Optics: image projectors – Relief illusion
Patent
1998-06-12
2000-08-01
Dowling, William
Optics: image projectors
Relief illusion
353 79, 359 15, 359 33, 359446, G03B 2114
Patent
active
060956526
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a projection system where the screen uses holographic optical elements.
In U.S. Pat. No. 4,799,739 there is disclosed apparatus for autostereoscopic display in which images are projected onto a screen comprising a diffused holographic optical element.
However, a disadvantage of such apparatus is that it is necessary to project the image at an angle of incidence in the range of 60 to 30 degrees to an axis normal to the screen, the normal axis extending at 90 degrees to a plane containing the screen. Such off-axis projection produces image distortions which are difficult to compensate for in any practical system.
In U.S. Pat. No. 4,799,739 there exist additional problems in obtaining a good colour response from the screen over a range of viewing positions.
It would be an advantage if a screen could be made which allows normal or near normal image projection with good colour response over a wide range of viewing positions. (The normal is a line perpendicular to the plane of the screen). Part of one possible approach is outlined in EP-A-0349947 and WO-A-840615. These inventions can produce a screen which allows normal image projection and on axis viewing with an achromatic response.
In certain applications, however, EP-A-0349947 and WO-A-840615 suffer from the intrinsic problem in that, although they allow achromatic on axis viewing, the viewing zones (also known as exit pupils) for every wavelength of the incident light form at different distances from the screen (this is illustrated well in FIG. 1. of EP-A-0 349947). This is not a problem in the case of on-axis viewing, in other cases--e.g. in autostereoscopic applications or where the viewing zone needs to be small it can be a severe problem as illustrated in plan view in our FIG. 1.
Referring to FIG. 1 (which closely resembles FIG. 1 in EP-A-0349947) it may be seen that a viewing zone for every different wavelength of the incident light forms at a different distances from the screen. In the illustrated case the viewing zones are quite narrow which means that an eye at position a will see just part of the screen through the red zone--as indicated by the ray r as the other colours are absent this will be perceived a red band to the left side of the screen. An eye at position b will observe a variety of colours--the line indicated by ray q passes through all the viewing zones so that part of the screen will appear white, ray s passes through the blue zone only so that side of the screen will appear blue. From eye position c. the screen will appear with a red band at the right as may be seen by considering ray p.
It is apparent that in certain applications (such as autostereoscopic viewing) the fact that the viewing zones of all relevant wavelengths are not completely registered one with another will pose great problems by introducing bad colour fringing.
According to the present invention, there is provided a projection system comprising a screen, the screen comprising a diffraction grating, means for attenuating zero order light, and a diffuse holographic optical element characterised in that the system further comprises a focusing system element with positive optical power and in that the aggregate optical power of the grating and the holographic optical element is substantially less than that of the focusing element whereby the projection system exhibits a substantially achromatic response in a direction substantially perpendicular to the plane of the screen.
By suitably arranging the grating and holographic optical element (HOE), chromatic dispersion perpendicular to the screen (in the z-direction) is minimised.
The invention provides for on-axis (or nearly on-axis) projection and both on- and off-axis viewing while maintaining an achromatic response, typically with little or no chromatic dispersion in any dimension.
Typically the grating and the holographic optical element are made so that if they were assembled together they have no effective optical power. That is to say that on their own they are incapable of forming a viewin
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Orr Edwina Margaret
Trayner David John
Dowling William
Richmond Holographic Research and Development, Ltd.
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