In Corona, when something isn't visible to the camera, it's pretty much non-renderable, so it will not affect the scene in any way. Do I understand correctly that you would like to have a volume grid which is emissive (casts some light into the scene), but is not visible directly? Why exactly do you need that? What's the use case? I think this may not be possible, but maybe I could offer some workaround once I understand the specific scenario better.