= Axial intensity gain of an amplitude zone plate
{title2=$I_f/I_0\simeq4N_o^2$}
For uniform coherent plane-wave illumination, an ideal binary amplitude <zone plate> transmits alternate <Fresnel half-period zones>. If $N_o$ complete odd zones are open, every one contributes approximately twice the unobstructed axial <complex amplitude>, in the same phase, and $I_f/I_0\simeq4N_o^2$. This is a local <light intensity> gain relative to removing the plate and outer stop. It differs from comparison with a finite clear <aperture>: an <aperture> containing an odd integer number of half-period zones has axial <intensity> $4I_0$, giving plate-to-aperture ratio $N_o^2$. Neither gain is a total <power> efficiency.
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