A chromatic aberration correcting element that is a simple lens having at least one aspheric surface, the radius of curvature of which increases from the optical axis toward the periphery. At least one of the surfaces is formed as a diffraction lens surface that consists of annular segments that shift...http://www.google.com/patents/US5883744?utm_source=gb-gplus-sharePatent US5883744 - Chromatic aberration correcting element and its application
