Focal length measurement based on lateral shearing interference

This paper proposes a new, to the best of our knowledge, method for measuring the focal length of lenses based on lateral shearing interference. In this method, two shearing interferograms are recorded at the initial CCD position and after a known axial displacement of the charge-coupled device camera (CCD). By using the resulting variation in normal fringe spacing of the shearing interferograms, the wavefront curvature radius at the initial CCD position can be determined without directly measuring the distance between the lateral shearing interferometer and the CCD. Subsequently, the wavefront curvature radius is combined with the vertical beam diameter ratio measured without and with the test lens, establishing a geometric relationship between the wavefront curvature radius and the lens focal length to calculate the focal length. Experimental results show that, for a biconvex lens with a focal length of 175.5784 mm and a biconcave lens with a focal length of -50.9785 mm, the relative measurement errors are 0.1018% and 0.3736%, respectively. The proposed method does not require complex optical path adjustment or an additional reference beam, providing a practical approach for rapid and stable focal length measurement.