A lens focusing light at two different wavelengths

A single element lens capable of focusing light from a source at two different wavelengths to two distinct foci in the image space is studied in this paper. Results are obtained concerning the existence and uniqueness of the lens. The investigation is centred around a system of functional differential equations derived from Fermat's Principle, and an asymptotic method is developed for the special case when the image space foci are close and the source nearly monochromatic. © 1992.