OPTICAL SYSTEM FOR AN ENDOSCOPE

Abstract

An optical system for an endoscope includes an objective and a reversal system arranged after the objective. The reversal system includes at least one reversal stage for projecting the distal intermediate image as a proximal intermediate image into a proximal intermediate image plane. The reversal system imprints on the proximal intermediate image a first longitudinal chromatic aberration referred to a predetermined wavelength from the visible spectrum and a predetermined wavelength from the near infrared range. The objective imprints on the distal intermediate image a second longitudinal chromatic aberration referred to the predetermined wavelength from the visible spectrum and the predetermined wavelength from the near infrared range. The second longitudinal chromatic aberration has the opposite sign relative to the first longitudinal chromatic aberration, which reduces the longitudinal chromatic aberration caused by the reversal system in the proximal intermediate image.

Claims

1. An optical system for an endoscope, comprising: an objective configured to image an object as a distal intermediate image in a distal intermediate image plane; and a reversal system arranged after the objective, including at least one reversal stage configured to project the distal intermediate image as a proximal intermediate image into a proximal intermediate image plane, wherein the reversal system is configured to imprint on the proximal intermediate image a first longitudinal chromatic aberration referred to a predetermined wavelength from a visible spectrum and a predetermined wavelength from the a near infrared range, wherein the objective is configured to imprint on the distal intermediate image a second longitudinal chromatic aberration referred to the predetermined wavelength from the visible spectrum and the predetermined wavelength from the near infrared range, and wherein the second longitudinal chromatic aberration has the opposite sign relative to the first longitudinal chromatic aberration such that the longitudinal chromatic aberration caused by the reversal system in the proximal intermediate image is reduced.

2. The optical system according to claim 1, wherein all curved material boundary surfaces of the objective are spherically curved.

3. The optical system according to claim 1, wherein the reversal system comprises several reversal stages arranged one behind the other.

4. The optical system according to claim 1, wherein at least one reversal stage comprises two lenses directly joined to each other, the materials of which are chosen such that the refractive indices of the materials differ by not more than 0.3.

5. The optical system according to claim 4, wherein each refractive index step of all lenses which are directly joined to each other from at least one reversal stage is smaller than or equal to 0.3.

6. The optical system according to claim 1, wherein at least one reversal stage comprises a lens with an aspherical boundary surface.

7. The optical system according to claim 1, wherein the predetermined wavelength from the visible spectrum is in a range of 400-700 nm and the predetermined wavelength from the near infrared range is in a range of 710-900 nm.

8. The optical system according to claim 1, wherein the predetermined wavelength from the visible spectrum is 540 nm and the predetermined wavelength from the near infrared range is 840 nm, and wherein the first longitudinal chromatic aberration lies in a range from 35 to 65 μm and the second longitudinal chromatic aberration lies in a range from −4 to −0.5 μm.

9. The optical system for an endoscope according to claim 1, wherein the predetermined wavelength from the visible spectrum is 460 nm and the predetermined wavelength from the near infrared range is 840 nm, and wherein the ratio of the first longitudinal chromatic aberration to the second longitudinal chromatic aberration lies in a range from −4 to −1.

10. The optical system according to claim 1, wherein the reversal system comprises a third longitudinal chromatic aberration for the wavelength of 540 nm relative to the wavelength of 460 nm and a fourth longitudinal chromatic aberration for the wavelength of 540 nm relative to the wavelength of 640 nm, and wherein the objective comprises a fifth longitudinal chromatic aberration for the wavelength of 540 nm relative to the wavelength of 460 nm and a sixth longitudinal chromatic aberration for the wavelength of 540 nm relative to the wavelength of 640 nm, and wherein a ratio of the third longitudinal chromatic aberration to the fourth longitudinal chromatic aberration lies in the range from −1.1 to −0.8 and a ratio of the fifth longitudinal chromatic aberration to the sixth longitudinal chromatic aberration lies in the range from −4.5 to −3.

11. The optical system according to claim 1, wherein the objective comprises a seventh longitudinal chromatic aberration for the wavelength of 460 nm relative to the wavelength of 840 nm and an image angle alpha in the object space, and wherein a product of sin(alpha/2) and the seventh longitudinal chromatic aberration lies in a range from 18 to 26 μm.

