Optical: systems and elements – Diffraction – From zone plate
Patent
1996-12-13
1999-11-09
Henry, Jon W.
Optical: systems and elements
Diffraction
From zone plate
359566, 359571, 359742, 351159, G02B 2744, G02B 518, G02B 308, G02C 702
Patent
active
059825430
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a zoned lens.
BACKGROUND OF THE INVENTION
Lenses are in this case generally understood to be both refractive lenses and diffractive lenses (for example, Fresnel zone plates). A diaphragm provided with a small hole can also be provided with effective powers (effective refractive powers), as described with reference to the drawings. Optical elements with a zero nominal power but with a limited aperture such as, for example, circular or annular plates with parallel front and rear surfaces can therefore also be understood here as "lenses".
Different zones of a lens are represented by areas of a lens which can be differentiated by physical properties, wherein between the zones the physical properties alter very rapidly or abruptly over a relatively small area. In particular, step-like sudden changes in the thickness of the lens material being used, for example, steps on the front or rear surface of the lens, delimit different zones of a lens. The use of different lens materials in different areas of the lens also results in a zoned lens in the present sense. The geometrical arrangement of the different zones can be configured in many different ways. For example, a central circular zone can be provided, to which outer concentric annular zones are adjacent. The surface areas of the different zones can be the same or different.
SUMMARY OF THE INVENTION
The object of the invention is to provide a zoned lens with novel optical properties. This is achieved according to the invention in that it comprises at least two adjacent zones, wherein the differences in the optical path lengths between an object point and an image point, of light rays passing through these two adjacent zones of the lens, are at least equal to half the coherence length of the light used, preferably at least equal to the coherence length of the light used. If there are optical materials with different refractive indices in the different zones, in such a configuration it can advantageously be provided that any two rays parallel to the axis through two adjacent zones pass through optical path lengths within the lens which differ by at least the coherence length. If adjacent zones are made from the same lens material, the optical path lengths of any two rays parallel to the axis through the two adjacent zones have to differ by at least CL n.sub.c /(n.sub.c -n.sub.i), wherein CL is the coherence length of the light used, n.sub.c is the refractive index of the lens material and n.sub.i is the refractive index of the medium surrounding the lens. The values n.sub.c =1.5 and n.sub.i =1 produce, for example, 3 CL. This condition is satisfied when, for example, steps of a height resulting from CL/(n.sub.c -n.sub.i) are provided between the zones. The values selected for n.sub.c and n.sub.i and the coherence length of, for example, 2 micrometers, produces a step height of 4 micrometers.
In the case where the differences in the optical path lengths of the light rays which pass through adjacent zones of the lens between an object point and an image point are at least equal to the coherence length of the light used, there is no interference of the light rays passing through the adjacent zones, as will be described in more detail with reference to the drawings, whereby in various applications advantages can be obtained compared to conventional lenses. For example, in bifocal lenses disturbing interferences between the areas with different focal lengths can be eliminated.
If the steps are configured so that the optical path length differences of the light rays through adjacent zones between an object point and an image point are shorter than the coherence length but at least equal to half the coherence length of the light used, the interference of the light rays passing through adjacent zones is reduced but not completely suppressed, which may be sufficient for some applications. Zoned lenses with such reduced steps are thus also the subject-matter of the invention.
In an embodiment of the invention it is pro
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Encyclopaedic Dictionary of Physics, Pergamon Press, 1961, pp. 266-267.
H. Haumann/G. Schroder, Bauelemente der Optik, Carl Hanser Verlag, pp. 150-151.
Meyers Lexikon der Technik und der exakten Naturwissenschaften, Bibliographisches Institut, p. 1041.
Bifocon Optics Forschungs-Und Entwicklungsgmbh
Chang Audrey
Henry Jon W.
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