  to viewing
    (observer)         location


   Note that if the target-observer vector, the target normal vector
   at the surface point, and the target-illumination source vector
   are coplanar, then phase is the sum of the incidence and emission
   angles. This rarely occurs; usually

      phase angle  &lt;  incidence angle + emission angle

   All of the above angles can be computed using light time
   corrections, light time and stellar aberration corrections, or no
   aberration corrections. In or