[SOLVED] One-sided mesh lamps in Cycles

Hi, no doubt this dumb question has been asked before, but I can’t find an answer.

How do I set a plane to emit on only one side? Do I create a box and apply to one face?

Thanks for reading.

You cannot, Cycles direct light function invert normal depending on side, and indirect (when main ray hit mesh) it use same algorithm. I have trouble with that trying to make bi-dir emission photon chain from mesh. For now, make another face close enough to dark side (but keep gap between more then shadow probe epsilon, IIRC it 0.00005 units or so).

You can mix it with a plain diffuse material using the geometry node > backfacing output as the factor.

This will make the material shine light in one direction while showing as a diffuse surface on the opposite side.

Thanks Folks. Much appreciated.

Here is the noodle for anyone interested

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