Image focusing in endoscopic systems

Abstract

Conventional endoscopes use expensive and damageable rod lenses to transport the image through a long rigid tube. Outside the patient's body a camera may be mounted onto the telescope to capture the image and to display it on a video screen. However, rather often difficulties arise to sterilize these cameras by using autoclaves at high temperatures. Present developments in endoscopy aim for placing the optical imaging system together with the digital image sensor in the tip of the endoscope. A major drawback of these chip-on-thetip endoscopes are their fixed lenses. Neither focusing nor optical zooming is possible. This leads to a reduced depth of focus and a minimal object distance of typically twenty millimeters. These limitations can be overcome by integrating miniaturized linear motors to move the lenses. We developed a drive which incorporates a permanent magnet slide with a central hole for fixing the lenses along its rotational axis. Lorentz forces move the slide onto focusing positions and stop it, thereby creating a locking mechanism which keeps the lens position resistant to external forces, so that manipulation of the endoscope during surgery does not affect the optical adjustment. The high positional accuracy of less than ten microns excels the requirement of the optical system. Image focusing may be obtained by simple knob regulation. However, to keep the surgeons hands free, image contrast is evaluated by use of a fast microprocessor which feeds the motors power supply and drives the slide to the position of the sharpest image. Due to the small mass of the slide, an electromechanical fast feed back system is achieved which focuses in less than 0.2 seconds. Regardless whether macro or micro imaging is required for minimally invasive surgery, the auto focus function provides an image with high resolution and contrast.