Design example for the use of hybrid optical elements in the infrared.

The Petzval objective is an effective design form for IR camera systems. It has excellent performance characteristics for high-aperture applications. Its field coverage is limited to a few degrees by the uncorrected Petzval sum.

Using a hybrid aspheric/diffractive front element, it is possible to achieve broadband color correction over the 3-5 µm or 8-12 µm region using only one lens material. For the long wavelength region, Germanium is typically used (except when the application environment is expected to exceed 60° C where Ge becomes opaque). This hybrid combination allows for the correction of the spherical and chromatic aberrations.

The advantage of the high precision obtainable with diamond turning makes it possible to generate simultaneously the diffractive phase profile superimposed on the aspheric surface. To protect the diffractive profile from the environment, it is placed on the rear surface of the first element.