Lens assembly for image transmission, rigid endoscope, and image transmission system

Abstract

Provided is a lens assembly for image transmission with which it is possible to excellently perform image transmission. A lens assembly (8A) for image transmission comprises: a plastic optical fiber (1) for image transmission, the plastic optical fiber (1) being composed of a transparent light guide part having a refractive index distributed in a convex curve shape that is larger toward the center with respect to the radial direction, and an uneven part that is formed on the surface of the light guide part and absorbs light that has reached the surface from the interior of the light guide part; a protective pipe (2) that covers the outer circumferential surface of the plastic optical fiber for image transmission; and an outer shell pipe (3) that accommodates therein from the proximal end to the distal end of the plastic optical fiber for image transmission covered by the protective pipe.

Description

Image transmission lens assembly, rigid endoscope, and image transmission system

[0001] The present invention relates to an image transmission lens assembly, a rigid endoscope, and an image transmission system.

[0002] Plastic optical fiber (hereinafter referred to as POF), a representative plastic optical material, uses plastic as a raw material and therefore has advantages such as low cost, light weight, and excellent processability compared to silica-based optical fiber. Various methods for manufacturing POF have been known. For example, Patent Document 1 discloses a POF in which two cladding layers, an inner cladding and an outer cladding, are provided around a core in order to reduce bending loss due to bending.

[0003] Furthermore, research is being conducted into a technology for using a POF as a relay lens for image transmission by controlling the refractive index distribution of the core serving as an optical transmission path with high precision.

[0004] Japanese Patent Application Publication No. 2007-163910

[0005] Research into controlling and adjusting the refractive index profile of POF to reduce transmission loss has been ongoing, and the inventor discovered a manufacturing technique for a plastic optical fiber containing a core with a refractive index profile suitable for image transmission. He then proceeded with the development of various image transmission devices equipped with this plastic optical fiber, and completed the present invention.

[0006] An object of one aspect of the present invention is to provide an image transmission lens assembly, a rigid endoscope, and an image transmission system that include a plastic optical fiber that is capable of performing good image transmission.

[0007] In order to solve the above problems, an image transmission lens assembly according to one embodiment of the present invention comprises: an image transmission plastic optical fiber comprising a transparent light-guiding section having a refractive index distributed in a convex curve that is larger radially toward the center, and an uneven section formed on the surface of the light-guiding section to absorb light that reaches the surface from the inside of the light-guiding section; a protective pipe that covers the outer surface of the image transmission plastic optical fiber, the protective pipe being made of a moisture-proof material; and an outer shell pipe that houses the image transmission plastic optical fiber covered by the protective pipe from the proximal end to the distal end.

[0008] In order to solve the above problems, a rigid endoscope according to one aspect of the present invention is provided with the above-described image-transmitting lens assembly at its tip.

[0009] In order to solve the above problem, an image transmission system according to one aspect of the present invention is an image transmission system that transmits images of the inside of a patient's body to outside the body using the above-mentioned rigid endoscope, and includes a display unit that displays the images transmitted outside the body.

[0010] According to one aspect of the present invention, it is possible to provide an image transmission lens assembly, a rigid endoscope, and an image transmission system that include a plastic optical fiber that can perform good image transmission.

[0011] 1 is a diagram of an image transmission lens assembly (first image transmission lens assembly) according to one embodiment of the present invention. It is a cross-sectional view of the first image transmission lens assembly taken along the line A-A' in FIG. 1. It is a cross-sectional view of a POF lens 7 included in the first image transmission lens assembly of FIG. 1, and a partially enlarged cross-sectional view. It is a cross-sectional view of an image transmission plastic optical fiber lens in the first image transmission lens assembly of FIG. 1. It is a schematic diagram showing how light incident on the tip of the image transmission plastic optical fiber of the first image transmission lens assembly of FIG. 1 meanders through the fiber while tracing a sine curve. It is a diagram showing the optical transmission characteristics of the image transmission plastic optical fiber in the first image transmission lens assembly of FIG. 1. It is a diagram showing the optical transmission characteristics of an image transmission plastic optical fiber of a comparative configuration. It is a diagram showing the refractive index profile of the image transmission plastic optical fiber in the first image transmission lens assembly of FIG. 1. It is a cross-sectional view of a preform of the image transmission plastic optical fiber in the first image transmission lens assembly of FIG. 1. It is a diagram showing a flow of a manufacturing method for the first image transmission lens assembly of FIG. 1. 12 is a diagram of an image transmission lens assembly (second image transmission lens assembly) according to another embodiment of the present invention. FIG. 13 is a cross-sectional view of the second image transmission lens assembly shown in FIG. 11 , taken along the axial direction of the fiber, at the distal end side of the image transmission plastic optical fiber. FIG. 14 is a cross-sectional view of the second image transmission lens assembly shown in FIG. 11 , taken along a direction perpendicular to the axial direction of the image transmission plastic optical fiber. FIG. 15 is a view showing the opening of the outer shell pipe and its surroundings in the second image transmission lens assembly shown in FIG. 11 . FIG. 16 is a cross-sectional view of the second image transmission lens assembly shown in FIG. 12 , taken along the axial direction of the fiber, at the proximal end side of the image transmission plastic optical fiber. FIG. 17 is an exploded view of a rigid endoscope provided in an image transmission system according to an embodiment of the present invention. FIG. 18 is a diagram showing how an image of the inside of a patient's body is transmitted to the outside of the body using an image transmission system according to an embodiment of the present invention. FIG. 19 is a graph showing the bending index distribution measured using a POF lens of an example. FIG. 19 is a graph showing the bending index distribution measured using a POF lens of an example.

