WO1995035522A1

WO1995035522A1 - Relay lens assembly and method of manufacture

Info

Publication number: WO1995035522A1
Application number: PCT/US1995/007798
Authority: WO
Inventors: John E. Kennedy; Yury Kazakevich.
Original assignee: Smith & Nephew Endoscopy Inc.
Priority date: 1994-06-20
Filing date: 1995-06-19
Publication date: 1995-12-28
Also published as: AU2867195A

Abstract

Each of a plurality of lens assemblies (22) for relaying an image along an axis (24) comprises a liquid rod lens (36a) disposed between a pair of solid lenses (32a, 32b) spaced from each other along the axis. An endoscope (10) includes a plurality of lens assemblies, each as described above, disposed within an elongated tube. A method for making a plurality of lens assemblies, each as described above, and disposing the assemblies in an elongated tube is also provided.

Description

RELAY LENS ASSEMBLY AND METHOD OF MANUFACTURE

This invention relates to lens assemblies, and in particular to lens assemblies for relaying images from the distal end to the proximal end of a relatively long optical instrument of relatively small cross-sectional area, such as an optical endoscope.

Optical endoscopes typically include a small-diameter tube within which a series of lens assemblies are arranged end-to-end at fixed, periodic intervals along the longitudinal tube axis. Each plurality of adjacent lens assemblies translating a real image of the object along the endoscope optical train is termed a "relay lens set." An objective lens assembly at the distal end of the endoscope forms an image of the viewing site, which is then relayed through each relay lens set in series. At the proximal end of the endoscope, a viewing assembly, such as an eyepiece lens assembly or a camera, receives the relayed image, enabling the user, such as a surgeon, to observe the viewing site either directly or as a televised image for, eg. diagnostic or therapeutic purposes.

In some optical endoscopes, the lens assemblies are singlets, cemented doublets and/or triplets, and the like. Endoscope lens assemblies often include one or more rod lenses, eg. as disclosed in U.S. Patent No. 3,257,902 to Hopkins, lenses having an axial thickness greater than their diameter. Rod lenses are generally arranged in alternating fashion with one or more thin lenses to form multiple-lens assemblies. In a general aspect of the invention, each of a plurality of lens assemblies for relaying an image along an axis comprises a liquid rod lens disposed between a pair of solid lenses spaced from each other along the axis.

Among other advantages, the liquid rod lens in each lens assembly intrinsically conforms to the shape of the solid lenses. Thus, the mating optical surfaces of adjacent rod and solid lenses need not be carefully matched, such as by precision grinding or molding. Further, adjacent lenses need not be bonded to one another, and the rod lens self-aligns with the two solid lenses. Because expensive matching, bonding, and aligning steps are unnecessary, the lens assembly may be manufactured in relatively high volume at relatively low cost.

Moreover, each lens assembly has five characteristic parameters, the length of the liquid rod lens and the curvatures of the four optical surfaces of the solid lenses, that may be tailored to reduce various of the aberrations that distort the relayed image.

In a preferred embodiment of the present invention there is provided apparatus comprising: a plurality of lens assemblies for relaying an image along an axis, each of said lens assemblies comprising; a pair of solid lenses spaced from each other along said axis, and a liquid rod lens disposed between said solid lenses.

Preferred embodiments include the following features.

In a particularly useful embodiment, a plurality of relay lens sets, each including a pair of adjacent lens assemblies symmetrical with respect one another about a plane of symmetry, are arranged along the axis. Images relayed through each relay lens set are thus intrinsically free of third-order coma, distortion, and lateral color aberrations. In each lens assembly, the solid lenses (comprised of, eg. a plastic or a glass) are secured to opposite ends of a sleeve, and the liquid rod lens (comprised of, eg. a siloxane composition or a gel) extends between the optical surfaces of the solid lenses. The optical surfaces may be concave, convex, or planar, depending on the desired optical properties of the lens assembly. If convex or concave, the optical surfaces may be spheric or aspheric.

In another aspect of the invention, each of a plurality of lens assemblies for relaying an image along an axis comprises a rod lens composed of a deformable material disposed between a pair of solid lenses spaced from each other along the axis.

Among other advantages, the optical surfaces of the rod lens match the mating optical surfaces of the solid lenses. In a particularly useful embodiment of this aspect of the invention, the deformable material comprises liquid, such as a siloxane composition or a gel.

