(under subclass 558) Subject matter comprising a plate of glass, usually a photograph, on which there is a central spot surrounded by concentric annular zones, alternately opaque and transparent, the radii of the boundaries between the zones being proportional to the square roots of the natural numbers 1,2,3, etc. It has the property of forming a real image of a point on the axis, as does a lens, but by a process of diffraction instead of refraction.
(1) Note. Zone plates are also known as "Fresnel zone plates".
(2) Note. The diffraction occurs from a circular diffraction grating of variable spacing having focal properties, and also including means for forming such a circular focal grating.
(3) Note. Phase zone plates are also included in this subclass.
