 emissn      is the emission angle at `spoint', as seen from `obsrvr'
               at time `et'.  This is the angle between the surface
               normal vector at `spoint' and the spoint-observer
               vector.  Units are radians.  The range of `emissn' is
               is [0, pi].
 
               See Particulars below for a detailed discussion of the
               definitions of these angles.

               Note that if the target surface is non-convex, an emission
               angle l