I find something interesting in this slide. Considering only the amplitude of the light, it is obvious scattering function cannot go minus value. But actually, besides the amplitude there is phase also in the light, which means two nearby units which reflect some specific lights, the output light in certain direction is dark (no light at all). Is there any model taking the light phase into consideration?
I'd think that light sources generally have a random distribution of phases, so this wouldn't be something to take into account usually...
A similar line of thought would be how to take into account polarization of light.
Most path tracers used in computer graphics assume geometric optics; that is, light can be represented by rays, and the radiance associated with those rays are real-valued, non-negative quantities. There are, however, ways to generalize path tracers to model the polarization of light, or even to model the effect of interference (by taking into account phase). For example, one of our CMU faculty members, Ioannis Gkioulekas, recently wrote a renderer to simulate laser speckle by tracking the complex phase of light. (Strictly speaking, this still involves geometric optics since we are still modeling light as rays, but tracking phase or polarization is a step towards modeling the wave-like nature of light.)