Germdiff illustrates the use of a diffractive surface to correct the chromatic aberration of an infrared system, as well as an aspheric to correct spherical aberration. One of the first practical applications of diamond-turned computer-generated holograms was a single element achromat for the infrared. This lens, designed by Reidl and McCann, is discussed in detail in the Optics Reference manual. It combines a conventional aspheric on the front surface with a diffractive surface on the back of a Germanium singlet to achieve good performance at f/1. Note that the system, while achromatic, has secondary spectrum that cannot be corrected using this scheme.
