Area lamps and emissive materials emit light rays that spread out. How do I make an emission shader that only emits light rays exactly straight to the object's face normals? Is it also possible to adjust the amount of light spread?

  • $\begingroup$ A variation on the answer here blender.stackexchange.com/questions/53548/… might work, with an area light instead of a point light, and the plane in which emission is turned on rotated so it's not edge - on to the scene? $\endgroup$ – Robin Betts May 5 '18 at 19:00

That's not something you can do AFAIK. You could try to block out the feathering of the light with a shadow created by another mesh aperture, but even then you have to deal with diffraction of light around the corners. That just isn't how light works.

For example, see this monochromatic light that diffracts when passing through a small aperture:

enter image description here

I'd suggest you look at the rest of the Wikipedia article where that image came from.

The Cycles behavior is relatively consistent (although it appears to only have the central diffraction maxima):

enter image description here

As far as the amount of light spread, you may be interested in the lamp size property. From the Blender manual:

Size of the lamp in Blender Units; increasing this will result in softer shadows and shading.

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