Important Materials Theory - Photographic Rendering with V-Ray for SketchUp - page 257

Understanding translucency
Of course, not all transmissive surface types allow so much light through that they
can rightly be called transparent or see-through. Some, while clearly allowing light
to pass into the volume, retain (to a greater or lesser degree) a measure of opaque-
ness to them.
For instance, if we were to hang thin cotton curtains at a window that has bright sun-
light pouring through, as well as being able to see any color or pattern information
found on the interior surface of the fabric, we would also be able to clearly see the
outline or shape of any solid object on the opposite side of the curtains. We see the
same effect at work when we hold a sheet of paper or a very thin plastic object up
to a bright light. We tend to refer to these particular transmissive effects as translu-
cency.
What we are seeing at work of course is a combination of both reflectance and trans-
mittance, whereby light is being transmitted into the volume of the object, but then
rather than being allowed to pass straight through as with glass, the physical makeup
of the substance causes a measure of diffuse reflection or light bounce to take place
inside the volume before at least some of the light wavelengths are allowed to exit.
How subsurface scattering is different
Now you may well wonder given that description, what the difference is between
translucency and another light interaction phenomenon commonly referred to as
subsurface scattering? Well, the honest answer is, there really isn't any difference,
as both terms are in reality describing the same basic process.