Subsurface scattering (SSS) is the phenomenon when light enters a translucent material and upon entering it the light scatters inside the material before either being absorbed or leaving the material at a different location. I know this sounds like a hard idea to grasp but it’s a good concept to keep in the back of your mind when painting skin or other translucent materials.
Subsurface scattering is mostly used in 3D applications, but I apply it when illustrating organic characters such as people. Adding a red tint to parts of the character such as the fingertips, nostrils, and ears really brings a character to life. SSS occurs most often when three conditions are met: translucent flesh, thin forms, and backlighting. Any organic surface where the light doesn’t have to travel very far to emerge out the other side is where this phenomenon occurs. Why do you add this additional color to these areas? Because of SSS many organic and inorganic materials are not totally opaque at the surface, so light does not just bounce off the top of there surfaces like it would in say a material like metal. Opaque materials allow light to enter the surface and once inside the light scatters around. As the light scatters around it takes on the color of whatever is inside the material. Finally, the light emerges out at a different location, but it projects a hint of the color of what is on the inside. For example, human skin is opaque. Blood is trapped within the skin. The light enters the skin. Then the light enters the blood and emerges out the other side with a reddish tint. A great example of SSS, hold up your hand in a dark room and using a flashlight shine the light back through your hand and watch how the light travels through your skin and gets scattered, taking on a reddish color.
Subsurface scattering not only occurs with skin but other surfaces like the skin of fruits, leafs, marble, milk and gels. Without knowledge of SSS photo-realism really cannot be achieved so next time you’re drawing keep in mind the principles of subsurface scattering and watch how your characters are brought to life.