Lens System for a Video Endoscope, Endoscope Objective, Video Endoscope, and Assembly Method

Abstract

A lens system (1) for a video endoscope comprises, in order from an object side, a cover glass (20), a first lens (40), a second lens (60) and one or more further lenses, wherein all lenses are single lenses. An aperture stop (21) is arranged at the object side of the first or the second lens (40, 60), all lenses on an image side of the aperture stop (21) are aspherical, all lenses are made of glass or of a crystalline material, and at least one lens has a refractive index n approximately equal to or exceeding 1.66. The invention also relates to an endoscope objective, to a video endoscope, and to a method for assembling an endoscope objective.

Claims

1. A lens system for a video endoscope comprising, in order from an object side, a cover glass, a first lens, a second lens and one or more further lenses, wherein all lenses are single lenses, an aperture stop arranged on the object side of the first or the second lens, all lenses on an image side of the aperture stop are aspherical, all lenses are made of glass and/or of a crystalline material, and at least one lens has a refractive index n approximately equal to or exceeding 1.66.

2. The lens system of claim 1, wherein the at least one lens having a refractive index of approximately equal to or exceeding 1.66, has a refractive index approximately equal to or exceeding 1.7, preferably approximately equal to or exceeding 1.8.

3. The lens system of claim 1 wherein at least one lens has an Abbe number ν exceeding 70, preferably exceeding 80.

4. The lens system of claim 1, wherein the lens system comprises at most 3 lenses.

5. The lens system of claim 1, wherein in that the first lens has positive refractive power.

6. The lens system of claim 1, wherein at least one of the lenses has an aspherical surface having a turning point in surface inclination with respect to an optical axis.

7. The lens system of claim 1, wherein the aperture stop is arranged on an image-side surface of the cover glass.

8. The lens system of claim 1, wherein the lens system comprises a plane glass plate arranged between a last lens, in order from the object side, of the one or more further lenses and an image plane of the lens system.

9. The lens system of claim 1, wherein the second lens has a larger diameter than the first lens.

10. The lens system of claim 1, wherein an overall shape of the lens system is approximately frustoconical or frustopyramidal.

11. The lens system of claim 1, wherein at least the first lens and the second lens each have a functional rim having a plane surface outside their respective optical surfaces.

12. The lens system of claim 1, wherein each lens has a diameter, and each lens diameter is defined as a larger one of a diameter of the lens's object-side optical surface and a diameter of its image-side optical surface, as measured from the optical axis; and the diameters of the lenses increase from the object side to the image side.

13. The lens system of claim 1, wherein each lens is rotationally symmetric about an optical axis of all the lenses.

14. An endoscope objective for a video endoscope, characterized in that the endoscope objective comprises a lens system comprising, in order from an object side, a cover glass, a first lens, a second lens and one or more further lenses, wherein all lenses are single lenses, an aperture stop arranged on the object side of the first or the second lens, all lenses on an image side of the aperture stop are aspherical, all lenses are made of glass and/or of a crystalline material, and at least one lens has a refractive index n approximately equal to or exceeding 1.66.

15. The endoscope objective of claim 14, wherein at least the first lens and the second lens each have a functional rim having a plane surface outside their respective optical surfaces, wherein the functional rim of the second lens is mounted on the functional rim of the first lens or on a spacer mounted on the functional rim of the first lens.

16. The endoscope objective of claim 14, wherein at least one of the lenses has an aspherical surface having a turning point in surface inclination with respect to an optical axis.

17. A video endoscope having an elongate shaft, an objective arranged in a distal end section of the shaft, and an electronic image sensor arranged in an image plane of the objective, wherein the objective comprises, in order from an object side, a cover glass, a first lens, a second lens and one or more further lenses, wherein all lenses are single lenses, an aperture stop arranged on the object side of the first or the second lens, all lenses on an image side of the aperture stop are aspherical, all lenses are made of glass and/or of a crystalline material.

18. The video endoscope of claim 17, wherein at least one lens has a refractive index n approximately equal to or exceeding 1.66.

19. The video endoscope of claim 17, wherein at least one of the lenses has an aspherical surface having a turning point in surface inclination with respect to an optical axis.

