An interferometric method for measuring short focal length refractive lenses and diffractive lenses

The possibility that an interferometric method, previously developed for measuring long focal length systems, can be equally well applied to measuring very short focal length optics and a 1 mm aperture radius diffractive lens is demonstrated. The measurement can be easily performed despite some limiting factors such as small aperture, which drop the available light intensity, and very short focal length, which magnified impurities on the optical elements, both resulting in high noise in the fringe pattern. It is easy to predict that other approaches would fail in the presence of such limiting factors. The method is based on a reflective grating interferometer and measurement of the effective focal length is performed within the limit of paraxial approximation. Focal length is obtained by the knowledge of the spatial frequency of two interferometric fringe patterns imaged by the lens under test. The spatial frequencies are evaluated by using an FFT algorithm.