© CMG Lee • Dec2003
Basic 3D 
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Cam Display 

The Cambridge Display

  1. Overview

    The Cambridge Display is a time-multiplexed autostereoscopic display invented by Dr Adrian Travis at the Cambridge University Engineering Department [Travis,1995]. The prototypes created provide only horizontal parallax.

    The original design which uses a high-speed CRT to display each view sequentially. It is synchronised to LCD shutters which direct the view through a Fresnel field lens at the appropriate angle.

    The Cambridge Display
    Cambridge Displays using a CRT and another using an LCD panel

    It is also possible to separate the light-source from the imaging system by using simple strip lighting for illumination and a semi-transparent display panel in front of the field lens. The following discussion applies to this model.

  2. Theory of perspective vision

    N.A.Dodgson has analysed the viewing zones for a general Cambridge Display [Dodgson,2002] given the following parameters:

    The parameters f, wl, db and wb determine the optimal viewing area, called the “eyebox”. It is a rectangular region of width Eyebox width formula and distance Eyebox distance formula in front of the field lens.

    Eyebox construction and viewing zones Viewing zones legend

    The viewable region can be divided into two categories:

    Obviously, both of the viewer’s eyes should be in the full viewing region to fully appreciate the display. Outside the viewable region, and for portions of the partial viewing region without a view, the user will see the device housing which is usually kept dark.

    wo and wl determine the viewing zones. If wo is less than wl, the full viewing region expands towards infinity. Conversely, if wo is more than wl, this region shrinks to a point. The following figures illustrate this phenomenon.

    Eyebox and viewing zones Eyebox and viewing zones

    Dr Travis noted that it may be better to set the eyebox at infinity (by putting light sources at the focal plane of the field lens). The display is then compatible with views rendered from infinity (orthogonal projection).

    Eyebox and viewing zones Eyebox and viewing zones

    All manifestations of The Cambridge Display exhibit tiling of the image from all viewing positions other than the eyebox. The tiling corrects for perspective as the viewer moves towards or away from the scene.The image appears to be made from vertical strips of adjacent views. The implication is that views should be uniformly lit, and adjacent views should be similar enough that the transitions are smooth.

    Siegel and Nagata found that the differences between views need not be large in order to provide sufficient depth perception [Siegel,1999]. This phenomenon should minimise the effects of discontinuities in the image.

    The figures below illustrate the tiling observed from several arbitrary positions. The scene shown is a computer-generated simulation of a room. Displacements are exaggerated so changes can be more easily observed.

    Image tiling simulation Image tiling simulation

    These correspond to a typical viewing position for both right and left eye. Note that each view is tiled from several images.

    Image tiling simulation

    This shows the view from the eyebox where no tiling occurs.

    Image tiling simulation

    This shows the view from a position between the eyebox and field lens. Comparing the above, one can see that the perspective effect increases as the viewer approaches the lens due to the tiling.

    Image tiling simulation Image tiling simulation

    These show partial-viewing zones on both sides of the eyebox.One side of the view is dark.

    Image tiling simulation

    This shows the view from the small triangular partial-viewing zone with maximal tiling and dark bands on both sides.

    Image tiling simulation

    This is taken from outside the viewing zones. The user will see the device housing which is usually kept dark.

    The figures were taken from a Java applet the author programmed that allows the user to manipulate these parameters and observe the effect on the viewing zones and image. It is available at http://www2.eng.cam.ac.uk/~cmgl100/CamDisp/.

  3. Overview of prototypes

    Several prototypes were subsequently built, as shown below.

    Prototype overview

    The 7-view 25-inch prototype was a component of the 28-view 25-inch one.The latter is a hybrid using both spatial and temporal multiplexing. Although feasible, with faster LCD displays, it may be preferable to stick to just temporal multiplexing to simplify construction and alignment.

    The 50-inch prototype uses a concave spherical mirror in place of a Fresnel lens shared by two sets of projectors, each having a separate CRT for each primary colour.It can provide 15 VGA (640x480) resolution views at 30 Hz [Dodgson,2000]. It was sold to an entertainment company in the United States.

Copyright CMG Lee & ARL Travis, Photonics and Sensors Group, Cambridge University Engineering Department