Passive autofocus for digital endoscopic imaging systems

摘要

In handheld medical endoscopes the use of active autofocusing systems is to be avoided because of the added weight. Autofocusing should rather be performed passively, furthermore, in applications which are particularly critical with respect to safety, e.g., in the eye, a stable and reliable operation in real time is necessary. Passive autofocus strategies applied to date and known to the authors lead however to algorithms which are either too slow for a real time implementation and/or are influenced by the structure of the object which is to be brought into focus. Accordingly, a new autofocus criterion is proposed which exhibits a stable and reliable operation in real time under all circumstances of interest. It is based on the squared differences of the intensity of adjacent points in both dimensions of a plane image (Square Plane Sum Modulus Difference, SPSMD) and as such particularly suitable for digital camera systems and realtime needs (typically, 30 evaluations per second on an image of 1024 $MUL 1024 pixels). The SPSMD criterion is more sensitive and has a larger SNR than other focus criteria known to the authors. As it includes intensity differences in both (perpendicular) directions in the image plane, to a large extent independence of image structures is obtained.