Surfacing - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 205

At this point you should have a decent
skin material with three texture channels,
one each for color, specular intensity,
and normal variation. Try a test-render
to see how we're faring. Figure 7.41
shows the result.

Human skin, as well as a host of other
substances (marble; jade; lots of foods like
milk, apple, and chicken) exhibit an
effect called subsurface scattering
(SSS). It is most obvious when holding
a flashlight against your fingers or putting
one in your mouth and puffing your
cheeks! The red glow that you see is
caused by the light passing through your
skin and other tissues, which are semi-
translucent, bouncing around a bit, then
coming back out. Without this important
effect, rendered materials for these types
of substances will not look believable.

Figure 7.39 A face map prepared for the Normal channel.

In the Material properties for the
head, enable the Subsurface Scattering
panel. From the Presets menu, select
Skin 2 . The provided presets are good
starting points, and skin usually only
requires a tweak or two to get it looking
right. Figure 7.42 shows the SSS panel
with the final settings for the example
skin. The three main values that you will
need to adjust are:

Scale: This is the most important (math
warning!). This refers to the scale of
your object, as opposed to any kind
of scaling factor or tool. Calculating
the correct value involves some
serious mathematical voodoo, so
we'll just resort to the rule of halves.
Try 0.5 first. If there is too much of
the SSS effect, reduce by half (0.25).
Too little, increase by half (0.75). Figure 7.40 A specular map for the head.