ion
                              \    phase      /    source vector
                               \   .    .    /
                               .            .
                                 \   ___   /
                            .     \/     \/
                                  _\ inc./
                           .    /   \   /
                           .   |  e. \ /
       *             &lt;--------------- *  surface point on
    viewing            vector            target body
    location         