Diffractive grazing-incidence interferometer

I propose a symmetric geometry for grazing-incidence interferometry of flat surfaces using diffraction gratings for beam splitting and recombination. The geometry employs a reference mirror to correct for relative inversion of the measurement and reference wave fronts. Preliminary testing with a 4-microm equivalent-wavelength system shows a 3 sigma repeatability of 20 nm for both smooth and rough surfaces, including a variety of precision-engineered metal, ceramic, and glass objects. The system has a comfortable working distance and large field of view, suitable for production testing.