Remark#1: Radius of a Zone
A pattern of a Fresnel zone plate consists of alternatively placed transparent and opaque zones. Both patterns with a transparent central zone (a positive zone plate) and with an opaque central zone (a negative zone plate) are permissible. The radius of the outer circular boundary of the m-th transparent zone is given by the following equations where
are the wavelength of light and a designed focal length.
In the case of a positive zone plate the first zone (m=1) is the central circle and the radius is given as
. The radius is equal to that of a pinhole with a focal length
derived by a simplified method (
Pinhole Salon Remark#4). As described previously if the pinhole radius is optimized with respect to the divergence due to diffraction the pinhole radius is given as
. But for the calculation of the zone plate radius we don’t consider a similar condition and, therefore, an equation with different coefficient was obtained. The difference of the coefficients has little effect on taking a pinhole photograph.