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I am trying to create a light that only illuminates volume scatters.

The input > light path node has "is X ray" outputs for all the various types of rays coming off a surface. However I'd like something like an is volume ray output to tell if a ray is coming from a volume. Is this possible?

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    $\begingroup$ Unfortunately I don't think it's possible atm.. :/ $\endgroup$ – gandalf3 Aug 12 '15 at 3:15
  • $\begingroup$ This is a great question, and although an "Is Volume Ray" feature would be very handy, I'm wondering - could you use the "Is X Ray" output to determine that it is not a surface output (and therefor presumably a volume output) as a solution for your particular use case? $\endgroup$ – Mentalist Aug 12 '15 at 19:56
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Okay I am not sure about using some sort of 'Is Volume Ray' value to make a light source only illuminate a volume, but if you simply want a lamp/meshlight to only illuminate a volume scatter material, in the ray visibility settings of that light, you can leave only the 'Volume Scatter' ticked and the rest unticked:

enter image description here

Keep in mind that the light source may still end up illuminating the scene due to multiple volume bounces and the density of the volume.

Here is a simple scene with a point lamp directly above suzanne with a volume scatter material and a maximum of 32 volume bounces. The ray visibility is of the lamp directly above suzanne.

enter image description here

Here is the same scene except with only the 'Volume Scatter' ticked for the point lamp.

enter image description here

Here is the same scene with only 8 volume bounces. You can see how there is more light exiting suzanne with 32 volume bounces than with 8, though it is a very small difference.

enter image description here

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