An extreme example of how the incident surface radiance may be different from the exiting surface radiance is subsurface scattering. In my favorite 3D graphics software Maya, the standard Arnold renderer shader has a feature called subsurface scattering, where it allows light to go beneath the surface. Some light becomes absorbed completely into the surface while others come out in distorted ways. I often use this feature to shade skin, wax, and rubber. It helps create the effect of when you shine a flash light behind a thin piece of rubber and it looks semitransparent and glowy at the same time.