o reported symptoms of such stress, such as blurred vision." The article goes on to describe the primary reason for the stress - the difference between the image focal depth and the disparity. Normally, the when your eyes look at a close object they focus (accommodate) close and rotate inward (converge). When they accommodate on a far object, the eyes also diverge. However, a stereoscopic display does not change the either the effective focal plane (set by the optics) and the disparity depth. The eyes strain to decouple the signals. The article discusses some potential solutions, but notes that most of them (dynamic focal/disparity) are difficult to implement. It mentions monoscopic HMDs only to say that while they would seem to avoid the problems, they were not tested. The article does not discuss non-HMD stereoscopic devices at all, but I would extrapolate that they should show some similar problems. The full article is available from CyberEdge Journal for a small fee. There has been a fair bit of discussion ongoing in the sci.virtual-worlds newsgroup (check the Sept./Oct. 93 archives) about this and some other studies. One contributor, Dipl.-Ing. Olaf H. Kelle, University of Wuppertal, Germany reported only 10% of his users showing eyestrain. His system is setup with a focal depth of 3m, which seems to be a better, more comfortable viewing distance. Others have noted that long duration monitor use often leads to the user staring or not blinking. It is common for VDT users to be cautioned to look away from the screen occasionally to adjust their focal depth and to blink. Another contributor, John Nagle provided the following list of other potential problems with HMDs: electrical safety, Falling/tripping over real world objects, simulator sickness (disorientation due to conflicting motion signals from eyes and inner ear), Eye Strain, Induced post-HMD accidents ("some flight simulators some flight simulators, usually those for military ...