20. The video endoscope of claim 17, wherein a micro-lens array is arranged on a distal side of the image sensor having a non-linear CRA function and the micro-lens array is arranged on a proximal side of a most proximal lens of the objective.

21. The video endoscope of claim 20, wherein the image sensor comprises a sensor plane, and wherein the sensor plane is arranged essentially perpendicular to an optical axis of the objective.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 shows a lens system according to a first embodiment of the present invention in an axial sectional view;

[0048] FIG. 2 shows a lens system according to a second embodiment of the invention in an axial sectional view;

[0049] FIG. 3 shows a lens system according to a third embodiment of the invention in an axial sectional view;

[0050] FIG. 4 shows a lens assembly according to an exemplary embodiment, in an axial sectional view;

[0051] FIG. 5 shows in a simplified manner an endoscope objective and an electronic image sensor according to a first variation; and

[0052] FIG. 6 shows in a simplified manner an endoscope objective and an electronic image sensor according to a second variation.

DETAILED DESCRIPTION OF THE INVENTION

[0053] In FIGS. 1, 2, and 3 exemplary embodiments of a lens system in accordance with the present invention are shown in an axial sectional view. According to each of the embodiments shown the lens system 1 comprises optical surfaces 2-11, which are, in order from an object side, plane surfaces 2, 3 of a cover glass 20, aspheric surfaces 4, 5 of a first lens 40, aspheric surfaces 6, 7 of a second lens 60, aspheric surfaces 8, 9 of a third lens 80, and plane surfaces 10, 11 of a glass plate 100. Further to the image side an electronic image sensor 110 is arranged having its sensor plane in the focal plane of the lens system 1. The image sensor 110 may be a CCD, CMOS or a MOSFET sensor, for example.

[0054] Between the glass plate 100 and the image sensor 110 a micro-lens array is arranged (not shown). On the image-side surface 3 of the cover glass 20 an aperture stop 21 is formed, for example, by a ring-shaped diaphragm or by a coating on the surface 3 of the cover glass 20. FIGS. 1-3 also show exemplary rays entering into the lens system 1 from an object field, forming an image of the object field on the image sensor 110.

[0055] Each one of the lens systems 1 as depicted in FIGS. 1-3 comprises only three lenses, which are the first, second and third lens 40, 60, 80, each lens having two aspheric optical surfaces. The lenses 40, 60, 80, as well as the cover glass 20, the glass plate 100 and the image sensor 110 are arranged along the optical axis 12 of the lens system 1. The optical surfaces 4-9 of the lenses 40, 60, 80 are symmetric with respect to the optical axis 12. In the examples shown, the plane surfaces 2, 3 of the cover glass 10 and the plane surfaces 10, 11 of the glass plate 100 are perpendicular to the optical axis 12. As can be seen in FIGS. 1-3, the diameters of the lenses 40, 60, 80 increase with increasing lens number, i.e. from the object to the image side of the lens system 1. The diameter of each lens 40, 60, 80 is defined as the larger one of the diameters of its both optical surfaces, as measured from the optical axis 12. The diameter of the glass plate 100 exceeds that of the third lens 80. The cover glass 10 may have a diameter exceeding that of the first lens 40, however an outer portion of the cover glass 10 may be employed for mounting the lens system 1 or an endoscope objective (see below). The lenses 40, 60, 80 and the glass plate 100 are made of glass, while the cover glass 20 consists of sapphire.

[0056] Parameters describing the optical surfaces of the respective lens systems 1 according to the embodiments shown are given in Tables 1a-3b. In particular, Tables 1a, 2a, and 3a give the radius R of the inner portion of the respective refractive surfaces, the thickness d of the respective optical element or air gap, as measured on the optical axis 12 starting on the respective optical surface, the refractive index n, and the Abbe number ν of the respective optical element. The refractive index n and the Abbe number ν are defined in the conventional manner (see above). Tables 1b, 2b, and 3b give the coefficients of the aspherical surfaces 4-9, as defined in the conventional manner, indicating the displacement of a surface point in an axial direction as a function of various powers of r, where r is the distance from the optical axis 12.

