your ‘translucent’ material emissive instead.
Take a look at this thread.
Should work if you just mix diffuse and translucency shader (then fresnel mix in glossy if desired). Actual translucency don’t rely on caustics, and subsurface scattering don’t transport light at all. I use this setup for thin geometry based curtains all the time.
It is prone to producing noise simply for being indirect, so fake it with emission if you can, i.e. baking it in.
Thanks - the emission could work. I was hoping to be able to place lights, but I think I could figure out emission maps that would accomplish pretty much the same thing…
hmmm…
Thanks, emission is looking pretty good.
If you do want to go the translucency route, check your light clamping settings:
Oh yeah, absolutely. Forgot about that. I have mine turned off as default, it’s such a dangerous setting.