Therefore, if an electric field E i n c ( x , y ) is incident on the aperture, the field produced by this aperture distribution is given by the surface integral Ψ Ψ ( r ) ∝ ∝ ∬ ∬ a p e r t u r e E i n c ( x ′ , y ′ ) e i k | r − − r ′ | 4 π π | r − − r ′ | d x ′ d y ′ ,