Double-pass oblique-incidence interferometer for the inspection of nonoptical surfaces

Interferometric testing of the flatness or straightness of ground surfaces of metals, granite, ceramics, and glasses has always proved difficult because of the extremely low specular reflection from these optically rough surfaces. At oblique angles of incidence the specular reflection coefficient of nonoptical surfaces increases rapidly, typically from near 0.01 at 75 degrees up to, say, 0.50 at 85 degrees and near 1.00 at 90 degrees angle of incidence. By taking advantage of this enhanced reflection at oblique incidence and the brightness and spatial coherence of visible gas lasers, it becomes practical to use interferometric inspection of nonoptical surfaces. A novel double-pass oblique-incidence interferometer is described which enables testing of large pieces having a specular reflection as low as 1%.