An Online Calibration Method for Microsoft HoloLens

An optical see-through (OST) head-mounted display (HMD) is an appropriate option for an image-guided surgical navigation system (IGS) based on augmented reality (AR). However, the calibration of OST-HMDs has been challenging, since the augmented results can only be observed by the users wearing it. In this paper, we proposed an online calibration method based on the single-point active alignment method (SPAAM) for a HoloLens using a virtual tracking system. The transformation matrices between the world coordinates and the virtual holographic coordinates are calculated using a linear model and decomposed by the singular value decomposition (SVD) algorithm. The surgical workspace is analyzed, and there are nine sampling points for the offline calibration: eight at the vertices and one at the center of a cube of the workspace. 20 groups of data at each sampling point are collected. Five alignments should be implanted in the online calibration. The transformation matrices are compensated for the offset of every use of a HoloLens. To assess the accuracy of the calibration method, in accordance with the RANSAC algorithm, eight groups of additional data at random non-sampling locations are collected. Three different users that are familiar with the calibration procedures performed the calibration. The average distance error of our calibration was below 6 mm, and the rotation error was up to 5°. It is a user-friendly, simple method for the online calibration of OST-HMDs.