I've been playing around with SSS shaders in the new cycles engine, attempting to understand the logic behind it. I think I grasp the concept and I am on my way to build a decent skin shader. But I've come across a strange artifact and I am skeptical that I've arrived at the correct answer of the source.

On this character, the SSS creates a red ring around the eyes. (My SSS is, of course, red. The eyes are missing in this image, as I was trying to debug the issue. The red appears when the eyes are present also.) I can decrease the SSS scale, but that defeats the purpose. The scale is supposed to represent the depth the light ray penetrates and is diffused. I've set it at .003 here, which I guess would be equivalent to 7cm converting from poser units. Since there is no light inside the head, there should not be this effect.

Here is a shot of the back of his head. I've adjusted the SSS scale so that the light passes through the ears just enough to give a realistic impression. The scale is uniform across the entire body.

So my first theory was that they may be reflections from the eyes, such as the eye surface. So I made those invisible and the problem continued. So I put a camera inside the head to see what it looked like. This was interesting, because there is a skin flap here for the eye to rest against and it showed up bright red, like the ears.

So, my new theory is that these skin flaps create a radiation of red color that reflects back out the front of the face. One possible solution would be to create a morph and eliminate the skin flap so that it does not react to the light and create an ambient red to reflect out the front of the face. This seems difficult and may not even work. Another possible solution, without eliminating the SSS entirely across the entire face, is to create an alpha mask and just turn off SSS around the eyes.

Any other bright ideas?

FYI - lighting is an HDR of an abandoned warehouse, using an EZDome saved from P11. No other lights. This is an G2 character imported from DAZ. The cycles shaders are my own using the techniques from this tutorial. Lance Pham: Skin Material in Blender 2.8