12. The optical system according to claim 1, wherein the predetermined wavelength from the visible spectrum is 540 nm and the predetermined wavelength from the near infrared range is 840 nm and the reversal system comprises n optical elements, wherein a product of the number n of optical elements and the first longitudinal chromatic aberration lies in a range from 1500 to 2000 μm.

13. The optical system according to claim 1, wherein the objective is configured to imprint on the distal intermediate image such a longitudinal chromatic aberration that the focus position of a wavelength of 840 nm lies between the focus positions for 540 nm and 640 nm.

14. An endoscope, comprising an optical system according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 is a schematic representation of a first embodiment of the optical system in an endoscope.

[0050] FIG. 2 is a schematic representation of a second embodiment of the optical system in an endoscope.

[0051] FIG. 3 is a representation of the dependence of the focal position on the wavelength for a conventional optical system.

[0052] FIG. 4 is a schematic representation of the structure of the reversal system of the optical system.

[0053] FIG. 5 is a schematic representation of the focal position caused by the reversal system according to FIG. 4 as a function of the wavelength.

[0054] FIG. 6 is a schematic representation of the optical structure of the objective of the optical system according to certain embodiments.

[0055] FIG. 7 is a representation of the focal position caused by the objective as a function of the wavelength.

[0056] FIG. 8 is a schematic representation of the focal position caused by the optical system as a function of the wavelength.

[0057] FIG. 9 is a schematic representation of the focal position caused by a reversal system of a known endoscope as a function of the wavelength.

[0058] FIG. 10 is a schematic representation of the focal position caused by an objective of a known optical system as a function of the wavelength.

[0059] FIG. 11 is a schematic representation of the focal position of a known optical system as a function of the wavelength.

[0060] While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular example embodiments described. On the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0061] In the following descriptions, the present invention will be explained with reference to various exemplary embodiments. Nevertheless, these embodiments are not intended to limit the present invention to any specific example, environment, application, or particular implementation described herein. Therefore, descriptions of these example embodiments are only provided for purpose of illustration rather than to limit the present invention.

[0062] In the embodiment shown in FIG. 1 the optical system 1 is arranged in a rigid shaft 2 of a schematically represented endoscope 3.

[0063] The optical system 1 comprises a schematically represented objective 4, which images an object 6 located in front of a distal end 5 as a distal intermediate image in a distal intermediate image plane 7, and a schematically represented reversal system 8, which projects the distal intermediate image as a proximal intermediate image into a proximal intermediate image plane 9. The objective 4 and the reversal system 8 have a common optical axis OA.

[0064] Furthermore, the endoscope 3 comprises a main part 10 in which a further optical system can be arranged, such as e.g. the schematically represented eyepiece 11, as well as a camera connection 12. A camera 13 can be detachably connected to the main part 10 at the camera connection 12, as is represented schematically in FIG. 1. The camera 13 can comprise an optical system (not shown). Furthermore, the camera 13 contains a flat image sensor 14, which is formed, for example, as a CCD sensor or as a CMOS sensor. The camera may not only be connected to the camera connection 12 directly, as is shown in FIG. 1. It is also possible that a coupler (not shown) is connected between the camera connection 12 and the camera 13, which for its part can contain an optical system.

[0065] The endoscope 3 shown in FIG. 1 is formed as a forward-viewing endoscope, since an object 6 can be captured with it which is positioned in front of the distal end 5 in the direction in which the endoscope shaft 2 extends. However, it is also possible that the endoscope 3 looks in almost another direction than the direction in which the endoscope shaft 2 extends. This is shown schematically in FIG. 2, wherein here the direction of view is inclined upwards by 45° compared with the direction of extension of the endoscope shaft 2. In the endoscopes according to FIGS. 1 and 2, the reversal system 8 is in each case formed the same. Only the objective 4 differs in that the desired deflection, where necessary, takes place in the objective, as will be described in detail in the following.

[0066] Furthermore, the endoscope can comprise e.g. on the main part 10 an illumination connection 15, via which the desired illumination radiation can be supplied to the endoscope 3. The illumination connection 15 is connected to a schematically drawn in light guide 16, which extends to the distal end 5 of the endoscope 3 and emits the guided radiation for the illumination of the object 6.

