Diffractive optics: the gradial zone lens.
A gradial zone lens has the diffractive component within the bulk lens rather than as a surface-relief profile. It is constructed by spin molding annular zones from microliter volumes of monomers injected into a rotating mold. Plastic lenses have micrometer width zones with dimensions that give a phase difference of modulo 2pi. A gradial lens made with flat surfaces shows a series of foci similar to those of a radially blazed grating; however, when zones are placed within a plano-convex lens, there is an additive contribution from both refractive and diffractive focusing.
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Hamblen