The operation and performance of a six degree-of-freedom (DOF) shared-aperture tracking system with image overlay is described. This unique tracking technology shares the same aperture or scanned optical beam with the visual display, virtual retinal display (VRD). This display technology provides high brightness in an AR helmet-mounted display, especially in the extreme environment of a military cockpit. The VRD generates an image by optically scanning visible light directly to the viewer's eye. By scanning both visible and infrared light, the head-worn display can be directly coupled to a head-tracking system. As a result, the proposed tracking system requires minimal calibration between the user's viewpoint and the tracker's viewpoint. This paper demonstrates that the proposed shared-aperture tracking system produces high accuracy and computational efficiency. The current proof-of-concept system has a precision of +/− 0.05 and +/− 0.01 deg. in the horizontal and vertical axes, respectively. The static registration error was measured to be 0.08 +/− 0.04 and 0.03 +/− 0.02 deg. for the horizontal and vertical axes, respectively. The dynamic registration error or the system latency was measured to be within 16.67 ms, equivalent to our display refresh rate of 60 Hz. In all testing, the VRD was fixed and the calibrated motion of a robot arm was tracked. By moving the robot arm within a restricted volume, this real-time shared-aperture method of tracking was extended to six-DOF measurements. Future AR applications of our shared-aperture tracking and display system will be highly accurate head tracking when the VRD is helmet mounted and worn within an enclosed space, such as an aircraft cockpit.

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