[0067] In the operation of the endoscope 3, the object 6 to be imaged is illuminated with the radiation from the light guide 16 and the illuminated object 6 is imaged via the objective system 1 in the proximal intermediate image plane 9 and there in the plane of the image sensor 14 by means of the eyepiece 11, such that the image sensor 14 can capture a sharp image of the object 6. Known endoscopes are usually designed for the wavelength range of from approx. 400 to 700 nm, which is visible to the human eye. The wavelength-dependent longitudinal chromatic aberration is to be as small as possible for the visible wave range. In FIG. 3, the focal position produced in a conventional endoscope is plotted along the x-axis in μm as a function of the wavelength along the y-axis in nm. A focal position of 0 μm means that the location of the sharpest image coincides with the plane in which the image sensor 14 lies. A focal position of greater than 0 μm means that the location of the sharpest image is behind (and thus further to the right in FIGS. 1 and 2) the plane in which the image sensor 14 lies. Correspondingly, a negative focal position is a deviation from the optimal focal position in the direction towards the distal end 5. As can be learned from this representation, deviations in the range of from approx. −100 μm to 400 μm are in the range of from 460 to 700 nm. This is still acceptable for a sufficiently good imaging by means of the sensor 14.

[0068] However, in FIG. 3, the focal positions are also shown for wavelengths of greater than 700 nm and in particular for the range of from 700 to 840 nm. This range from 700 nm and, for example, from 700 to 900 nm or also from 780 to 3000 nm, is referred to as the near infrared range and is of increasing interest for endoscopy, since e.g. in addition to imaging with light from the visible wavelength range, fluorescence diagnostics can also be carried out. For example, a fluorescent substance, such as e.g. ICG (indocyanine green), can be injected into tissue to be examined or a vascular system to be examined, which substance emits a fluorescent signal with longer wavelengths, the wavelengths of which are e.g. 830 to 840 nm, when excited with light of a wavelength of approx. 805 nm. It is therefore desired also to be able to capture an image with radiation from the near infrared range by means of the camera 13. As can be learned from the representation in FIG. 3, however, the longitudinal chromatic aberration based e.g. on the wavelength of 540 nm and the wavelength of 840 nm (and thus the difference of the focal positions for these two wavelengths) is about 1200 μm. In the case of the wavelengths 460 nm and 840 nm, the longitudinal chromatic aberration is about 1000 μm. This results in it not being possible to carry out a sharp imaging with the fluorescent signals.

[0069] The longitudinal chromatic aberration described is mainly produced by the reversal system, since, because of the necessary length of the shaft 2, many rod lenses with different materials are necessary, which all contribute to the longitudinal chromatic aberration. The optical system 1 is therefore designed such that the longitudinal chromatic aberration caused by the reversal system 8 is reduced as much as possible by an opposite longitudinal chromatic aberration of the objective 4. As a rule, the objective 4 hardly contributes to the longitudinal chromatic aberration because of the very much shorter extension in comparison with the reversal system 8. The objective 4 is now designed such that it has a very much larger longitudinal chromatic aberration than usual, which, however, is opposite to the longitudinal chromatic aberration of the reversal system 8, with the result that the resulting longitudinal chromatic aberration in the intermediate image in the proximal intermediate image plane 9 is reduced to the case in which the objective would cause no longitudinal chromatic aberration at all and the longitudinal chromatic aberration would only be caused by the reversal system.

[0070] In the optical system 1 according to the invention, the reversal system 8, as is shown in FIG. 4, is formed from three reversal stages 17, 18, 19 arranged one behind the other, which in each case project an intermediate image into a next intermediate image plane. The first reversal stage 17 thus projects the intermediate image lying in the distal intermediate image plane 7 into a second intermediate image plane 20. The second reversal stage 18 projects the intermediate image lying in the second intermediate image plane 20 into a third intermediate image plane 21. The third reversal stage 19 projects the intermediate image from the third intermediate image plane 21 into the proximal intermediate image plane 9.

[0071] The three reversal stages 17-19 are thus arranged one behind the other such that an intermediate image lying in the distal intermediate image plane 7 is projected (via in each case the next intermediate image plane 20 and 21) into the proximal intermediate image plane 9. Since each reversal stage 17-19 during the projection of the intermediate image produces a reversed intermediate image and an odd number of reversal stages 17-19 are provided, the intermediate image of the object 6 lying in the distal intermediate image plane 7 is projected into the proximal intermediate image plane 9 as a reversed intermediate image.

