A NEW APPROACH TO HIGH-ACCURACY MEASUREMENT OF THE FOCAL LENGTHS OF LENSES USING A DIGITAL FOURIER-TRANSFORM

A new approach is proposed for measuring the focal length of a lens. T he lens is illuminated by an interference pattern with a regular pitch produced by an interferometer that uses a reflective grating. The foc al length is determined by measuring the number of fringes falling ins ide a fixed aperture both with and without the test lens inserted in t he set-up. To measure the number of fringes, three images of the inter ference pattern are digitized by a CCD camera. A one-dimensional FFT a lgorithm is applied to the digitized image to reconstruct the phase mo dule 2 pi along each row. The phase is unwrapped along each row and av eraged to calculate the number of fringes that fall inside the apertur e of 512 pixels. This technique is alternative to methods based on the moire effect and a higher accuracy is obtainable. The focal lengths o f both positive and negative lenses have been measured.