Optical: systems and elements – Projection screen – With lens
Patent
1992-09-04
1995-05-30
Ben, Loha
Optical: systems and elements
Projection screen
With lens
359458, 359566, 359615, 353 38, 353 81, 354112, 354115, G02B 2744, G02B 504
Patent
active
054207185
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical elements and is particularly concerned with retro-reflecting, retro-imaging, and optical transmission inversion elements.
An important aspect in the field of the transmission or reflection of images to provide three-dimensional information for viewing or for recordal, is resolution.
All previous systems employing retro-reflecting/retro-imaging or optical transmission inversion screens to relay a true scale equal conjugate image of a scene have a resolution limit set by the pitch and/or packing density of micro-optical elements which retro-reflect or optically transmit the ray bundles impinging on their foremost surfaces. The number of micro elements within the viewing zone determines the lateral and hence the depth resolution in any given direction. Various methods have been described to reduce the effect of the micro-screen texture, but these require either gauged optical networks which are expensive and difficult to align or a physical movement of the micro-screen in the focal plane of a lens or lens array, or the image to be scanned across the micro surface. The latter two methods prevent real-time systems being realised and this reduces a system's usefulness when used with fast moving objects.
The object of the present invention is to provide improved resolution by substantially reducing if not eliminating the effect of micro-screen texture in a relatively simple and cost-effective manner.
According to the present invention, an optical element comprises a retro-reflecting or auto-collimating transmission screen having one or more lenses and an associated means of displacing wavelengths to different degrees and whereby to spread a reflected or transmitted image into a spectral streak. The conventional auto-collimating transmission screen is a back-to-back arrangement of microlens arrays in which the microlenses are aligned and of equal focal length as illustrated. The microlenses are themselves aligned to produce the auto-collimation, so that the combination is afocal.
The displacement means is preferably a grating that may be a blazed grating produced mechanically or optically, or it may be formed by a number of prisms. Equally a diffraction or zone plate could be used and then there is diffraction in all directions.
The use of a displacement means such as a grating or the like in combination with an imaging lens or lenses and the spreading of the projected/transmitted image into a spectral streak over a wide area with displacement of wavelengths to different degrees has the result that the recombined image is the resultant of a number of wavelengths from a number of screen locations each having its own, and different, screen pattern or uniquely displaced pattern, with the most beneficial effect that the original screen pattern is suppressed to a degree sufficient to prevent it from forming any part of the projected or transmitted image.
An important advantage of the optical element of the invention is that because a number of screen locations are used simultaneously, the lateral and hence depth resolution thereby is increased.
For screens with random micro-optical component distribution it need only be necessary to spread the image in one axis perpendicular to the imaging lens/lenses axis/axes, whereas a number of discrete angular displacements could be required to eliminate the screen texture of say a precise honeycomb array where for example images might be displaced in axis at 60.degree. to each other.
Another important advantage of the optical element of the system is that because the lensed image is spread to a number of screen locations simultaneously any localised break in the screen structure or any local obstacle on the screen surface is rendered less visible in the final retro-directed or transmitted image.
Two embodiments of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which:
FIG. 1 shows an optical reflective system embodying the optical element of the invention; and
FIG. 2 shows an opti
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Davies Neill
McCormick Malcolm
Stevens Richard
Ben Loha
De Montfort University
Jr. John Juba
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