Laplacian-based focus measure allows rapid focus estimation of annular regions in gray-scale images

Important ophthalmic imaging techniques, such as fundus camera imaging, utilize light sources that need to be focused on an annular section of an eye surface. In order to be performed dynamically, this requires real-time control of the focus plane of the illumination optics. When considering adaptive focusing in ophthalmic instruments, liquid lenses represent the best compromise in terms of tunability, tuning range, compactness, numerical aperture, and speed. In recent years, several compact eye imaging devices using liquid lenses have been described in the literature. These would benefit from low computational complexity algorithms for adaptive local auto-focus on an annular region, to allow the illumination optics to be controlled by low-cost and low-power electronics. In this paper, we propose a novel radial-focus evaluation method based on a revised version of a more traditional Laplacian-based focus estimator. This radial focus evaluation is targeted to focusing annular sections of an image, with the advantage of a drastic reduction of computational complexity.