In another aspect of the present invention, an endoscope includes a plurality of lens assemblies, each as described above, disposed within an elongated tube.

In a further preferred embodiment of the present invention there is provided an endoscope comprising: an elongated tube that extends along an axis between a distal end and a proximal end, a plurality of lens assemblies disposed in said tube for relaying an image from said distal end to said proximal end, each of said lens assemblies comprising; a pair of solid lenses spaced from each other along said axis, and a liquid rod lens disposed between said solid lenses.

Among other advantages, because individual lens assemblies may be manufactured in relatively high volume at relatively low cost, it may prove economic in some applications, particularly in the medical area, to dispose of the endoscope after a single use, thus avoiding the need for re-sterilization.

Yet another aspect of the invention involves making a plurality of lens assemblies, each as described above, and disposing the assemblies in an elongated tube.

Among other advantages, this aspect of the invention provides for the production of an optical instrument that includes lens assemblies having a liquid rod lens disposed between a pair of solid lenses.

In a particularly useful embodiment of this aspect of the invention, liquid is injected into a sleeve after the pair of solid lenses has been secured to opposite ends of the sleeve.

In another particularly useful embodiment of this aspect of the invention, a sleeve is substantially filled with liquid before at least one of the pair of solid lenses has been secured to the sleeve.

In another aspect of the present invention, a lens assembly for relaying an image along an axis comprises a liquid rod lens disposed between a pair of solid lenses spaced from each other along the axis, where the axial thickness of the liquid rod lens is substantially greater than its diameter. Among other advantages, the lens assembly exhibits reduced Petzval curvature.

Thus in another preferred aspect of the present invention there is provided a lens assembly for relaying an image along an axis, said lens assembly comprising: a pair of solid lenses spaced from each other along said axis, and a liquid rod lens disposed between said solid lenses, said liquid rod lens having an axial thickness substantially greater than its diameter.

In a further aspect of the present invention there is also provided a method of making an apparatus for relaying an image along an elongated tube that extends along an axis between a distal end and a proximal end, said method comprising: making a plurality of lens assemblies each of which includes a pair of solid lenses and a liquid rod lens disposed between said solid lenses, and disposing said plurality of lens assemblies in said tube.

Other features and advantages of the invention will become apparent from the following detailed description, and from the claims.

Fig. 1 shows an endoscope according to the invention;

Fig. 2 shows a relay lens set according to the invention;

Fig. 3 schematically shows light rays passing through the relay lens set of Fig. 2;

Fig. 4 shows a set of polychromatic diffraction modulation transfer function curves for the relay lens set of Fig. 2; Fig. 5 shows another set of polychromatic diffraction modulation transfer function curves for the relay lens set of Fig. 2;

Fig. 6 shows another set of polychromatic diffraction modulation transfer function curves for the relay lens set of Fig. 2;

Fig. 7 shows the polychromatic diffraction modulation transfer function curves of Figs. 4-6 superimposed on a single graph;

Fig. 8 shows field sag curves for the relay lens set of Fig. 2; and

Fig. 9 shows longitudinal spherical aberration curves for the relay lens set of Fig. 2.

Referring to Fig. 1 , an endoscope 10 suitable for observing a confined region, eg. the interior of a joint space or of a hollow human organ such as a stomach, includes a rigid tube 12. An objective lens assembly 14 is located at the distal end 16 of tube 12, and a viewing assembly 18 is located at the proximal end 20 of tube 12. Viewing assembly 18 may comprise, eg. an eyepiece lens assembly that enables the user, such as a surgeon, to view the relayed image of the surgical site directly. Alternatively, viewing assembly 18 may include, eg. imaging optics and a charge-coupled device or other type of camera unit that generates an electrical signal representative of the relayed image, allowing the user to view the image on a television-type monitor (not shown).

A series of relay lens sets 22 (only two of which are fully shown in Fig. 1 ), each of which is concentric with the longitudinal axis 24 of endoscope 10, are spaced at fixed, regular intervals within tube 12. In turn, tube 12 is disposed within tube 26. The gap between tube 12 and tube 26 contains an illumination fiber optic bundle (not shown) that delivers light from a light source (not shown) to distal tip 16 of endoscope 10. When the light source is activated, the fiber optic bundle illuminates a surgical site 28 near distal tip 16 of endoscope 10, an image of which is formed by objective lens assembly 14.

