where θ denotes the angle between r_{01} and z.
\bold U(x_0,y_0) = \frac{1}{j\lambda z}\int\!\int \bold U(x_1,y_1) \frac{\exp(jkz[1+(\frac{x_0-x_1}{z})^2+(\frac{y_0-y_1}{z})^2]^{1/2})}{1+(\frac{x_0-x_1}{z})^2+(\frac{y_0-y_1}{z})^2}dx_1 dy_1 .