[0057] According to the first embodiment and as can be seen in FIG. 1, the first lens 40 is, relating to respective inner portions of the optical surfaces 4, 5, a bi-convex lens having an almost plane object-side surface 4 and a convex image-side surface 5. The second lens 60 is a positive meniscus lens, having its convex surface on the image side. The third lens 80 is a negative meniscus lens, having its concave surface on the image side. Each of the lens surfaces 4-9 has a turning point in surface inclination to the optical axis, as seen in an axial section. That is, in a portion of the surface near the optical axis 12 the optical surface increasingly inclines in an object-side direction, but in an outer surface portion inclination in the object-side direction decreases with increasing distance radial distance r from the optical axis 12, or in an inner portion the surface inclination increases in the image-side direction with increasing radial distance r, and in an outer portion decreases with increasing distance radial distance r. The optical parameters of the embodiment of FIG. 1 are given in Tables 1a and 1b. The f-number of the first embodiment is 6.0, and the angle of view (2ω) is 73°.

TABLE-US-00001 TABLE 1a Optical parameters of embodiment of FIG. 1 Surface R [mm] d [mm] n ν 2 Infinity 0.50 1.77 72 3 Infinity 0.10 4 2.5 0.40 1.50 81 5 −0.57 0.22 6 −3.7 0.46 1.69 53 7 −0.47 0.05 8 2.6 0.40 1.90 21 9 0.97 0.53 10 Infinity 0.40 1.51 63 11 Infinity 0.05

TABLE-US-00002 TABLE 1b Surface parameters of embodiment of FIG. 1 Coefficient Surface 4 Surface 5 Surface 6 Surface 7 Surface 8 Surface 9 Conic −4.1E+01 −2.8E+00  −3.3E−01  −8.8E−01  −3.4E+00  −1.3E+00  Coefficient on r.sup.2  0.0E+00 0.0E+00 0.0E+00 0.0E+00 0.0E+00 0.0E+00 Coefficient on r.sup.4 −1.4E+00 5.1E+00 6.2E−01 −4.6E−01  −6.7E−01  −9.5E−01  Coefficient on r.sup.6  1.2E+01 −5.4E+01  7.6E+00 3.0E+00 8.7E−01 1.2E+00 Coefficient on r.sup.8 −1.5E+02 1.8E+02 7.2E+01 4.2E−01 −1.2E+00  −1.7E+00  Coefficient on r.sup.10 −5.4E+01 −6.6E−09  9.4E+01 7.3E−03 1.8E−02 1.2E+00 Coefficient on r.sup.12  2.2E+01 0.0E+00 −1.8E+02  0.0E+00 0.0E+00 0.0E+00

[0058] According to the second embodiment and as depicted in FIG. 2, the first lens 40 is, relating to respective inner portions of the optical surfaces 4, 5, a bi-convex lens having an almost plane object-side surface 4 and a convex image-side surface 5. The second lens 60 is a bi-concave lens. The third lens 80 is a positive meniscus lens, having its concave surface on the image side. Each of the lens surfaces 4, 6-9 has at least one turning point in surface inclination to the optical axis, as seen in an axial section. In particular, surface 7 has two turning points, and surfaces 8 and 9 each have three turning points. The optical parameters of the second embodiment are given in Tables 2a and 2b. The f-number of the second embodiment is 5.0, and the angle of view (2ω) is 81.5°.

TABLE-US-00003 TABLE 2a Optical parameters of embodiment of FIG. 2 Surface R [mm] d [mm] n ν 2 Infinity 0.50 1.77 72 3 Infinity 0.10 4 2.6 0.46 1.43 95 5 −0.42 0.20 6 −0.80 0.40 1.69 53 7 0.00 0.10 8 1.4 0.40 1.69 53 9 2.9 0.25 10 Infinity 0.40 1.51 63 11 Infinity 0.05