Referring also to Fig. 2, each relay lens set 22 is comprised of two lens assemblies 30a, 30b. Each lens assembly 30a, 30b is a lens "triplet," in that it includes three coaxial lenses: a liquid rod lens 36a, 36b extending between two solid lenses 32a, 34a; 32b, 34b. Characteristically, a rod lens has an axial thickness greater than its diameter. Each of solid lenses 32a, 34a; 32b, 34b has a large-diameter portion, which is slightly smaller than the inner diameter of tube 12, and a reduced diameter portion. The step transition between the two portions of each sold lens defines a circumferential shoulder. The ends of liquid rod lenses 36a, 36b conform intrinsically to the mating optical surfaces of the adjacent solid lenses 32a, 34a; 32b, 34b. Thus, each relay lens set 22 has eight characteristic curvatures, 38a, 40a, 42a, 44a, 44b, 42b, 40b, 38b, as provided by the two optical surfaces of each of the four solid lenses 32a, 34a; 32b, 34b. Liquid rod lenses 36a, 36b extend between optical surfaces 40a, 42a and 42b, 40b, respectively.

Solid lenses 32a, 34a; 32b, 34b of endoscope 10 are formed, eg. by molding or grinding, of polymethylmetacrylate (PMMA), which has a refractive index (n_d) of 1.4918 and a dispersion (V_d) of 57.2 for light having a wavelength of 0.588 microns. Liquid lenses 36a, 36b of endoscope 10 are composed of laser liquid code 1057 (Cargille liquid), a siloxane composition available from R.P. Cargille Laboratories, Inc., 55 Commerce Road, Cedar Grove, N.J. Code 1057 Cargille liquid has a refractive index of 1.5780 and a dispersion of 29.0 for light having a wavelength of 0.588 microns.

In each lens assembly 30a, 30b, a sleeve 54a, 54b maintains the desired relative orientations between solid lenses 32a, 34a; 32b, 34b. Likewise, the longitudinal spacing between lens assemblies 30a, 30b is maintained by a sleeve 56, and the longitudinal spacing between adjacent relay lens sets 22 is maintained by a sleeve 58.

Each sleeve 54a, 54b, 56, 58 is a short section of tubing (constructed from, eg. metal or plastic) having an outer diameter slightly smaller than the inner diameter of tube 12. The inner diameter of each of sleeves 54a, 54b, 56, 58, which determine the diameter of the optical train of endoscope 10, is slightly larger than the reduced-diameter portion of solid lenses 32a, 34a; 32b, 34b.

The reduced-diameter portions of solid lenses 32a, 34a are long enough to ensure that solid lenses 32a, 34a are coaxially aligned with sleeve 54a, and thus also each other, when the lenses are secured at opposite ends of sleeve 54a. Each solid lens 32a, 34a is inserted into sleeve 54a until the circumferential shoulder on each lens abuts the respective ends of sleeve 54a, fixing the relative longitudinal orientations of the two lenses. The relative radial and longitudinal orientations of solid lenses 32b, 34b are similarly maintained by sleeve 54b.

In each relay lens set 22, lens assemblies 30a, 30b are symmetrical with respect to one another about a plane of symmetry 60 that bisects sleeve 56. As such, relay lens set 22 is intrinsically free of third-order coma, distortion, and lateral color aberrations. At the point where it is bisected by plane of symmetry 60, the inner diameter of sleeve 56 serves as the aperture stop 62 of relay lens set 22.

Because lens assemblies 30a, 30b are symmetrical, solid lenses 32a, 34a are identical to, and oppositely disposed in tube 12 with respect to, solid lenses 32b, 34b, respectively. Accordingly, curvatures 38a, 40a, 42a, 44a are equal and opposite to curvatures 38b, 40b, 42b, 44b, respectively. Likewise, liquid rod lens 36a is identical to, and oppositely disposed with respect to, liquid rod lens 36b. Sleeve 54a is therefore identical to sleeve 54b. Thus, symmetric relay lens set 22 has five characteristic parameters: the thickness of liquid rod lenses 36a, 36b and the four unique curvatures of solid lenses 32a, 34a; 32b, 34b. For a given field of view, f-stop number, optical train diameter, and distance between surgical site 28 and objective lens assembly 14, these characteristic parameters are selected to provide relay lens set 22 with the desired optical properties.

