A surface has two sides; usually, only the side facing the light source is illuminated, the other side remains in
double_illuminate is used, the other side is also illuminated.
This is useful for
simulating effects like translucency (as in a lamp shade, sheet of paper, etc).
double_illuminate only illuminates both sides of the same surface,
so on a sphere, for example, you will not see the effect unless the sphere is either partially transparent, or if the
camera is inside and the light source outside of the sphere (or vise versa).