Focal lengths measurements using digital speckle interferometry

An application of the Digital Speckle Pattern Interferometry (DSPI) technique to obtain the focal length of positive lenses is proposed. The lens to be measured is placed in an interferometer, under a Mach-Zehnder configuration. A plane wavefront, emerging from the lens, illuminates a diffuser. A lateral displacement of the lens rotates this wavefront, generating, after processing, a set of fringes displayed directly upon a monitor. The required information is therefore codified in the spatial frequency of these fringes. Experimental results are presented, showing agreement with the theoretical calculations. (C) 1997 Elsevier Science B.V.