Three of the five characteristic parameters of relay lens set 22 (the length of the liquid rod lens and two of the curvatures) are chosen to satisfy three first-order requirements. First, the paraxial image must be located symmetrically relative to the object. That is, the magnification of relay lens set 22 must be -1. Second, the paraxial entrance and exit pupils must be at infinite conjugates. That is, relay lens set 22 must be telecentric. Third, the rim ray height cannot exceed the outside diameter of the optical train anywhere in relay lens set 22. A fourth characteristic parameter of relay lens set 22, a lens curvature, is used to satisfy the conditions necessary to reduce axial chromatic aberration, provided the appropriate selection of materials for liquid rod lenses 36a, 36b, and solid lenses 32a, 32b, 34a, 34b.

After satisfying the above conditions and requirements, only a single unique lens curvature remains to reduce spherical aberration, astigmatism, and Petzval curvature. Typically, little can be done about Petzval curvature without introducing sizable high-order aberrations. However, satisfying the three first-order conditions generally results in rod lenses 36a, 36b having an axial thickness substantially greater than their diameter (i.e., the rod lens axial thickness is greater than about twice the lens diameter, and preferably is greater than about four or five times the diameter). Such an aspect ratio intrinsically yields reduced Petzval curvature.

To correct for the remaining two optical aberrations, spherical aberration and astigmatism, the fourth unique lens curvature is made aspheric, providing two design variables. In particular, providing optical surfaces 40a, 40b of solid lenses 32a, 32b with the proper aspheric curvature both reduces third order spherical aberration and introduces an overcorrected astigmatism into the optical train, which further offsets Petzval curvature. These aspheric surfaces define the solid lens-liquid lens interfaces located farthest from aperture stop 62. Because of the deformable nature of the liquid of which they are comprised, the mating optical surfaces of rod lenses 36a, 36b inherently conform to match the aspheric curvatures of surfaces 40a, 40b of solid lenses 32a, 32b.

Note that although spherical aberration may instead be corrected by making the optical surfaces closest to aperture stop 62 aspheric (i.e., curvatures 44a, 44b of solid lenses 34a, 34b), it then becomes difficult to introduce a positive astigmatism into the optical train.

In addition to the five characteristic parameters of symmetric relay lens set 22, the length of the air gap separating lens assemblies 30a, 30b (i.e., the gap provided by sleeve 56) and the length of the air gap separating adjacent relay lens sets 22 (i.e., the gap provided by sleeve 58) also affect the optical characteristics of endoscope 10.

In particular, increasing the air gap separating lens assemblies 30a, 30b decreases Petzval curvature, but increases fifth-order astigmatism. Moreover, because light diverges in the air gap, increasing gap size increases also the aggregate light loss through each relay lens set 22. On balance, ideally this air gap is small. However, the smaller the air gap, the shorter sleeve 56 must be, and, accordingly, the more difficult it becomes to install sleeve 56 into tube 12 (a method of assembling endoscope 10 is described in further detail below). Thus, and because slightly increasing the air gap separating lens assemblies 30a, 30b results in only moderately diminished endoscope performance, practical considerations favor making sleeve 56 large enough for easy handling and installation.

Concerns regarding light divergence also militate in favor of decreasing the air gap separating adjacent relay lens sets 22. As with sleeve 56, however, decreasing this air gap makes sleeve 58 more difficult to handle during assembly. Further, because the real image is formed in this air gap, scratches, dust, and other imperfections on the lens surfaces become increasingly visible in the relayed image as the gap size decreases. As with the air gap separating lens assemblies 30a, 30b, these practical considerations must be taken into account when determining the size of sleeve 58.

The specifications of a relay lens set 22 constructed in accordance with the above-described conditions and parameters are listed in Table I below. The f-stop number of the relay lens set is 5.000, the diameter of aperture stop 62 (and hence also the diameter of the optical train) is 2.0 mm, and the diameter of the object image formed by objective lens assembly 14 is 1.1 mm. The signs of the radii listed in Table I are consistent with convention: The sign is positive if the center of the curvature is to the right of the surface, and negative if the center is to the left of the surface. 'Thickness" refers to the axial thickness between the surface and next surface to the right. "Medium" refers to the type of medium separating the surface from the next surface to the right.

