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turn at a maximum speed of about 800 /s. This means that theoretically it is necessary
to have a field of vision of 170 horizontally and 145 vertically on the images. In
practice, the field of vision in the head-mounted displays is much smaller (in the top-of-
the-range head-mounted displays, we can reach 120 horizontally). The impression of
visual immersion is thus more or less strong with these devices. It is therefore necessary
to study in depth what the sensorimotor I 2 require at the visual level. The total field of
vision with movement of the head and eyes is very wide (horizontally > 200
temple
side and 130 nose side while vertically it is 140 up and 170 down).
Remember that IPD, the Inter-Pupillary Distance, is variable in humans. On aver-
age, it is 65mm in adult men and a little less in adult women; the differences can be
significant, between around 50 and 70mm, in Europeans. This size should be taken
into account while using the head-mounted displays, as the two optics must be cor-
rectly centred with respect to the eyes and thus have an optimum field of vision. For
fine settings of a head-mounted display, we have measured the IPD for each user using
a pupillometer. These measurements make it possible to correctly set the IPD, rather
than letting the user make approximate adjustments.
3.2.3.5 Maximum temporal frequency in vision
Temporal frequency of images to perceive a flow of movements is critical for values
below 25 images per second (a value depending on the type of images). The movements
will be free-flowing on monoscopic screens at 50Hz (an image is made of two frames).
In case of stereoscopic screens with time-division multiplexing, the scanning rate must
be double, 100Hz (or 96Hz) to get 25 (or 24) images per second and per eye.
3.2.3.6 Psychophysical characteristics of stereoscopic vision
As explained earlier, the visual system can merge the two retinal images for all the
points located in the Panum's area. In other cases, the brain either cannot merge the
two images, or only does so with a certain amount of strain. For a three-dimensional
vision, the aim is to create stereoscopic images by creating small retinal disparities in
the observer's eyes while maintaining an effect of depth. What are the limits of merging
generally acceptable for human beings?
We have previously defined retinal disparity. For 3D images displayed on a stereo-
scopic screen, the horizontal disparity is (approximately) equal to the horizontal
parallax. It is defined by the DPG angle, formed by the two homologous points of
left and right images, seen from the viewpoint of the observer.
Taking the horizontal parallax as the parameter influencing the binocular vision,
experimental studies (Valyus, 1962) have demonstrated the difficulty in merging two
plane images having horizontal parallaxes (and hence retinal disparities) of an angle
higher than 1.5 degrees. We carried out tests in working conditions as part of the
remote-control operations with real stereoscopic images. Our results give 1.2 (Fuchs
et al., 1995) as the limit for horizontal parallax, a value that is a little below the limit
defined by Valyus. These variations are normal as they vary from person to person,
depending on their tolerance for 3D images and on whether they force their visual
system to merge the images. It should be noted that 3 to 5% (possibly more) of the
population cannot merge 3D images on a screen.
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