other technologies need to be developed for high-performance HMD-tracking applications requiring both accurate registration and fast dynamics. A particularly promising combination is an all-inertial orientation tracker combined with a hybrid inertial-acoustic position tracker.
Eye-movement trackers An ideal eye tracker would satisfy three requirements: linear response over a large range (roughly 50 deg), high bandwidth (1 kHz), and tolerance to relative motion of the head. To date, no devices satisfy all three requirements. The CCD imaging systems such as ISCAN have a reasonably linear response, but the sampling rates are too low and range is limited to about 20 deg. The infrared reflection devices have the bandwidth, but are linear only in a small range; calibration to overcome their nonlinear responses is difficult.
Characterization methods One of the most confusing issues when considering which position-tracking solution to adopt is the lack of agreement on the meaning of the performance specifications and how they should be measured. Standards need to be set defining how to measure accuracy, resolution, latency, bandwidth, sensitivity to interference, and jitter. Equipment for making in-house measurements should be made commercially available, and the establishment of an independent testing laboratory would also be beneficial so that consumers would not be forced to rely on manufacturer's specifications, which usually have little relation to actual performance.