Lens inspection using Multiple Aperture Shear Interferometry: Comparison to Wave Front Sensing Methods

Multiple Aperture Shear Interferometry (MArS) is a new approach for lens inspection. It is based on measuring the spatial coherence function of light being reflecting by the specimen under investigation using a shear interferometer. Since the result of the measurement is the mutual intensity function rather than a wave field, the method offers a number of unique and beneficial advantages. First and foremost, it allows the use of several independent light sources (illumination apertures) at the same time, thus mitigating the prominent problem of a limited observation aperture in interferometric lens testing. Additionally, it has the potential to eliminate disturbing reverse side reflections in many cases and to inspect front and reverse surfaces at the same time. Along with a detailed discussion on the fundamentals of the method, we present a comparison between the results of MArS and those obtained from standard wave front sensing. As an example, we investigate the shape of a cylindrical lens in reflection mode. For the specific case of just a single light source, we show that MArS and wave front sensing yield equivalent results.