Calibration of an Optical See Through Head Mounted Display with variable Zoom and Focus for Applications in Computer Assisted Interventions

摘要

During the last few years head mounted displays (HMD) became an more important display device in computer assisted surgery (CAS). Head movements of the surgeon require to change the focal plane and the zoom value without loosing the calibration. Starting from a previously developed optical see through head mounted display we adapted our HMD to measure the focal and zoom values. We made a series of planar calibrations with seven different fixed zoom and focus values using Tsais algorithm for camera calibration. Then we used the Polaris optical tracking system (Northern Digital, Ontario. Can) to measure the transformation from the planar calibration grid to a tracker probe rigidly mounted to the HMD. Then least squares approximating polynomial surfaces were derived for the seven calibration parameters. Minimizing the object space error in the last step of the procedure we achieved a mean object space error of 0.85 +/- 0.5 mm. Calibration of the HMD is maintained during minor changes in zoom and focus. Employing an automated calibration procedure which utilizes more zoom and focus steps and more accurate measurement of the position of the zoom lenses and the focal plane should reduce the error significantly, improving the accuracy needed for CAS.