Correction of aperture aberrations of hybrid lens

The possibilities of aberration correction in the case of a single lens are limited. It is well known that, if classic glasses are used, it is impossible to compensate spherical aberration. It can be, however, minimized by proper choice of the ratio between the first and second surface radii of curvature (referred here as (zetz) ). It is possible also, in cost of uncorrected spherical aberration, to compensate III order coma. Additional possibilities of aberration correction occur, however, if a thin diffractive structure is deposited on one of the lens surface. Such lens is usually referred as a hybrid (diffractive-refractive) lens. The diffractive structure typically corresponds to the holographic lens generated by the interference of two spherical waves. The ratio of these waves radii of curvature is treated as a parameter (called here (beta) ) describing fully aberration properties of this structure. A focusing power of the diffractive part typically is only a small fraction ((eta) ) of total focusing power of a hybrid lens, so the diffractive part acts mainly as aberration corrector. Aberration properties of hybrid lens are determined by two parameters: (zetz) and (beta) so it is possible to achieve simultaneous correction of aperture aberrations: spherical aberration and coma. In the paper formulas describing the III-order aberration coefficients were used for calculating the values of parameters (zetz) and (beta) assuring such correction for several values of parameter (eta) and different locations of object plane. The calculations were performed with help of the MATHCAD programme. Basing on the results a number of hybrid lenses (collimating and imaging) were designed. Their imaging quality was then evaluated by numerical calculation of aberration spots. Estimated values of such image characteristics as the aberration spot moment of inertia or third order moment of the spots distribution enable to compare the imaging quality.