= Zone plate
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= Fresnel zone plate
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A concentric diffractive focusing element with successive zone boundaries chosen to differ by half a <wavelength> of optical path to a focus. Alternating transmitting and absorbing zones suppress cancellation between neighboring half-period zones. For an incident <plane wave>, the paraxial boundaries satisfy $r_j^2=j\lambda f$. The outermost zone width controls the finest transverse scale, and the <focal length> varies with <wavelength>.
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