Abstract

It has been proven several times recently that a purely diffractive optical system, no matter how complex, cannot form an achromatic real image. An achromatic virtual image is possible, but not a real one. Nonetheless, there are two ways to cheat. Farn and Goodman have described in JOSA how enormous amounts of spherochromatism between two separated surfaces can be used to form a real, achromatic, perfect axial point image for nonparaxial rays. This scheme cannot work at the paraxial level. Here I will describe a different approach, which does work at the paraxial level but which is not achromatic in the conventional sense. Although it can bring an arbitrary number of wavelengths to a common real focus, by using purely diffractive elements, the focus position is never stationary with wavelength.

© 1992 Optical Society of America