                      observer.

   The diagram below illustrates the geometric relationships
   defining these angles. The labels for the incidence, emission,
   and phase angles are &quot;inc.&quot;, &quot;e.&quot;, and &quot;phase&quot;.


                                                    *
                                            illumination source

                  surface normal vector
                            ._                 _.
                            |\                 /|  illuminat