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<PRE>
 

   The term &quot;illumination angles&quot; refers to following set of 
   angles: 
 
 
      solar incidence angle    Angle between the surface normal at the
                               specified surface point and the vector
                               from the surface point to the Sun.
 
      emission angle           Angle between the surface normal at the
                               specified surface point and the vector
                               from the surface point to the