TABLE-US-00004 TABLE 2b Surface parameters of embodiment of FIG. 2 Coefficient Surface 4 Surface 5 Surface 6 Surface 7 Surface 8 Surface 9 Conic  1.6E+01 −3.2E−09 −8.8E−01 −5.6E+23 3.1E−01  5.2E+00 Coefficient on r.sup.2 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00  0.00E+00 Coefficient on r.sup.4 −2.1E+00  4.6E+00  3.9E+00  1.5E+00 6.2E+00  4.9E+00 Coefficient on r.sup.6  7.7E+01 −9.0E+01 −1.1E+02 −4.2E+00 −6.9E+01  −4.1E+01 Coefficient on r.sup.8 −5.3E+03  1.1E+03  1.6E+03 −2.7E+00 2.9E+02  1.3E+02 Coefficient on r.sup.10  1.3E+05 −9.0E+03 −1.3E+04  5.2E+00 −6.2E+02  −2.0E+02 Coefficient on r.sup.12 −1.6E+06  3.6E+04  4.9E+04  1.5E+01 7.1E+02  1.4E+02 Coefficient on r.sup.14  1.0E+07 −5.3E+04 −7.1E+04  1.2E+01 −4.1E+02  −4.1E+01 Coefficient on r.sup.16 −2.3E+07 −4.7E+00 −3.3E+02 −3.7E+01 9.6E+01 −2.2E−02

[0059] According to the third embodiment and as shown in FIG. 3, the first lens 40 is, relating to respective inner portions of the optical surfaces 4, 5, a bi-convex lens having an almost plane object-side surface 4 and a convex image-side surface 5. The second lens 60 is a positive meniscus lens, having its convex surface on the image side. The third lens 80 is a negative meniscus lens, having its concave surface on the image side. Each of the lens surfaces 4, 7-9 has at a turning point in surface inclination to the optical axis, as seen in an axial section. The optical parameters of the third embodiment are given in Tables 3a and 3b. The f-number of the third embodiment is 5.5, and the angle of view (2ω) is 79°.

TABLE-US-00005 TABLE 3a Optical parameters of embodiment of FIG. 3 Surface R d n ν 2 Infinity 0.50 1.77 72 3 Infinity 0.10 4 4.7 0.42 1.5 81 5 −0.75 2.32 6 −0.64 0.45 1.81 41 7 −0.83 0.49 8 8.6 0.70 1.88 35 9 0.43 0.35 10 Infinity 0.40 1.51 63 11 Infinity 0.05

TABLE-US-00006 TABLE 3b Surface parameters of embodiment of FIG. 3 Coefficient Surface 4 Surface 5 Surface 6 Surface 7 Surface 8 Surface 9 Conic −7.1E−03 −4.1E−06 2.8E−03 −1.4E−02 2.0E−03 −8.8E−01 Coefficient on r.sup.2 −7.1E−03 −4.1E−06 2.8E−03 −1.4E−02 2.0E−03 −8.8E−01 Coefficient on r.sup.4 −1.3E+00  1.2E+00 1.6E+00  9.8E−01 −6.9E−02  −1.6E−02 Coefficient on r.sup.6  8.3E+00 −7.6E+00 −4.9E+00  −7.9E−01 −1.4E−01   2.3E−02 Coefficient on r.sup.8 −1.0E+02  1.4E+01 5.2E+00  2.6E−01 1.0E−01 −7.4E−03

[0060] In each of the embodiments shown in FIGS. 1-3 the lens system 1 is adapted to a full-HD image sensor, for example an OV5670 image sensor having a 1920×1080 pixel array. The image circle diameter is, for example, about 2.5 mm. The object distance is between about 10 and 100 mm, for example about 15 mm or 70 mm.

