Subaperture Interferometric Testing of Aspheric Optics

Aspheric optical surfaces fill the need to generate non-standard wavefront profiles in many modern optical systems. Interferometric tests have been developed to measure the extreme phase errors, relative to a spherical reference, which occur near the outer regions of the aperture. Several null compensation techniques have been described. For example, a null lens designed to generate an aspheric reference wavefront can be used to negate the effect of the surface figure resulting in measurement of deviations from the desired asphere. Alternatively, computer generated holograms have been used in two-beam interferometers such that the wavefront from the aspheric test surface is compared to a "perfect" wavefront generated by the hologram. A second class of techniques based on shearing interferometry have also been described. For example, a lateral shear interferometer, in which two displaced wavefronts from the same test surface are interfered, does not require compensating optics or a reference surface to measure aspheric figure errors.