TABLE I

Surface Radius Thickness Medium N_ v_ NOTE (mm) (mm

38a 8.982 3.000 PMMA 1.4918 57.2

40a 1.658 10.117 Liquid 1.578 29.0 Aspheric* (k=-1.663)

42a 3.870 3.000 PMMA 1.4918 57.2

44a -5.608 0.5 Air

62 0.5 Air Aperture Stop

44b 5.608 3.000 PMMA 1.4918 57.2

42b -3.870 10.117 Liquid 1.578 29.0

40b -1.658 3.000 PMMA 1.4918 57.2 Aspheric* (k=-1.663)

38b -8.982 2.000 Air ^"Optical surfaces 40a, 40b are rotationally symmetrical hyperboloids. The shape of the surfaces is described by the following equation:

where

-f_* '+/

and where c is the surface curvature (i.e., the reciprocal of the radius listed in Table I); x, y, z are the cartesian coordinates, where z coincides with optic axis 24; and k is the conic constant listed in Table I.

Note from Table I that the axial thickness of rod lenses 36a,

36b is substantially greater than the diameter of the rod lenses, by approximately a factor of five.

Fig. 3 is an optical schematic view depicting light rays passing through relay lens set 22 constructed in accordance with Table I. As can be seen from Fig. 3, in accordance with the first-order conditions, the magnification of relay lens set 22 is -1. Typically, objective lens assembly 14 also inverts the image. If viewing assembly 18 is an eyepiece lens assembly through which the user views the relayed image directly, it is typically desirable to have an odd number of relay lens sets 22 in endoscope 10, in order that the image perceived by the user will be properly oriented. If viewing assembly 18 instead includes a camera, it is generally immaterial whether there are an even or an odd number of relay lens sets 22, as the camera can be rotated to display the image in the desired orientation.

Figs. 4-6 show the analytic polychromatic diffraction modulation transfer function (MTF) curves, at fractional object heights of 0.0, 0.7, and 1.0, respectively, of relay lens set 22 constructed in accordance with Table I. The tangential, sagittal, and ideal MTF curves reflect the performance of relay lens set 22 for wavelengths of 0.588, 0.656, and 0.486 microns, where each wavelength is weighted equally. Fig. 7 shows the curves of Figs. 4- 6 superimposed on a single graph.

Figs. 8 and 9 show field sag and longitudinal spherical aberration, respectively, for relay lens set 22 constructed in accordance with Table 1.

The endoscope shown in Fig. 1 is constructed using a suitable number of pre-manufactured lens assemblies 30a, 30b. The manufacture of a single lens assembly involves coating the entire outer surface of the reduced-diameter region of solid lens 32a with a sealing adhesive, eg. epoxy, and inserting the reduced-diameter region completely into one end of sleeve 54a. The sealing adhesive cures to form a liquid-tight seal between lens 32a and sleeve 54a. After orienting the lens-sleeve assembly vertically, with the open end of sleeve 54a facing up, the interior of sleeve 54a is then completely filled with Cargille liquid. Although sleeve 54a has a small hole 64a extending completely through the sleeve wall near the midpoint of the sleeve, the diameter of the hole is small enough that only minute amounts of liquid leak out hole 64a during this step of the manufacturing process. The entire outer surface of the reduced-diameter region of solid lens 34a is then coated with a sealing adhesive and inserted . into the open end of sleeve 54a. As solid lens 34a is inserted into sleeve 54a, the assembly is turned horizontally to orient hole 64a vertically, allowing all air and excess Cargille liquid to be forced out hole 64a. Hole 64a is then sealed using a sealant, or by wrapping lens assembly 30a with tape (for clarity, holes 64a, 64b are shown enlarged and unsealed in Fig. 2). Of course, solid lenses 32a, 34a may be attached to sleeve 54a in any order, as long as sleeve 54a is filled with liquid after the first solid lens is attached.

Alternatively, sleeves 54a, 54b may be provided with two holes (the second hole is not shown) through the sleeve wall. After both solid lenses are secured at either end of the sleeve in the manner described above, liquid is introduced, eg. with a syringe, into the empty sleeve interior through one of the holes. The second hole serves as a vent, allowing air to escape as it is displaced by the entering Cargille liquid.

Because relay lens set 22 is symmetric, lens assembly 30b is identical to lens assembly 30a; only their respective orientations within tube 12 differ.

