As the visual scene was visible only to the patient, we needed to provide a secondary screen display for the clinician so that they could see what the patient was doing and check that they were attending to the display with both eyes. The box was designed such that, when placed on a standard desktop, and used in conjunction with an adjustable chair, the viewing holes are at a height suitable for children. By using an LCD monitor we could make the entire display system small and light enough to be fitted into one box. Only by using each eye can the patient see what time is shown on the clock. The left eye is also presented the clock numbers whilst the right eye is presented with the clock hands. In this example, both eyes are presented with an image of the clock surround. The mirrors in the Cyberscope rotate the images back the right way up and send them, one to each eye (as illustrated in Figure 2). The monitor display is divided vertically down the middle, and in each half the image to one of the eyes is displayed rotated through 90o to give the images the desired proportions. This device uses a combination of mirrors to take the image from a standard monitor and present one half of it to one eye and one to the other eye. A cyberscope is a plastic hood which fits onto the front of the 15' PC monitor to allow it to be used as a stereo display. The system we developed incorporated a device called a Cyberscope (Wired, 1993). We decided that we needed to develop our own display in order to conduct preliminary case study trials. However, although the Cy-Visor could be worn for 30 minutes of treatment, it needed to be held in place by the clinician whilst the child was slightly reclined on the seat.

The Cy-Visor DH-4400 binocular headset, although not designed specifically for children, is much more lightweight than the V8, it has a less enclosed design the IPD was small enough for the children we tested. In addition the inter- pupillary distance (IPD the distance between the two pupils) did not go down to small enough levels for children. The centre of gravity is at the front which pulls the headset forward, causing neck strain. HMDs such as the Virtual Research V8 were too heavy for extended use by our expected user population and children found the headset too claustrophobic. We needed to find a 3D viewer that would be suitable for children in the range 5 to 8 years of age. desktop computer alone does not provide a 3D experience unless attached to a head mounted display (HMD) or an alternative display to provide a 3D effect.

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