[0072] The two reversal stages 17 and 18 comprise in each case two rod lenses of type A. The reversal stage 19 comprises a rod lens of type A and a rod lens of type B. All rod lenses of type A have spherical radii and are constructed identically. The rod lens labelled with −A in FIG. 4 is identical to the rod lens of type A, merely mounted the other way around. The rod lenses of type A comprise the boundary surfaces A1 (positively spherically curved), A2 (negatively spherically curved), A3 (negatively spherically curved), A4 (positively spherically curved) and A5 (negatively spherically curved). The material between the surfaces A1 and A2 has a refractive index n.sub.d=1.8 and an Abbe number v.sub.d=29.9. The material between the boundary surfaces A2 and A3 has a refractive index n.sub.d=1.5182 and an Abbe number v.sub.d=58.9. The material between the boundary surfaces A3 and A4 has a refractive index of n.sub.d=1.6377 and an Abbe number of v.sub.d=42.4. The material between the boundary surfaces A4 and A5 has a refractive index of n.sub.d=1.65 and an Abbe number of v.sub.d=50.9.

[0073] The rod lens of type B comprises the boundary surfaces B1 (positively aspherically curved), B2 (negatively spherically curved), B3 (negatively spherically curved), B4 (positively spherically curved) and B5 (negatively spherically curved). The material between the boundary surfaces B1 and B2 has a refractive index n.sub.d=1.75 and an Abbe number of v.sub.d=45.4. The material between the boundary surfaces B2 and B3 has a refractive index of n.sub.d=1.523 and an Abbe number of v.sub.d=59.5. The material between the boundary surfaces B3 and B4 has a refractive index n.sub.d=1.6377 and an Abbe number v.sub.d=42.4. The material between the boundary surfaces B4 and B5 has a refractive index of n.sub.d=1.65 and an Abbe number of v.sub.d=50.9.

[0074] A reversal system 6 formed in this way has a longitudinal chromatic aberration behaviour as is shown in FIG. 5. In FIG. 5, in the same way as in FIG. 3, the focal position is plotted along the x-axis in μm and the wavelength is plotted along the y-axis in nm. From this representation in FIG. 5 it can be learned that the longitudinal chromatic aberration for a first predetermined wavelength of 540 nm from the visible wavelength range in relation to a second predetermined wavelength of 840 nm from the near infrared range is approx. 59.05 μm. For the wavelengths 460 nm and 840 nm, the longitudinal chromatic aberration is approx. 88.08 μm. The longitudinal chromatic aberration behaviour can be described as a positive longitudinal chromatic aberration.

[0075] The objective 4 has the structure shown in FIG. 6 with an optical flat 22, a prism 23 and lenses 24-30. In the representation in FIG. 6, the prism 23 is represented as an extended glass block, which, in the formation of the optical system for a forward-viewing endoscope, as is shown in FIG. 1, is realized as the glass block shown. In the case of the implementation of the optical system 1 for an inclined direction of view, as is shown in FIG. 2, the prism 23 is then actually formed as a prism. The representation chosen in FIG. 6 serves only for simplification of the representation, since the description of the longitudinal chromatic aberration depends on the glass paths as well as on the curved boundary surfaces.

[0076] The curvature properties and materials used are indicated in the following table (Table 1) for the surfaces F1-F16 of the objective 4, wherein the specification of the refractive index n.sub.d and of the Abbe number v.sub.d for a surface means that this material is present between the surface in the line of which the specifications are and the next following surface.

TABLE-US-00001 TABLE 1 Refractive Abbe Surface Curvature index n.sub.d number v.sub.d F1 flat 1.768 72.2 F2 flat F3 positive spherical 1.892 37.1 F4 positive spherical F5 flat 1.835 42.7 F6 flat 1.8502 32.2 F7 negative spherical F8 positive spherical 1.529 77 F9 negative spherical F10 negative spherical 1.855 24.8 F11 negative spherical F12 positive spherical 1.603 65.4 F13 negative spherical 1.6377 42.4 F14 positive spherical F15 positive spherical 1.529 77 F16 negative spherical

