Optical: systems and elements – Compound lens system – With relay
Reexamination Certificate
1998-12-31
2001-10-09
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Compound lens system
With relay
C359S654000, C600S172000
Reexamination Certificate
active
06301043
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an endoscope in accordance with the introductory clause of Patent claim
1
.
PRIOR ART
An endoscope of the claimed general type is known from the U.S. Pat. No. 4,783,154. That known endoscope comprises a distally disposed lens and an optical transmission system which transmits the image of the lens disposed on the distal end to the proximal end, i.e. to that end of the endoscope which is located outside the cavity. The transmission system includes at least one reversing system consisting of two symmetrical elements whereof each is formed by a optically thin piano-concave lens. The rod lenses, which may include, inter alia, plane faces, consist of a material having a varying or inhomogeneous refractive index.
For an explanation of all the aspects not described here in more details explicit reference is made, by the way, to the U.S. Pat. No. 4,783,154 and the prior art documents listed on the cover sheet of that patent document.
For the purposes of the present application the term “reversing system”—which is also referred to as relay lens system—is to be understood to denote an optical system which produces (at least) one 1:1 image. In endoscopes, as a rule, several reversing systems—preferably in an odd number—are combined to form one transmission system with a number of image reversals in correspondence with the number of image reversals.
In known endoscopes with lenses exclusively consisting of a homogeneous material the two elements of each reversing system are frequently composed of more than two elements.
In the endoscope of the claimed general type, as it is known from the U.S. Pat. No. 4,783,154, the principal focal point is exclusively the substitution of several lens elements of conventional endoscopes by at least one lens element having an inhomogeneous refractive index, rather than the improvement of the compared against known endoscopes.
From the article “Correction of chromatic aberrations in GRIN endoscopes” by Dennis C. Leiner and Rochelle Prescott, published in APPLIED OPTICS, vol. 22, No. 3, from page 383 onwards, it is moreover known to reduce chromatic image aberrations by the application of lenses consisting of a material having an inhomogeneous, i.e. varying, refractive index.
In endoscopes of the type used in medicine or in engineering for the observation of cavities such as body cavities, combustion chambers in etc., it is not the correction of chromatic aberrations which is the predominant focal point but rather the following problem:
Depending on the specific application, endoscopes of different length must be used. In order to be able to “standardise” the structure of the endoscopes it is generally common to provide endoscopes of different lengths with a different number (1, 3, 5, . . . ) of reversing systems, with the reversing systems always being identically configured, independently of the length of the endoscope.
The reversing systems which have so far actually been employed present a defined field curvature or camber. The cumulated field curvature of the transmission system is hence dependent on the number of the employed reversing systems. In the known endoscopes which are presently commercially available the field curvature of the transmission system, i.e. the “added or cumulated field curvatures” of the reversing system, are “compensated” by an opposite field curvature of the respective lens. This means that the endoscope manufacturers must use and hence keep stocks of different lenses for endoscopes of “different lengths” in order to be able to produce, at short notice, endoscopes have a length and hence a different number of reversing systems in correspondence with the respective application as ordered.
Another cost factor which is anything but negligible results from the calculation of a plurality of lenses which are, actually speaking, equal—e.g. in terms of the angle of view and/or the image angle—which are distinguished from each other only in view of their field curvature, so that they are usable with a different number of reversing systems.
These stock-keeping and calculation problems could be avoided if each reversing system and hence the transmission system, too, had no or only a negligibly small field curvature. Then the same lens could constantly be used independently of the length of the endoscope.
In technical literature and particularly patent literature various proposals have been disclosed for reversing systems for endoscopes having no or a negligibly small field curvature. The known proposals present, however, the disadvantage that they require a comparatively great number of lenses for each reversing system so that the production costs are too high for commercial applications in engineering or medicine.
BRIEF DESCRIPTION OF THE INVENTION
The present invention is based on the problem of providing an endoscope including an optical transmission system consisting of reversing or relay lens systems having a simple design and a field curvature so small that a compensation of the field curvature or camber of the transmission system by the distally disposed lens is not required even with a strongly different number of reversing or relay lens systems, so that the same lens can always be used independently of the number of the reversing or relay lens systems.
One inventive solution to this problem is defined in Patent claim
1
. Improvements of the invention are the subject matters of claims
2
et seq.
In accordance with the invention it has been found that the defined problem may be solved in a way that, starting out from an endoscope in accordance with the introductory clause of Patent claim
1
, this endoscope is improved by the provision that the rod lenses consist of a material having a positive dispersion and a radially inhomogeneous refractive index. Moreover, the optically thin lenses are plano-convex lenses.
The refractive index which varies along the radial direction (i.e. orthogonally on the optical axis) decreases as a function of the radius in radial symmetry as the distance from the optical axis increases. Moreover, on account of the positive dispersion the refractive index decreases as the wavelength increases.
The invention starts out from the following fundamental consideration:
A conventional reversing system with a locally non-varying refractive index, which comprises two elements symmetrical relative to the aperture, whereof each includes a rod lens—as has been proposed, for instance, by Hopkins and as it is presently produced by a number of manufacturers—presents a strong field curvature and hence a substantial astigmatism. These aberrations must be compensated by a complex lens design.
On the other hand, reversing systems with rod lenses or rods having an inhomogeneous refractive index (GRIN rods) present chromatic aberrations which must equally be compensated by the lens.
The advantages of the known reversing systems are avoided in accordance with the invention by the provision that a GRIN rod is used instead of a rod lens with a conventional design, which rod displays a positive dispersion and hence an over-corrected longitudinal chromatic aberration. Due to the inhomogeneity of the refractive index it is now possible to achieve the desired optical effect with planar faces and to compensate yet the astigmatism and the field curvature practically completely. The thin plano-convex lens, which presents an under-corrected longitudinal chromatic aberration, serves substantially to correct the longitudinal chromatic aberration of the GRIN rod.
With the inventive configuration of the reversing or relay lens systems it is hence possible to correct not only the chromatic aberrations of transmission systems in endoscopes but to keep also the field curvature or camber of each reversing system so small that different lenses are not required even with a strongly differing number of reversing systems and thus over a wide range of varying endoscope lengths: in practical operation the same lens may be used with 1 to 15 reversing or relay lens systems, on account of the
Karl Storz GmbH & Co. KG
Robinson Mark A.
Spyrou Cassandra
St. Onge Steward Johnston & Reens LLC
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