 The range of `phase' is [0, pi]. See
               Particulars below for a detailed discussion of the
               definition.

               [1,1] = size(incdnc); double = class(incdnc)

      incdnc   is the illumination source incidence angle at `spoint',
               as seen from `obsrvr' at time `et'. This is the angle
               between the surface normal vector at `spoint' and the
               spoint-illumination source vector. Units are radians.
               The range of `incdnce' is 