Phase-shifting point diffraction interferometer.

We describe a novel interferometer design suitable for highly accurate measurement of wave-front aberrations over a wide range of wavelengths, from visible to x ray. The new design, based on the point diffraction interferometer, preserves the advantages of the conventional point diffraction interferometer but offers higher efficiency and improved accuracy through phase shifting. These qualities make it applicable to at-wavelength testing of many optical systems, including short-wavelength projection lithography optics. A visible-light prototype was built and operated.