The FireFly is an optical motion capture system capable of absolutely localizing multiple infrared tags at a rate of 900 Hertz with a latency of less than 5 milliseconds. The system has a resolution of 0.1mm throughout its range with an absolute accuracy ranging from 0.8mm to 2 mm depending on the distance to the camera array. We used this motion capture system to determine the orientation of the operator’s head, which can then be used to determine absolute gaze orientation in real time.