Interferometric method for testing focal lengths using a digital Fourier transform

We describe an interferometric method for measuring effective focal length of lenses. Measurement of the effective focal lengths of macroscopic refractive lenses and of a diffractive microlens are performed within the limit of paraxial approximation. The technique uses a reflective grating interferometer to produce a fringe pattern with a regular pitch. The lens under test is illuminated by the interference pattern and the focal length is obtained by the knowledge of the spatial frequency in a fixed aperture. Three imags of the interference pattern are stored by using a CCD camera: two of them are imaged by the lens under test placed at two different position, an one is the reference pattern with no lens inserted in the set-up. A 1D fast Fourier algorithm is applied to the digitized image and the phase modulo 2 (pi) is reconstructed. The phase is unwrapped and averaged to calculate the spatial fringe frequency inside the chosen aperture. The method has been applied for measuring the focal length of macroscopic refractive lenses and of a 1mm aperture diffractive microlens.