[0061] As shown in FIG. 4, each of the lenses 40, 60, 80 has a functional rim 41, 61, 81, which may be integral with the respective lens or may be formed separate and cemented to the respective lens. Each of the functional rims has plane surfaces on the object side as well as on the image side, the plane surfaces extending perpendicular to the optical axis 12. Further, ring-shaped spacers 42, 62 are arranged between the respective functional rims 41, 61, 81. Moreover a further spacer 82 is provided between the glass plate 100 and the functional rim 81 of the third lens 80. The spacers 42, 62, 82 have plane surfaces on both sides and are cemented with their plane surfaces to the respective adjacent plane surfaces of the functional rims 41, 61, 81, and the glass plate 100. Moreover, a functional rim 22 of the cover glass 20 that may be an outer portion of the cover glass 20 is cemented to the object-side surface of the functional rim 41 of the first lens 10. The functional rims 41, 61, 81 and the spacers 42, 62 have a thickness to hold the cover glass 20 and the lenses 40, 60, 80 at respective distances as indicated in the Tables. The spacer 82 has a thickness to hold the glass plate 100 at a distance such that the image sensor 110 is in the focal plane of the lens system 1.

[0062] In FIG. 4 the lens system 1 itself is configured as the second embodiment. However, the other embodiments may comprise functional rims and may be mounted in a corresponding manner. The assembly 13 of lenses 40, 60, 80 and cover glass 20, including the respective functional rims 41, 61, 81 and spacers 42 and 62, and the glass plate 100, is part of or forms an endoscope objective 15. Due to the lenses having increasing diameters in the proximal direction, the lens assembly 13 shown in FIG. 4 has an overall frustoconical or frustopyramidal shape with the tip of the cone or the pyramid pointing in the distal direction. The lens assembly 13 may be enclosed in a casing 14, thereby forming an endoscope objective 15, which may be a hermetically sealed unit. This is shown in schematic, simplified manner in a perspective view in FIGS. 5 and 6.

[0063] In a first variation and as depicted in FIG. 5, the casing 14 has frustoconical shape, the cover glass 20 being located in the truncated end of the cone. According to another variation shown in FIG. 6, the casing has frustopyramidal shape, the cover glass 20 being arranged in the truncated end of the pyramid. In this case the cover glass 20, as well as the lenses 40, 60, 80, and/or the glass plate 100 may be quadratic, rectangular, or circular, for example; in the latter case the functional rims 41, 61, 81 may have a quadratic outline in a radial sectional view. At the base end of the cone or pyramid the casing 14 is mounted on the image sensor 110, or on a carrier or packaging of the image sensor 110. In this way a compact imaging unit may be formed that can easily be inserted and mounted in the distal end section of an endoscope shaft. In an alternative embodiment the endoscope objective 15 may have overall cylindrical shape (not shown).

[0064] According to an exemplary assembly method for a video endoscope objective 15, a first lens 40, a second lens 60 and a third lens 80 are provided, all of which are single lenses, i.e. none of the lenses 40, 60, 80 is a compound lens or a cemented doublet, triplet, or multiplet (see FIGS. 1-3). The first, second, and third lenses 40, 60, 80 consist of glass or of a crystalline material, such as sapphire, for example. The second lens 60 and the third lens 80 each have a refractive index n exceeding 1.66, and the first lens 40 has an Abbe number ν exceeding 80, as indicated for the exemplary embodiments in Tables 1a, 2a, and 3a. Each of the lenses 40, 60, 80 is aspherical, having rotationally symmetric aspherical surfaces on both sides, as given in an exemplary manner in Tables 1b, 2b, and 3b. The lenses 40, 60, 80 may have been made by forming lens blanks for example by molding, cutting or grinding, with the aspherical surfaces being embossed on the lens blanks.

[0065] Moreover, as shown in FIG. 4, each one of the lenses 40, 60, 80 has a functional rim 41, 61, 81 having plane surfaces on both sides, the plane surfaces being formed outside the optical surfaces, i.e. at a larger radial distance from an axis of symmetry of the respective lens 40, 60, 80. The plane surfaces of the functional rims 41, 61, 81 are perpendicular to the axis of symmetry of each respective lens 40, 60, 80. The diameter of the second lens 60 exceeds the diameter of the first lens 40, i.e. in particular the second lens 60 has optical surfaces 6, 7 both of which have larger diameters than the optical surfaces 4, 5 of the first lens 40, and the outer diameter of the functional rim 61 of the second lens 60 is larger than the outer diameter of the functional rim 41 of the first lens 40. In a similar manner, the diameter of the third lens 80 exceeds the diameter of the second lens 60, referring to the optical surfaces as well as to the functional rims 61, 81.

