OBJECTIVE LENS SYSTEM

FIELD OF THE INVENTION

This invention relates to an objective lens system for an endoscope.

BACKGROUND OF THE INVENTION

There are known various endoscopes employing in their frontal, insertion part optical heads for viewing the interior of the body cavity or lumen (hereinafter "the object"). Such optical head normally includes at least an illumination means for illuminating the object, an objective lens system and a solid state image sensor. Most the endoscopes make use of small-sized image sensors such as CCD/CMOS sensors (hereinafter referred to as CCD. Figs. IA and IB illustrate one such CCD known in the art, based on on-chip microlens technology. The CCD consists of an array of microlenses 11 and a light sensitive surface having a plurality of pixels 13, each microlens being associated with one such pixel. The number of pixels defines the resolution of the resulting image and the information the image will contain. As shown in Fig. IB, the sensor is covered by a photo-shielding film 15, so that the light energy is concentrated in the center of each pixel. This improves the signal-to- noise ratio and increases the light utilization efficiency. However, this also causes the CCD to be sensitive to incident angles between the light rays which have passed the microlens array 11 and the optical axis of the system. As shown in Fig. IB light rays 17, which have small incident angles, pass through the photo-shielding film 15 to the center of the pixel 13. In the contrary, light rays 17' having relatively large incident angles will not reach the pixel 13, leading to significant energy losses. The losses are maximized at the edges of the field of view, i.e. for light rays having incident angles close to that of the chief ray. US 6,956,703 discloses an objective lens for endoscopes comprising a front lens unit component and a rear lens unit component, between which a stop is located,

wherein a front lens unit component comprises, in order from the object side, a first lens having a negative refractive power and a second lens having a positive refractive power which directs a surface of the small radius of curvature toward the object side; wherein a rear lens unit component comprises a third lens having a positive refractive power which directs a surface of the small radius of curvature toward the image side, a fourth lens having a positive refractive power and a fifth lens having a negative refractive power; and wherein the fourth lens and the fifth lens are cemented, and satisfying the following condition: 2.0<]f3/f|<3.0 where f is the composite focal length of the total system and f3 is the focal length of the third lens. The system disclosed above may appear less effective in terms of homogeneity of the image brightness when used with the modern CCD sensors provided with the microlens array.

The object of the present invention is, therefore, to provide a system that will make use of modern CCD sensors, while being less sensitive to the incident angles and at the same time will keep the size of the optical head as small as possible.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is provided an objective lens system for an endoscope, adapted to form image of an object in the lens system's image plane, the lens system comprising: - a front and a rear sub-system and an aperture stop therebetween; and, in the rear sub-system with a compound achromatic component and a rear positive lens disposed between said component and the image plane of the system in close proximity to the image plane, said rear positive lens satisfying the following condition: f _R > 4f, where f is the composite focal length of the total system and f _R is the focal length of the rear positive lens.

- The system may comprise an additional rear positive lens in the rear sub-system between said aperture and said component, wherein said rear positive lens constitutes a first rear positive lens and said additional rear positive lens constitutes a second rear positive lens.

- The use of the rear lens (hereinafter L _R ) with positive power before the image plane allows to essentially reduce the incident angle of the chief ray which comes out of the rear lens and reaches the image plane, thereby allowing the system to be used with sensors highly sensitive to the angle of incidence of light incident thereon, to prevent undue light losses. Due to the very close disposition of L _R to the image plane of the system, and due to the relatively long focal length as defined above, the above is achieved without influencing the total focal distance of the system and, consequently, without essentially increasing the length of the total optical track of the system, and also without the deterioration of the image quality. L _R may have planar surface facing towards the image plane. In practice the thickness d _R of L _R may be selected to be no more than 60% of the focal length of the total system. In other words, the thickness is selected so that the length of the optical track of the system is not increased and so the focal length of the total system remains essentially the same.

L _R may further be adapted for immediate attaching to an image sensor of the endoscope, in which case it will function also as a protection cover for the sensor. In practice, for the sensor which is a CCD comprising a microlens array, L _R may be adapted for attaching to the microlens array. In particular, with the microlens array having a front side and a rear side, the planar surface of L _R may be adapted for attaching to the front side of the microlens array.

According to one embodiment of the system, comprising the the first rear lens and the second rear lens the focal length f _R of the rear lens, which is in vicinity of the sensor would be satisfying the following condition: 1.5<f _R /f<2.

According to another embodiment of the system, this focal lens would be satisfying the following condition: 3<f _R /f<3.5.

The expression f _R /f, mentioned above, may vary according to different parameters of the lenses of the system, i.e. radiuses of curvature, thicknesses, glass materials and diameters, and positions of the lenses. In any case, the parameters of the lenses are chosen to ensure optimal aberration correction.

The system may further comprise a meniscus lens located closer to the object than the front sub-system, oriented with its concave surface facing the object and preferably

- A - this meniscus lens may have a diameter essentially greater than the largest dimension of the rear sub-system in the direction perpendicular to the optical axis (this largest diameter would normally be the one of LR). The meniscus geometry of the lens, especially when its opposite surfaces are selected to be approximately concentric and when their center of curvature is located near the center of the aperture stop results in production of a more homogeneous image in terms of homogeneity of the image brightness, i.e. the image having brightness at the image periphery approaching that at its center.,

The meniscus geometry of the lens also reduces the losses of chief incident rays entering the system

In addition, the meniscus lens improves the correction of the field aberrations.

