I appreciate that in reality a light source is always emitted from a physical object, but for the sake of a creative concept, this may not be the case. In the attached images there is a hollow sphere (solid; thickness) with coincident lenses on the front, back, left, right, top, and bottom. There is a Keyshot sphere at the exact center of the object with an area light attached.
If 'visible to camera' is disabled, but 'visible to reflections' is enabled, the image does not change, i.e. the area light sphere remains visible.
If 'visible to camera' and 'visible to reflections' are disabled, the area light sphere becomes invisible (as intended, in this case), but the interior of the hollow sphere is no longer illuminated.
Is there a way to make the area light sphere invisible, but illuminate the interior of the hollow sphere?
Model and Keyshot scene are attached.
That the Area Light is not illuminating the interior of the "sphere_hollow" object is because it is a Metal.
Here's some physics for you to help explain the behavior...
Metals have a very high absorption of incoming light. This absorption, perhaps counter-intuitively, causes (almost complete) re-emission of incident light. This explains the high reflectivity of metals. As a consequence, metals do not have diffuse reflections, because light cannot enter them.
So what you are seeing in KeyShot is physically correct. :)
If you want the interior surface to be illuminated, then you can change the material of part "sphere_hollow" to e.g. a Plastic (or any other non-metal material).
I hope that helps.
Dries
Wow, that was a horrible oversight on my part, thanks again Dries
Well, I guess the optical differences between metals and dielectrics are not entirely trivial. :)
Dries
Definitely not trivial, I'm glad you pointed it out.
The reason I put together the scene was to troubleshoot the 'light dots / noise' artifacts that can occur when rendering GI and transmissive materials in Keyshot.
For example, why does the jewelry preset use the Interior rendering mode, rather than product mode? For example, what is it about the interior rendering algo that helps resolves (denoise) caustics better than product mode?