Self-imaging pitch variation applied to focal length digital measurements

Several approaches were developed in the literature for focal length measurements. In our experimental arrangement, a collimated laser beam illuminates a test lens with a Ronchi grating placed against this lens. This procedure gives rise to a series of classical Talbot images, although magnified due to the non-parallel illumination. In this configuration, we propose to relate the self-images positions and pitches to obtain the focal length. We designed a software that helps first to precisely focus each self-image plane by a best-contrast algorithm and then to take an intensity histogram along a direction perpendicular to the grating lines. A theoretical explanation, experimental results and error analysis are presented.