Aspheric and diffractive surfaces in one, two and three element lenses

Abstract

The use of surfaces other than spheres in optical systems has become increasingly practical due to advances in manufacturing technology. Two such alternate surface types are aspheres and diffractives. Aspheric surfaces are typically used to control the Seidel (and higher order) aberrations. Diffractive surfaces, because of their high dispersion, can be used in broadband systems to provide chromatic aberration correction as well. The aim of this work is to develop general statements about the application of aspheric and diffractive surfaces to photographic and digital imaging lenses. The use of such complex surfaces can reduce the number of elements in an imaging system while maintaining equivalent image quality. General rules regarding this design tradeoff are developed. The improvement in performance achieved by adding aspheric and diffractive surfaces, alone or in combination, to one, two and three element lenses is examined. A measure of performance is defined based upon the transverse ray errors calculated from real ray tracing. Using this, lenses of equal performance are designed for various combinations of numerical aperture and field angle. Contours of equal performance are compared for lenses of different constructional parameters. As an example application of the use of aspheric and diffractive surfaces, the design of an objective lens for a digital still camera is considered. Possible configurations for one, two and three element lenses are discussed. The use of diffractive surfaces in broadband imaging systems brings with it the associated cost of stray light due to the variation of diffraction efficiency with wavelength. Under the condition of a low contrast object, the effect of diffraction efficiency is included in the measure of performance and the systems containing diffractive surface reevaluated. The single axis symmetry of the aspheric or diffractive surfaces used results in the inability to remove surface to surface decenter in the lens element during the final edging process. The sensitivity of the systems containing aspheric surfaces to a decenter error is examined and compared to that of a conventional system.