I've been trying to model a light-pipe also, and I've gotten internal
reflection to work, but only if I turn conserve_energy off. If it's
enabled, I get no visible internal reflection at all. My photons are all
emitted in a very narrow beam (spotlight of 0.04 degrees divergence), which
requires an extremely small photon-spacing on the glass pipe target, but it
doesn't seem to make any difference how small the spacing is unless
conserve_energy is off (i.e. you get no visible effect from photons, even
with no other lighting in the scene).

My light pipe and light source are configured such that I should be getting
total internal reflection. So perhaps conservation of energy isn't
important in this case, assuming that 100% of the incoming light gets
reflected (internally). But I wanted to look at step-index light pipes, so
ideally there would be partial reflection at the interface between two
slightly different indices of refraction. I think this will probably
require that conserve_energy is enabled for realistic behavior to be seen.

Dave

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