3D display technology - The Cambridge Display

`© CMG Lee • Dec2003`
		Home			Basic 3D			Projection			Holographic			Optical addressing			Flat panel			Survey			Accommodation			Autostereo			Perspective			Principles			Simulation			Cam Display			References

The Cambridge Display

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.

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.
Theory of perspective vision

N.A.Dodgson has analysed the viewing zones for a general Cambridge Display [Dodgson,2002] given the following parameters:
- Focal length of field lens, f
- Width of field lens, w_l
- Separation of light source array and field lens, d_b
- Total width of light sources, w_b
- Number of light elements, N
- Position of viewer�s eye, (z, x)
The parameters f, w_l, d_b and w_b determine the optimal viewing area, called the �eyebox�. It is a rectangular region of width and distance in front of the field lens.

The viewable region can be divided into two categories:
- Full viewing region, where the whole field lens appears to be illuminated, and
- Partial viewing region, where part of the lens is unilluminated.
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.

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

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).

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.

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

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

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.

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

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

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/.
Overview of prototypes

Several prototypes were subsequently built, as shown below.

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.