Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
1999-04-02
2001-10-09
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C359S007000, C359S035000
Reexamination Certificate
active
06300986
ABSTRACT:
This invention relates to displays, particularly to the video display of three-dimensional images.
Video displays commonly present moving two-dimensional images for use in television and as computer monitors. It is becoming possible to display three-dimensional images on such displays, and a variety of designs have been proposed.
It is well known that holograms are capable of displaying three-dimensional images. A crude hologram can be written by a high-resolution liquid-crystal display, so such displays are capable of displaying video three-dimensional images. But it is difficult to make liquid-crystal displays with pixels less than five microns wide, and holograms produced on such displays have a narrow field of view.
Just as video two-dimensional images can be displayed by raster-scanning a two-dimensional screen, a three-dimensional image can be displayed by raster-scanning a three-dimensional volume. One way of doing this is to scan a volume of a suitable material with a pair of laser beams. If the material is transparent to both laser beams, and if the material emits light at any point where the laser beams intersect, then it is possible to cause the point of intersection of the laser beams to sweep out the whole volume so as to write the three-dimensional image. The problem with displays like this is that the images which they produce are necessarily transparent, i.e. there is no provision for images at the front of the volume to block light from images at the rear.
More common is the technique whereby a conventional video display screens left and right views alternately of a three-dimensional object, and the viewer wears a pair of spectacles which make one of the pair visible to each eye. If this is done quickly enough for flicker to be imperceptible, the viewer sees a stereo image which allows the perception of depth.
Spectacles however get lost. An alternative system is to place an array of lenslets over a two-dimensional display and provide two pixels under each lenslet. Provided the viewer is positioned correctly it can be arranged that one pixel under each lenslet is imaged to the viewer's right eye and the other to the viewer's left eye. The viewer then sees a stereo image without having to wear spectacles.
The difficulty with screening only two views in this way is that the viewer must keep his head in the correct position. If more than two pixels are provided under each lenslet it becomes possible to screen several views of the three-dimensional image. This has the dual advantage that the viewer can see a stereo image over a wider range of positions, and that the viewer can inspect the three-dimensional image from a variety of angles.
For the lenslet system to work, the two-dimensional display must have a high resolution and this makes construction of the display difficult. A different approach is to adapt a rear-projection two-dimensional display which commonly comprises a video projector and translucent screen. If the translucent screen in such a display is exchanged for a lens then light from the video projector will as before form a video image which will lie in the plane of the lens, but after passing through the lens the light will converge to a zone at some point in space which in fact is an image of the video projector. If the viewer puts one eye in this zone they will be able to see a two-dimensional image but only with that eye. Put another video projector adjacent to the first and a view can be displayed to the viewer's other eye, and indeed further projectors can be added to display more views to adjacent positions into which the viewer's head might stray. Such a system is shown in FIG.
1
. This system can be assembled from projectors of conventional resolution, but the projectors must be positioned with great precision, and the projector lenses must be designed so that the aperture stop of each lens is directly adjacent to those of neighbouring lenses.
An alternative approach is to use a single video projector and lens, but to place a multi-element shutter in front of the video projector lens, as shown in FIG.
2
. As before, light from the video projector forms a video image on the lens and then is focused by the lens to a zone, but the zone now comprises an image of the video projector with the shutter in front. By choosing a lens with a different focal length the zone can be expanded until each element of the shutter is imaged to a size suitable for one eye so that if only one shutter is open and all others are closed, the image on the lens is visible to a single eye. Views of a three-dimensional image can be screened one by one on the video projector and a different element of the shutter opened and closed for each view in such a way that provided the sequence is repeated quickly, the viewer sees a three-dimensional image. The problem with this system is that it wastes light and is bulky.
Yet another approach (
FIG. 3
) is to use a liquid-crystal display which is illuminated in such a way that it is visible from only one position at any time. Views of the three-dimensional image can be screened one by one on the liquid-crystal display, and the direction of illumination switched as each view is screened in such a way that each view is visible to a different area. Provided this is repeated quickly enough the flicker of each view need not be perceptible, and the viewer sees a three-dimensional image much as with the lenslet system. A display of this type is shown in the inventor's earlier patent GB-B-2206763. Switching the illumination of a liquid-crystal display in this way means that the optics need not be precisely registered, and the liquid-crystal display need not have a high resolution. A switched illumination scheme requires a liquid-crystal display with a high frame rate and such displays have been demonstrated. But like most modern liquid-crystal displays a matrix of transistors is needed to make a high frame rate display, and it is expensive to manufacture even small versions of such displays.
Liquid-crystal displays are not the only flat-panel displays which are expensive to manufacture. Most flat-panel displays comprise a matrix of individually controlled pixels and since there may be almost a million of these great care is needed to ensure that none fails. The screen of a cathode ray tube by contrast is a uniform layer of material which, because it has no detail or structure, rarely fails and therefore makes the cathode ray tube rather simple to construct. It is because the picture is built up by raster-scanning that the screen of a cathode ray tube can be so unstructured, and it appears to have been assumed that one cannot raster-scan a flat-panel display because the display needs a certain depth for the scanning beam.
However if a cathode ray tube is designed to produce a video image composed of a single line of pixels it can be made essentially flat. Indeed any video system which is designed to produce a single line of pixels can be made flat. With such flat systems all the optical components can be usually be confined within the core of a slab waveguide, and even if the system is long or wide it can be packed behind a flat screen simply by using mirrors to fold the optical layout. All that is needed is some device at the end of the optical system to expand the height of the display in order to restore the lost dimension.
Of particular interest here are video projectors and three-dimensional displays. A video projector is a device which projects light that focuses to form a video image at some point in space, and a one-line video projector is for the purposes of this document defined to be a video projector which writes a video image comprising a single line of pixels. Similarly a three-dimensional display is a device capable of displaying a video three-dimensional image, and a one-line three-dimensional display is for the purposes of this document defined to be a three-dimensional display which produces an image that is a single line high.
According to the present invention there is provided a flat-panel
Dudek James A.
Nixon & Vanderhye P.C.
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