Surgical Microscope with Automated Focus Adaptation

Abstract

In a surgical microscope even small shifts lead to image blurring caused by improper focus adaptation. Thus, modern microscopes support the surgeon with an autofocus. In this work, a hybrid autofocus approach is presented for surgical microscopes based on laser guidance and imagecontrast based refinement. Two lasers are integrated into the microscope focus optics. Their spots are detected automatically from the microscope images acquired with a CCD camera. Hence, the focusing process is realized by a procedure of finding the object-lens parameters where both laser points merge. Subsequently, these parameters are refined using image-based contrast maximization. The results show that, given a flat organ surface, optimal focus parameters can be obtained by evaluation of the distance between two laser spots at two focus positions only. Correspondingly, the focus can directly be adjusted to match this optimum. On a curved surface this procedure is carried out iteratively. The laser point method offers a sufficient starting point for the final contrast-based refinements. Overall, the approach yields a stable procedure for a surgical microscope autofocus mechanism.