DIGITAL TECHNIQUE FOR HIGH-ACCURACY FOCAL LENGTH MEASUREMENTS

Digital speckle pattern interferometry (DSPI) is applied to obtain the focal length of both positive and negative lenses, and of a system of lenses. Reference correlation fringes are generated by rotating a pla ne wavefront in a Mach-Zehnder like DSPI interferometer. The lens to b e tested is placed close to the recording medium (a CCD array) after r ecombining the two branches of the interferometer, and can be axially displaced. The 'pitch' (number of pixels/cycle) of the fringes varies with the position of the lens and its absolute location can be elimina ted from the equations giving rise to a compact expression depending o nly on the introduced displacement and pitch measurement. A simple alg orithm allows accurate measurement of the pitch. Experimental results are presented for several lenses of known focal lengths showing very g ood agreement with the nominal values. Error analysis is also carried out, with the resulting experimental relative error being of the order of 2%. (C) 1998 Elsevier Science B.V. All rights reserved.