Zone Plates for X-Ray Microscopy

Zone plates are circular gratings with radially increasing line density. The imaging with zone plates of zone numbers n ≳ 100 obeys the same laws as imaging with thin refractive lenses. With r1= radius of the innermost zone (Fig.8.1), rn = radius of the n-th zone, n = zone number and m = number of diffracted order, the focal length of a zone plate is approximately given by fm =r1 2 λ−1 m−1. Because fm ∝1/λ, a zone plate has to be used with quasimonochromatic radiation with a bandwidth Δλ given by λ/Δλ ⋍ n◦m. The width of the outermost zone is drn = rn/(2n). The smallest distance of two point sources which can be resolved with a zone plate is — according to the Rayleigh criterion — given by d δ = 1,22 drn/m.