The meniscus lens also prevents direct contact of the system with the object to be viewed and thus protects the lens system from being contaminated. The meniscus lens may be disposable, and as such, may be adapted for being mounted in an aperture of a disposable cap of the endoscope, which cap is mounted on the optical head of the endoscope and includes the housing with the front and rear sub-systems of the objective lens system mounted therein as will be described later on.

In accordance with another aspect of the present invention, there is provided an objective lens system for an endoscope, adapted to form image of an object in the lens system's image plane, the lens system comprising:

- a front and a rear sub-system and an aperture stop therebetween; and, in order from the object side,

- a meniscus lens with any one of the features thereof described above, located closer to the object than the front sub-system; - in the front sub-system, a first front negative lens and a second front positive lens; and

- in the rear sub-system, a first rear positive lens, a compound achromatic component including a second rear positive lens and a third rear negative lens. In accordance with a still further aspect of the present invention, there is provided an optical head for an endoscope comprising an objective lens system adapted to form image of an object in the lens system's image plane, a image sensor having a microlens

array, and said optical head being covered by a disposable cap having an opening, the objective lens system comprising:

- a front and a rear sub-system and an aperture stop therebetween; and, in order from the object side, - a disposable meniscus lens mounted within said opening;

- in the front sub-system, a first front negative lens and a second front positive lens; and

- in the rear sub-system, a compound achromatic component and a rear positive lens assembled with said image sensor and constituting a protective cover therefor.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Figs. IA and IB illustrate CCD sensor known in prior art and provided with microlens array.

Figs. 2A and 2B illustrate an isometric view of an optical head without and with a disposable cap, respectively, according to the present invention;

Fig. 3 is a cross section of the optical head shown in Fig. 2A; Figs. 4 and 5 schematically illustrate two embodiments of objective lens system according to the present invention.

Figs. 6A and 6B graphs illustrating improvement of homogeneity of image brightness by virtue of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 2A schematically illustrates an optical head 21 according to the present invention, for use with an endoscope, which, in this particular non-limiting example, is a gastroscope. The optical head 21 comprises a body member 21a and a front member 21b. The body member comprises a frontal end 23, a middle section 25 and a rear end 27. The body member 21a further comprises a U-like depression U, extending through the frontal end 23 and the middle section 25. The U-like depression provides an exit for a working channel extending along an insertion tube (not shown). The body member

21a is responsible for retaining at least a part of components of the optical head 21, as will be described below. The front member 21b comprises an aperture 29 for an objective lens system 31, described herein below in detail, and two recesses 22 for mounting LED units therein. Fig. 2B schematically illustrates optical head with a disposable cap 24 detachably connected thereto. The cap 24 is a tubular housing comprising a rear open end 24a and a frontal end 24b. The interior of the cap 24 is configured and dimensioned to accommodate therein an optical head 21. The frontal end 24b has a frontal surface 26 comprising an opening 20 for passing therethrough of a surgical tool (not shown) when the tool is protracted distally and retracted proximally through the insertion tube. The frontal surface 26 further comprises an open region 70, covered by a window 32, as will be further described. There are provided also on the frontal end 24b a couple of oppositely situated guiding protrusions 80 configured to enable sliding attachment of a shield (not shown), which would direct a washing water jet from an irrigation channel (not shown) on the window 32.

Fig. 3 shows the interior of the optical head 21 covered by the cap 24. The main component retained within the optical head 21 is an optical system 30. The optical system 30 comprises the objective lens system 31 having a plurality of lenses 3 Ia to 3 If (indicated by letter L in figures 4 and 5) and an aperture stop S, and a sensor 33 electrically connected to a printed circuit board 35. The window in front of the objective lens system 31, may have meniscus geometry and thus to function as a meniscus lens M in addition to other lenses of the objective lens system 31. The lenses 31a to 3 Ie are situated within a metallic barrel 37 and secured therein for example by gluing. The lens 3 If is situated in a proximity to the sensor 33 and may be attached immediately thereto. A space D between the lenses 3 Ie and 3 If may be an empty space or may be preferably partially filled with glass or other optical non scattering medium. Also, filters may be placed within the space D. The sensor further comprises a grounding 39 connected by a cable (not shown) through which electrical signals are passed from the sensor 33 to a central control system unit of the endoscope. The sensor is supported by lateral supports 81.

Figs. 4 and 5 illustrate two examples of the optical system 30 according to the present invention. This examples of the optical system comprise respectively objective lens systems 41 and 51. The sensor used in the optical systems shown in Figs. 4 and 5 is

a CCD sensor. It should be borne in mind however that other types of solid state image sensor could be used instead of the CCD sensor.

