Design and automatic calibration of a head mounted operating binocular for augmented reality applications in computer-aided surgery

Abstract

In the last years we developed and tested a head mounted display (HMD) for augmented reality applications in computer aided surgery. This HMD was developed by adapting the Varioscope AF3 (Life Optics, Vienna), an operating binocular with variable zoom and focus. One of the drawbacks of the AF3 was the missing possibility to set the zoom and focus values automatically via a machine usable interface, necessary for automatic calibration of the device. The paper presents the successor of the Varioscope AF3, the Varioscope M5 adapted for augmented reality by our lab. This device has an interface for machine controlled setting of the zoom and focus lens groups via RS 232. This enabled us to develop an automated calibration using a calibration grid mounted on a linear positioner. The position of the grid was controlled using a stepping motor controller connected via IEEE 488. The calibration grid was equipped with automatically detectable fiducial points using varying cross values of consecutive points. The resulting point pairs were used for a camera calibration with Tsai's algorithm. Tracker probes (Traxtal, Toronto) were mounted on the HMD and onto the calibration grid to derive the transformation from the coordinate system of the HMD into the system of the displays. The error of this calibrations was measured comparing the position of the tip of a bayonet probe calculated by the algorithm and found in the image of a camera mounted at the eyepiece of the device. Averaging 16 positions of the probe this deviation was found to be 0.97 ± 0.22 mm.