To assemble endoscope 10, objective lens assembly 14 is first attached to distal end 16 of tube 12 in a conventional manner. A sleeve 66, the length of which is determined by the focal lengths of objective lens assembly 14 and lens assembly 30a, is then inserted into the interior of tube 12 until it seats against the proximal end of objective lens assembly 14. The rest of the optical train of endoscope 10 is then assembled, distally to proximally, by adding the requisite number of relay lens sets 22. For each set, lens assembly 30a is first inserted into tube 12 (with solid lens 32a at the distal end), followed in turn by sleeve 56 and lens assembly 30b (with solid lens 34b at the distal end). Sleeve 58 is inserted between adjacent relay lens sets.

At the proximal end of endoscope 10, viewing assembly 18 is attached in the normal manner. When the optical train is completely assembled, tube 12 is inserted into tube 26, along with the fiber optic bundle.

Among other things, because of the relatively low cost of manufacturing individual lens assemblies 30a, 30b and the relative ease of assembly, endoscope 10 can be produced inexpensively. Thus, it may prove economic in some applications, particularly in the medical area, to dispose of the endoscope 10 after a single use, thus avoiding the need to re-sterilize the endoscope.

Other embodiments are within the scope of the following claims.

For example, solid lenses 32a, 34a; 32b, 34b need not be composed of PMMA, and may be comprised of, eg. any suitable plastic or glass. All of the solid lenses in the lens assembly or assemblies need not be constructed of the same material. Similarly, liquid lenses 36a, 36b may be comprised of any suitable liquid, eg. a very viscous liquid such as a gel, or any other suitable deformable material that conforms intrinsically to the shape of a bounding solid surface. All of the liquid lenses in the plural lens assemblies need not be constructed of the same material. In general, the materials used for the solid and liquid lenses are selected so that their respective spectral curves of refractive indices provide the lens assembly with the desired optical properties.

Further, the optical surfaces of the solid lenses need not have the curvatures described herein and shown in the attached figures. Either or both of the optical surfaces of any of the solid lenses in the endoscope may be concave, convex, or planar. If concave or convex, the optical surfaces may be either spheric or aspheric. Moreover, the solid lenses need not be the conventional glass or plastic solid lenses described herein. Rather, any solid optical elements, such as gradient index (GRIN) lenses or binary or holographic diffractive optical media, can alternatively or additionally be employed. The solid lenses in the lens assembly or assemblies need ot all be of the same type.

In addition, a single lens assembly may have a plurality of liquid rod lenses, each extending between the optical surfaces of an adjacent pair of solid lenses.

While the invention has been described in terms of an endoscope for viewing the interior of the human body, a plurality of lens assemblies in accordance with the invention may also be used in the optical train of other types of optical equipment, such as telescopes, periscopes, and cameras.

Claims

1. Apparatus comprising: a plurality of lens assemblies for relaying an image along an axis, each of said lens assemblies comprising: a pair of solid lenses spaced from each other along said axis, and a liquid rod lens disposed between said solid lenses.

2. The apparatus of claim 1 wherein a pair of adjacently disposed ones of said lens assemblies are symmetrical with respect to each other about a plane of symmetry, said pair of lens assemblies defining a relay lens set.

3. The apparatus of claim 2 comprising a plurality of said relay lens sets arranged along said axis.

4. The apparatus of claim 1 wherein said liquid rod lens extends between a pair of optical surfaces of said solid lenses.

5. The apparatus of claim 4 wherein said pair of optical surfaces are configured so that said liquid rod lens has a convex optical surface and a concave optical surface at opposite ends of said liquid rod lens.

6. The apparatus of claim 4 wherein at least one of said optical surfaces has a spheric curvature.

7. The apparatus of claim 4 wherein at least one of said optical surfaces has an aspheric curvature.

8. The apparatus of claim 7 wherein a pair of adjacently disposed ones of said lens assemblies are symmetrical with respect to each other about a plane of symmetry, wherein in each of said pair of lens assemblies said optical surface disposed farthest from said plane of symmetry has an aspheric curvature.

9. The apparatus of claim 1 further comprising an elongated sleeve that encloses said liquid rod lens, said pair of solid lenses being secured to opposite ends of said sleeve.

10. The apparatus of claim 9 wherein each one of said pair of solid lenses includes a region that fits within one of said ends of said sleeve and a region that abuts said end of said sleeve.