[0066] Moreover, a cover glass 20 and a glass plate 100 are provided, each having opposing parallel plane optical surfaces 2, 3, 10, 11. An aperture stop 21 is arranged on an image-side surface 3 of the cover glass 20. The aperture stop 21 may be formed by providing a coating on the image-side plane surface 3 of the cover glass 20, or by mounting a diaphragm on the image side of the cover glass 20, for example. The cover glass 20 may also have a functional rim 22, which is formed by an outer peripheral portion of the cover glass 20.

[0067] The functional rim 41 of the first lens is mounted on the functional rim 22 of the cover glass 20 by cementing the object-side surface of the functional rim 41 to the image-side surface 3 of the cover glass 20 or the aperture stop 21 mounted on the cover glass 20. A first ring-shaped spacer 42 made of glass or metal is cemented to the image-side surface of the functional rim 41 of the first lens 40, the second lens 60 is centered with respect to the first lens 40 such that the respective axes of symmetry of both lenses 40, 60 coincide, and the functional rim 61 of the second lens 60 is cemented to the image-side surface of the first spacer 42. Moreover, a second ring-shaped spacer 62 is cemented to the image-side surface of the functional rim 61 of the second lens 60, the third lens 80 is centered with respect to the second lens 60 such that the respective axes of symmetry of both lenses 60, 80 coincide, and the functional rim 81 of the third lens 80 is cemented to the image-side surface of the second spacer 62. A third ring-shaped spacer 82 is cemented to the image-side surface of the functional rim 81 of the third lens 80, and the glass plate 100 is cemented to the image-side surface of the third spacer 82. The functional rims 41, 61, 81 and the spacers 42, 62, 82 each have an axial thickness that is adapted to form an air gap between the respective lenses 40, 60, and 80, as required for high-quality imaging.

[0068] The lens assembly 13 formed in this way has, depending on the outer circumferential shape of the functional rims 41, 61, 81, the spacers 42, 62, 82, and the glass plate 100, the shape of a truncated cone or a truncated pyramid, or is machined into an overall frustoconical or frustopyramidal shape. The lens assembly 13 is inserted into a casing 14 of corresponding shape, forming an endoscope objective 15.

[0069] An electronic image sensor 110 including a micro-lens array fixed to a sensor area of the image sensor can be arranged on an image side of the assembly 13 or the objective 15. The third spacer 82 and the glass plate 100 have thicknesses to define an axial distance to the image sensor 110 such that the sensor plane of the image sensor 110 is arranged in the focal plane of the lens assembly 13 when the image sensor or the micro-lens array is mounted directly adjacent to the image side of the glass plate 100, or when the casing 14 is fixed to a surface of the image sensor 110, or to a carrier or packaging of the image sensor 110 (see FIGS. 5 and 6). Alternatively, a gap between the lens assembly 13 or the casing 14 and the image sensor 110 may be adjusted such that the image sensor is in the focal plane of the lens assembly 13, and the image sensor 110 fixed in the corresponding distance to the casing 14 or fixed in the shaft of the endoscope.

[0070] For clarity not all reference numerals are displayed in all figures. If a reference numeral is not explicitly mentioned in the description of a figure, it has the same meaning as in the other figures.

REFERENCE NUMERALS

[0071] 1 Lens system [0072] 2 Surface [0073] 3 Surface [0074] 4 Surface [0075] 5 Surface [0076] 6 Surface [0077] 7 Surface [0078] 8 Surface [0079] 9 Surface [0080] 10 Surface [0081] 11 Surface [0082] 12 Optical axis [0083] 13 Assembly [0084] 14 Casing [0085] 15 Endoscope objective [0086] 20 Cover glass [0087] 21 Aperture stop [0088] 22 Functional rim [0089] 40 First lens [0090] 41 Functional rim [0091] 42 Spacer [0092] 60 Second lens [0093] 61 Functional rim [0094] 62 Spacer [0095] 80 Third lens [0096] 81 Functional rim [0097] 82 Spacer [0098] 100 Glass plate [0099] 110 Image sensor