Hang on.

You really need to show me, because you've brought up new issues now. (differential towards the center). The differential towards the center is because the inner iris fibers, near the center, become transparent and runny, not solid, and also because the iris is donut shaped.

I'm troubled by using words, now, instead of pictures, but I was referring to the idea that you observed the semi-circle that is farther from the light source being the brighter half. Total internal reflection isn't a complicated explanation, when you consider that TIR actually happens any time you're bouncing light inside a liquid. For most side lighting situations, fully 1/3 the interior of the cornea is a concave mirror, sort of like the light collector in a telescope. It focuses light onto the iris.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)