11. The apparatus of claim 1 wherein said liquid comprises a siloxane composition.

12. The apparatus of claim 1 wherein said liquid comprises a gel.

13. The apparatus of claim 1 wherein at least one of said pair of solid lenses is comprised of plastic.

14. The apparatus of claim 13 wherein said plastic is polymethylmetacrylate.

15. The apparatus of claim 1 wherein at least one of said pair of solid lenses is comprised of glass.

16. Apparatus comprising: a plurality of lens assemblies for relaying an image along an axis, each of said lens assemblies comprising: a pair of solid lenses spaced from each other along said axis, and a rod lens composed of a deformable material disposed between said solid lenses.

17. The apparatus of claim 16 wherein said deformable material comprises liquid.

18. The apparatus of claim 17 wherein said liquid comprises a siloxane composition.

19. The apparatus of claim 17 wherein said liquid comprises a gel.

20. An endoscope comprising: an elongated tube that extends along an axis between a distal end and a proximal end, a plurality of lens assemblies disposed in said tube for relaying an image from said distal end to said proximal end, each of said lens assemblies comprising: a pair of solid lenses spaced from each other along said axis, and a liquid rod lens disposed between said solid lenses.

21. The endoscope of claim 20 wherein a pair of adjacently disposed ones of said lens assemblies are symmetrical v/ith respect to each other about a plane of symmetry, said pair of lens assemblies defining a relay lens set.

22. The endoscope of claim 21 further comprising an aperture member disposed in said tube between said pair of lens assemblies.

23. The endoscope of claim 21 comprising a plurality of said relay lens sets disposed in said tube.

24. The endoscope of claim 23 wherein adjacent ones of said relay lens sets are spaced from each other by a selected amount in said tube.

25. The endoscope of claim 23 wherein each one of said lens assemblies includes an elongated sleeve, said pair of solid lenses being secured to opposite ends of said sleeve, said liquid rod lens being disposed within said sleeve and extending between a pair of optical surfaces of said solid lenses.

26. The endoscope of claim 25 wherein at least one of said optical surfaces has a spheric curvature.

27. The endoscope of claim 25 wherein at least one of said optical surfaces has an aspheric curvature.

28. The endoscope of claim 27 wherein in each of said lens assemblies, said optical surface disposed farthest from said plane of symmetry has an aspheric curvature.

29. A method of making an apparatus for relaying an image along an elongated tube that extends along an axis between a distal end and a proximal end, said method comprising: making a plurality of lens assemblies each of which includes a pair of solid lenses and a liquid rod lens disposed between said solid lenses, and disposing said plurality of lens assemblies in said tube.

30. The method of claim 29 wherein said step of making said lens assemblies includes providing an elongated sleeve, securing said pair of solid lenses to opposite ends of said sleeve, and disposing said liquid rod lens therebetween.

31. The method of claim 30 wherein said step of disposing said liquid rod lens between said pair of solid lenses includes injecting liquid into an interior volume of said sleeve through said sleeve after securing said pair of solid lenses to said sleeve.

32. The method of claim 30 wherein said step of disposing said liquid rod lens between said pair of solid lenses includes substantially filling an interior volume of said sleeve prior to securing at least one of said pair of solid lenses to said sleeve.

33. The method of claim 30 further comprising constructing each one said lens assemblies so that said liquid rod lens extends between a pair of optical surfaces of said solid lenses.

34. The method of claim 30 further comprising configuring at least one of said optical surfaces to have an aspheric curvature.

35. The method of claim 30 further comprising configuring at least one of said optical surfaces to have a spheric curvature.

36. The method of claim 30 further comprising: configuring a pair said lens assemblies to be symmetrical with respect to each other about a plane of symmetry, said pair of lens assemblies defining a relay lens set, and disposing said pair of lens assemblies adjacent to each other in said tube.

37. The method of claim 36 further comprising disposing a plurality of said relay lens sets in said tube.

38. The method of claim 37 further comprising spacing said plurality of said relay lens sets from each other by a selected amount.

39. A lens assembly for relaying an image along an axis, said lens assembly comprising: a pair of solid lenses spaced from each other along said axis, and a liquid rod lens disposed between said solid lenses, said liquid rod lens having an axial thickness substantially greater than its diameter.