Compensated shearing interferometry with spatially incoherent light sources

Summary form only given. Shearing interferometry is a common-path interferometric technique, which uses two different regions of a surface for the reference and object. It is widely used in demanding environments where both stability and interferometric accuracy are required. Our primary interest is in measuring imperfections on a regularly textured surface, such as magnetic storage media or machine parts. These surfaces present two significant challenges to interferometry-regular surface texture and low reflectivity. Laser light is ideal for illuminating the surface, but laser speckle is a problem. It is well known that speckle can be removed from interferometric images, by destroying the spatial coherence of the illuminating laser beam. This can be done by passing the laser light through a spinning ground glass disc. However loss of spatial coherence reduces the fringe visibility, unless the illumination shear is compensated for in the imaging optics. To address all these problems we have designed and built a novel shearing interferometer.