...On a ideal diffuse reflector : a very rough surface.
First, we will consider a particular type of surface
called an ideal diffuse reflector. An ideal diffuse surface is, at
the microscopic level, a very rough surface.
Because of the microscopic variations in the surface, an incoming ray of light is equally likely to be reflected in any direction over the hemisphere.
Diffuse reflections are constant over each surface in a scene, independent of the viewing direction.