Now the problem as I said is that there are curved bands of equal luminance. Our eyes detect and amplify these, as well as creating the illusion of varying luminance within each band.

To stop this, I introduce some random variation in luminance, using a Noise node connected to the Diffuse_Value. This is just one way - you could also accomplish this with some very slight bump noise.

The Noise min is .95 and the max is 1. So it produces luminance varying from 95% to 100% of the value specified in Diffuse_Value. You can adjust this as you like. You are trying to hide the banding without actually introducing a noisy looking texture. (Unless you want that.)

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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)