= Angular spectrum
{title2=$A(k_x,k_y;z_0)=\int u(x,y,z_0)e^{-i(k_xx+k_yy)}\,dx\,dy$}
The angular spectrum is the transverse <Fourier transform> of a monochromatic field on a reference plane. In a uniform medium, each transverse <wavevector> component propagates with a longitudinal wavenumber satisfying the <Helmholtz equation>. Components beyond the propagating wavenumber range are <evanescent waves>. The <outgoing angular spectrum> chooses the longitudinal branch that propagates or decays away from the plane.
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