[0077] The longitudinal chromatic aberration caused by the objective 4 is represented in FIG. 7 in the same way as in FIG. 5. The objective 4 was designed such that there is an essentially negative longitudinal chromatic aberration. Thus, for the wavelengths 540 nm and 840 nm, the longitudinal chromatic aberration is −2.3 μm. For the wavelengths 460 nm and 840 nm it is −32.7 μm. The longitudinal chromatic aberration imprinted by the objective 4 and the reversal system 8 as a whole during the projection from the distal intermediate image plane 7 into the proximal intermediate image plane 9 is shown in FIG. 8. Because of the opposite longitudinal chromatic aberrations of objective 4 and reversal system 8, the resulting longitudinal chromatic aberration is smaller than that of the reversal system 8 alone. Thus, for the wavelengths 540 and 840 nm, the longitudinal chromatic aberration is approx. 55.61 μm and for the wavelengths 460 and 840 nm approx. 54.05 μm. Because of this extremely small longitudinal chromatic aberration, sharp captures are possible without problems with light from the visible wavelength spectrum and with light from the infrared spectrum.

[0078] In FIGS. 9-11, the longitudinal chromatic aberration of a reversal system (FIG. 9), the longitudinal chromatic aberration of an objective (FIG. 10) and the resulting longitudinal chromatic aberration of objective and reversal system (FIG. 11) of a known comparison endoscope (which also comprises an optical system made up of objective and reversal system in a rigid endoscope shaft) are shown in the same way as in FIGS. 5-8. The comparison endoscope was designed with respect to as small as possible a longitudinal chromatic aberration of the reversal system. This is also slightly smaller than that of the reversal system 8 of the optical system according to the invention. Thus, for 540 and 840 nm, the longitudinal chromatic aberration of the reversal system of the comparison endoscope is approx. 59 μm and for 460 and 840 nm approx. 82.1 μm. This is achieved through the use of many different glasses, since the reversal system of the comparison endoscope comprises three reversal stages of two rod lenses with, in each case, 4-5 elements, with the result that in total 27 elements are provided. Furthermore, the refractive index steps between the different materials are greater than 0.5.

[0079] The objective of the comparison endoscope has the positive longitudinal chromatic aberration according to FIG. 10. Thus, for 540 and 840 nm, the longitudinal chromatic aberration is approx. 88.7 μm and for 460 and 840 nm approx. 115.5 μm. Although this is a small longitudinal chromatic aberration, it is positive with the result that the resulting longitudinal chromatic aberration, as is shown in FIG. 11, for 540-840 nm is approx. 140 μm and for 460-840 nm is approx. 186.7 μm. The resulting longitudinal chromatic aberration is thus considerably larger than in the case of the optical system according to the invention.

[0080] In addition, the use of more optical elements in the reversal system and of materials with refractive index steps of greater than 0.5 in the reversal system results in larger Fresnel losses occurring in comparison with the reversal system 8 according to the invention, in which the refractive index steps are not greater than 0.3. Because of the smaller refractive index steps and the lower number of lenses in the reversal system 8 according to the invention, the transmission of the optical system 1 according to the invention is considerably higher. Thus, the transmission in the optical system 1 according to the invention for the wavelengths 460 nm, 540 nm, 588 nm, 640 nm, 656 nm and 840 nm is 0.69, 0.77, 0.78, 0.77, 0.76 and 0.78, respectively. This gives a total transmission of 0.75. In the case of the optical system of the known endoscope, in contrast, the transmission for the same wavelengths is only 0.60, 0.71, 0.72, 0.72, 0.72 and 0.74. The total transmission is then 0.68. Thus, in the case of the optical system 1 according to the invention, there is a 16% greater brightness for blue (640 nm), a 9% greater brightness for green (540 nm) and a 5% greater brightness for near infrared 840 nm. Moreover, because of the higher transmission of blue, the image is whiter, which brings about a better image impression.

[0081] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it will be apparent to those of ordinary skill in the art that the invention is not to be limited to the disclosed embodiments. It will be readily apparent to those of ordinary skill in the art that many modifications and equivalent arrangements can be made thereof without departing from the spirit and scope of the present disclosure, such scope to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and products. Moreover, features or aspects of various example embodiments may be mixed and matched (even if such combination is not explicitly described herein) without departing from the scope of the invention.