The present invention relates to a system for alignment between real and virtual objects in a head-mounted optical see-through display. In an embodiment, the system includes a tracking system including a processor, a headgear attached with the head-mounted optical see-through display, the 5 head-mounted optical see-through display includes at least two cameras mounted on a rigid frame, at least one object may be fixed or mobile including a plurality of marker points, the tracking system is operatively coupled to the headgear and the object, the processor is configured for: capturing two perspective images of the substantially circular entrance pupil of at least one 0 eye and relaying the image data to the processor, a memory device coupled to the processor and containing the geometric calibration data of the at least two cameras and the pre-calibrated transformation between the cameras. The processor extracts the boundary between the entrance pupil and the iris, calculates the projected center of the boundary in the individual images and 5 using the calibration data estimates the center of the entrance pupil in three dimensional space in relation to the cameras.