Calibration of an optical see-through head-mounted display with variable zoom and focus for applications in computer-assisted interventions

Abstract

We are developing an optical see through head mounted display in which preoperative planning data provided by a computer aided surgery system is overlaid to the optical image of the patient. In order to cope with head movements of the surgeon the device has to be calibrated for a wide zoom and focus range. For such a calibration accurate and robust localization of a huge amount of calibration points is of utmost importance. Because of the negligible radial distortion of the optics in our device, we were able to use projective invariants for stable detection of the calibration fiducials on a planar grid. The pattern at the planar grid was designed using a different cross ratio for four consecutive points in x respectively y direction. For automated image processing we put a CCD camera behind the eye piece of the device. The resulting image was thresholded and segmented, after deleting the artefacts a Sobel edge detector was applied and the image was Hough transformed to detect the x and y axes. Then the world coordinates of fiducial points on the grid could be detected. A series of six camera calibrations with two zoom settings was done.The mean values of the errors for the two calibrations were 0.08 mm respectively 0.3 mm.

Automatic calibration of an optical see-through head-mounted display for augmented reality applications in computer-assisted interventions

“…A more detailed presentation of the design can be found in. 2,7 In our experimental setup the HMD was equipped with a CCD camera at the eyepiece to provide the image for the automated calibration. The transformation from the CCD camera to the plane where the image projected by the miniature displays can be found had been derived by another camera calibration, see 2.2.…”

“…The other transformations are either independent of the zoom and focus settings or are provided by the tracking system, details can be found in. 2,7 The derivation of T W C was done using Tsai's camera calibration algorithm as described in 18 using an open source implementation by Reg Willson * for the calculations.…”