f The short-term effects on binocular stabiliiy of wearing a conventional head-mounted display (HMD) to explore a virtual reality environment were examined. Twenty adult subjects (aged 19-29 years) wore a commercially available HMD for 10 min while cycling around a computer generated 3-D world. The twin screen presentations were set to suit the average interpupillary distance ofour subject population, to mimic the conditions of public access virtual reality systems. Subjects were examined before and after exposure to the HMD and there were clear signs of induced binocular stress for a number of the subjects. The implications of introducing such HMDs into the workplace and entertainment environments are discussed. IA recent development in graphical user interfaces for human-computer interaction has been the development of virtual reality (VR) systems. The central principle of such systems is that the user should be able to interact with a computer by using gaze and manipulative gestures within a 3-D computer environment. At the simplest level this can be through the use of a peripheral input device (e.g. a computer mouse) to "move" through a 3-D environment that is pictorially represented on a conventional computer screen. A more innovative and popular approach is to imbed the user within the 3-D VR by using a head mounted display (HMD). In addition to the pictorial depth cues that are presented on a screen representation, the HMD attempts to simulate binocularly overlapped images so that the fusion of disparate images can create the illusion of a three dimensional world. The advantages of the HMD, however, go beyond the provision of stereoscopic depth cues. A 6 degree of freedom tracking device is normally mounted on the HMD so that as the user moves his/her head new visual perspectives can be displayed and the user can scan through 360'-or walk through this new computer world. The HMD can provide the user with an impressive sense of presence and the ability to interact in a more natural way within the VR environment. Specifications of a typical head mounted displayOne of the most commonly used HMD systems is the VPL Eyephone LX (Redwood. CA. USA). This uses an adjustable headband to mount a 3 inch LCD screen in front of each eye. These are viewed through a -(-36D compound lens, created by the use of two -|-18 D Fresnel•MBCO Correspondence lo: VR project. Department ofPsychology. University of Edinburgh, Edinburgh, UK.lenses, and the LCD screens are placed close to the focal length of this lens. Because of the low resolution of such LCD screens (360 x 240 primary colour pixel equivalent to 208 X 139 RGB triads') a semi-opaque lens is placed between the lens and the screen to spatially filter the image. The Eyephone LX does not allow the user to adjust the lateral distance between the eyepieces which is fixed at 65 mm between the optical centres of the two lenses. The viewer can adjust the distance from the eyes to the lenses but the distance is invariably close, usually in the order of 2 cm. Precise mounting of ...