How do I create a volume emitter shader that is more intense towards the outside of the object? I would basically like to create tubes that look like the blue (not purple) tubes in this image:
(Figure is by Steve Burrows and the Kapteyn–Murnane Group, JILA. From this article.)
But, I cannot figure out how to get the "depth" coordinate of the image in order to feed this to the intensity of the emitter.
Any ideas on how to do this in cycles?