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Two main concepts of head mounted displays (HMD) for augmented reality (AR) visualization exist, the optical and video-see through type. Several research groups have pursued both approaches for utilizing HMDs for computer aided surgery. While the hardware requirements for a video see through HMD to achieve acceptable time delay and frame rate seem to be enormous the clinical acceptance of such a device is doubtful from a practical point of view. Starting from previous work in displaying additional computer-generated graphics in operating microscopes, we have adapted a miniature head mounted operating microscope for AR by integrating two very small computer displays. To calibrate the projection parameters of this so called varioscope AR we have used Tsai's (1987) algorithm for camera calibration. Connection to a surgical navigation system was performed by defining an open interface to the control unit of the varioscope AR. The control unit consists of a standard PC with an dual head graphics adapter to render and display the desired augmentation of the scene. We connected this control unit to an computer aided surgery (CAS) system by the TCP/IP interface. In this paper we present the control unit for the HMD and its software design. We tested two different optical tracking systems, the Flash-point (Image Guided Technologies, Boulder, CO), which provided about 10 frames per second, and the Polaris (Northern Digital, Ontario, Can) which provided at least 30 frames per second, both with a time delay of one frame.