The objective lens system 41,51 has an optical axis A and comprises a front optical sub-system 41a,51a, a rear optical sub-system 41b, 51b and an aperture stop S ₁₅ S ₂ therebetween. The system 41,51 further comprises a meniscus lens Mi,M ₂ located between the front sub-system 41a,51a and the object O. As previously explained, as meniscus lens M ₁₅ M ₂ can serve window 32 covering region 70 in the front end of the disposable cap.

The meniscus lens M ₁₅ M ₂ is oriented with its concave surface facing the object O and preferably having a diameter essentially greater than the largest dimension of the rear sub-system in the direction perpendicular to the optical axis.

The front sub-system 41a,51a comprises lenses L _F15 LV ₁ having a negative power, and L _F25 L ¹ _F2 , having a positive power, and the rear sub-system 41b,51b comprises lenses L _R15 L' _RI to L _R45 L R ₄ , wherein L _R1 ,L _R1 having a positive power, L _R25 U _R2 having a positive power, LR ₃ ,L' _R3 having a negative power and L _R45 L ¹ R ₄ having a positive power. The lenses L _R2 and L _R3 of the rear sub-system 41b and the lenses L ¹ _R2 and L ¹ _R3 of the rear sub-system 51b are cemented to compose an achromatic component.

The lens L _R25 L ¹ _R2 may be biconvex with radius of curvature of its front surface being smaller than radius of curvature of its rear surface, as indicated in Tables T ₁ ,T ₂ below. The lens L _R3 has a concave rear surface, while the lens L ¹ R ₃ has a convex rear surface.

The lens LR ₄₅ L R ₄ satisfies the following condition: f _R 4≥4f, where f is the composite focal length of the total system 41,51 and f _R4 is the focal length of the lens

L _R45 L ¹ _R4 . Tables T ₁ and T ₂ summarize the data of the objective lens systems 41 and 51, respectively, as follows:

R - radius of curvature* in mm;

Thick - thickness in mm;

Glass - lens glass type;

S-D - semi-diameter of the lens front surface in mm; f- effective focal length of the lens in mm;

* Radius of curvature equal to infinity is interpreted as planar.

Table T ₁

The lens L _R1 of the objective lens systems 41 has a focal length fm satisfying the following condition: 1.5<fiu/f<2, where f is the composite focal length of the total system. Particularly, for the data indicated in Table Ti the condition fiu=1.74f is satisfied and f = 1.338. Table T ₂

The lens L ¹ _R1 of the objective lens systems 51 has a focal length fju satisfying the following condition: 3 <f _R i/f<3.5. Particularly, for the data indicated in Table T ₂ the condition f _R i=3.25f is satisfied and f=1.389.

The lenses are further coated with an anti-reflection coating (AR coating as known in the art) for further improving the efficiency of the optical system 30.

Figs. 4 and 5 show schematically the propagation of four incident rays of light

R ₁ to R ₄ through the objective lens system 41,51 till the creating of an image of the object at an image plane P. The incident rays R ₁ to R ₄ , shown in Figs. 4 and 5 have angles of incidence αi to O ₄ , respectively, which are essentially equal to the following values: cti=O°, α ₂ =50°, α ₃ =60° and 0^=75°.

The use of the rear positive lens LR ₄₅ L ¹ R ₄ allows to essentially reduce the incident angles (βr β ₄ ) of the rays entering the microlenses 42, thereby allowing the optical system to be used with CCD highly sensitive to the angle of incidence of light incident thereon, and therefore to prevent undue light losses. The lens LR ₄₅ L R ₄ when attached to the microlens array 42, therefore will further function as a protection cover for CCD.

The meniscus lens Mi,M ₂ also makes a certain contribution to reduce the incident angles of rays entering therein. In practice is is advantageous to provide the meniscus lens with such a geometry that its opposite surfaces are concentric and their centres of curvature are located in vicinity of the aperture stop. In addition, the meniscus lens improves the correction of the field aberrations. Further, the meniscus lens M ₁₅ M ₂ also provides physical protection to the objective lens system 41,51.

With reference again to Fig. 3, the optical system 30 may be assembled by the following method comprising the steps of: - attaching the lens 3 If to the sensor 33, e.g. by gluing,; insertion of the elements of the frontal optical sub-system and the aperture stop S into the barrel 37 and securing them in place; insertion of the elements of the rear optical sub-system into the barrel 37 and securing them in place; - adjustment of the optical system location with respect to the image plane of the sensor and - enclosing the optical system 30 and sensor in a common housing.

Tables T ₃ and T ₄ summarize the improvement of image quality achieved by the optical system 30 in terms of transmission for the chief ray. As seen in the Tables and corresponding Figs. 6A and 6B, the transmission of the system 30 described above, whether without an AR coating (table T ₃ ) or with it (table T ₄ ), for the chief ray, is essentially greater than it would be if instead of meniscus lens M and rear positive lens 3 If the system 30 included merely corresponding plane-parallel transparent plates (columns 3 and 4 in the respective tables). In the corresponding Figs. 6A and 6B curves 1 and 2 are shown. The curves correspond to columns 1 and 2 in the above tables, whilst curve 3 corresponds to columns 4. Numbers of positions in the tables correspond to those in Figs. 6A and 6B Table T ₃

Table T ₄