Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1992-02-18
1994-11-29
Lerner, Martin
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 30, 359107, 359562, 359850, G02B 518, G02B 2742, G02B 508, G03H 104
Patent
active
053695119
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The instant invention is directed to methods of and apparatus for manipulating electromagnetic phenomenon. More particularly, the instant invention is directed to methods of and apparatus for manipulating electromagnetic phenomenon to convey information utilizing substantial portions of visible and invisible spectra to manipulate information, whether that information is in the form of pictorial displays, such as holographic displays; in the form of numerical information, such as digital or analog numerical information; or in the form of communication signals, wherein the signals are selected from any portion of the electromagnetic spectrum.
2. Prior Art
One of a number of fields to which the instant invention is pertinent is the field of holography. In order to create a holographic image, it is necessary to superimpose two coherent light beams which are incident on the same photographic plate or other suitable recording device. One beam is known as the "object beam" and the other beam as the "reference beam". In "off-axis holography", the beams are separated by an angle .theta., which is typically 45.degree.. The term "off axis" is utilized because the angle .theta. between the beams results in the axes of the beams not being coaxial. The general equation of holography is exceedingly complex and does not lend itself to any solution other than a numerical solution. The complexity of the equation is readily apparent when one considers two coherent waves S.sub.r (the reference wave) and S.sub.b (the object wave) with the waves being incident on the same photographic plate or other recording device, wherein:
Since the intensity at any point (x,y) in recording medium is given by the equation: exp{.theta.(x;y)+.alpha.(x;y)}+A.sub.b A.sub.r exp[-i(.theta.(x;y)-.alpha.(x;y)}] accordance with the practices of the prior art, one is a "plain wave", while the other is modulated with object information.
It can be readily seen that if .alpha.=.theta., waves S.sub.b and S.sub.r are coaxial, resulting in "in-line" holography. When .alpha.=.theta., the equation becomes greatly simplified. However, the prior art approaches have not produced viable holograms because prior art in-line devices produce double images, zero order aberrations, or both. The zero-order problem is a long-standing problem and is set forth in U.S. Pat. No. 3,944,322, incorporated hereby reference and assigned to Polaroid Corporation. An in-line system was demonstrated by the originator of holography, Dennis Gabor, who in 1971 received a Nobel Prize for his contribution. Gabor's device produced primitive holograms which, due to a lack of a coherent light source, were incapable of storing three-dimensional images. Off-axis configurations in which .alpha. and .theta. are different solve the double-image problem, but in many situations they are unsatisfactory because they are cumbersome and require a large air space to disperse the zero order light. In essence, off-axis configurations for off-axis holograms require considerably more space for operation than on-axis holograms. Exemplary of commercial off-axis holographic systems are the bar code readers used for checkout scanners in stores and disclosed in U.S. Pat. No. 4,415,224, incorporated herein by reference.
By substituting on-axis systems where off-axis systems are now used in the field of holography, considerable advantages will result in that if the art of holography is drastically simplified by using on-axis systems, holographic devices can be made much smaller and used for applications which in the past, if not unsuitable for holographic technology, have used that technology ineffectually.
A specific example of commercially available off-axis holographic technology is the use of a hologram head-up display to display information on the windshield of an automobile. For example, Nissan Corporation offers as an option a display of speedometer information in the left-hand corner of the windshield. This display is accomplished by an off-axis hologr
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