[0012] 1 is a side view of a first image transmission lens assembly 8A, which is an image transmission lens assembly according to one aspect of the present invention. The first image transmission lens assembly 8A includes a POF lens 7 and an outer pipe 3. The outer pipe 3 has a hollow tubular shape, and the linear POF lens 7 is inserted inside.

[0013] As an example, the outer shell pipe 3 can be made of stainless steel (SUS), but it may also be made of a material other than SUS, or may include other layers in addition to SUS.

[0014] Fig. 2 is a cross-sectional view of the first image transmission lens assembly 8A of Fig. 1 taken along the cutting line A-A' in Fig. 1. As shown in Fig. 2, a clearance may be provided between the inner peripheral surface of the outer shell pipe 3 and the outer peripheral surface of the POF lens 7. When such a clearance is provided, a separate linear structure similar to the POF lens 7 or a separate hollow cylindrical structure that covers the outer peripheral surface of the POF lens 7 may be disposed in the clearance. An example of such a structure is a light guide, which will be described later.

[0015] 2, the central axis of the outer pipe 3 and the central axis C of the POF lens 7 are arranged concentrically, generally aligned with each other. However, the present invention is not limited to this. That is, the POF lens 7 may be eccentric with respect to the outer pipe 3.

[0016] [POF Lens 7] The POF lens 7 is an image transmission lens. The POF lens 7 has a light incident surface 7a at a distal end located on the tip 8a side of the first image transmission lens assembly 8A shown in Fig. 1 , and transmits light (image) incident on the light incident surface 7a to a light exit surface 7b at a proximal end.

[0017] As shown in FIG. 2 , the POF lens 7 includes an image transmission POF 1 and a protective pipe 2. The image transmission POF 1 has a finely textured outer surface and a predetermined refractive index distribution in a single layer. The protective pipe 2 is a protective layer that covers the outer surface of the image transmission POF 1. The protective pipe 2 is moisture-proof to suppress the influence of humidity on the hygroscopic image transmission POF 1. As an example, the protective pipe 2 is made of a metal material, and as a specific example, it can be made of stainless steel (SUS).

[0018] Fig. 3 is a cross-sectional view of the POF lens 7 cut in a direction perpendicular to the central axis, and is the same cross-section as Fig. 2. Fig. 3 also shows an enlarged view of a portion. As shown in Fig. 3, a fine uneven portion 101k is provided on the outer peripheral surface of the image transmission POF 1. In order to provide a predetermined refractive index distribution in a single layer, the image transmission POF 1 is manufactured by a manufacturing method described below as an example, and the fine uneven portion 101k is provided on the outer peripheral surface during the manufacturing process.

[0019] 4 is a cross-sectional view of the POF lens 7 taken along the axial direction. The POF lens 7 has a cover glass 5 at its tip 7a. The cover glass 5 is fixed to the protective pipe 2. The cover glass 5 is made of glass and prevents moisture from entering the protective pipe 2, preventing the POF lens 7 from absorbing moisture, which could affect the refractive index distribution and adversely affect image transmission.

[0020] As an example, FIG. 5 shows a schematic diagram of light incident on the tip 7a of the POF lens 7 snaked through the POF lens 7 (the POF 1 for image transmission) while tracing a sine curve. When parallel light beams are incident on the POF 1 for image transmission, the light converges to a single point within the fiber and continues to transmit in this manner. The phenomenon of parallel light beams converging to a single point indicates the effect of a convex lens, and light transmitted through the POF 1 for image transmission, which is a GI-type POF, is equivalent to light traveling through a relay lens, which is an array of lenses, along the fiber axis. A relay lens is a device that transmits an image of an object over a long distance. The POF 1 for image transmission, which is a GI-type POF with the same lens effect, can focus an image of the object on the opposite side, just like a relay lens. The length of the POF lens 7 (POF1 for image transmission), i.e., the length along the central axis direction from the proximal end to the distal end of the POF1 for image transmission, can be, for example, a length that forms a sine curve with a total of three pitches as shown in Figure 5.

[0021] The uneven portion 101k (FIG. 3) formed on the outer peripheral surface of the image transmission POF 1 is a light absorbing portion that can absorb light incident from the tip of the image transmission POF 1 at an angle greater than the numerical aperture (NA) when the light reaches the outer peripheral surface of the image transmission POF 1. This allows the image of the transmission target to be transmitted clearly. This will be described with reference to FIG. 6.

[0022] FIG. 6 schematically illustrates the optical transmission characteristics of the image transmission POF 1. FIG. 6 illustrates the optical transmission characteristics of the image transmission POF 1, while FIG. 7 illustrates the optical transmission characteristics of a comparative image transmission POF that differs in that it does not include the concave-convex portion 101k. As shown in FIG. 6 , light incident from the tip 1a of the image transmission POF 1 at angles greater than the numerical aperture (NA) is absorbed by the fine concave-convex portion 101k on the outer peripheral surface of the core. This prevents light at angles greater than the NA from propagating as stray light. On the other hand, in the comparative configuration shown in FIG. 7 , light at angles greater than the NA is reflected by the peripheral surface (side surface) and becomes stray light.

[0023] Fig. 8 is a refractive index distribution diagram in the radial direction of the POF 1 for image transmission. A cross-sectional view of the POF 1 for image transmission in the radial direction is also shown at the top of Fig. 8. The horizontal axis of the refractive index distribution diagram in Fig. 8 represents the radial direction of the POF 1 for image transmission, and corresponds to the cross-sectional view of the POF 1 for image transmission in the radial direction shown at the top of the diagram. The vertical axis of the refractive index distribution diagram in Fig. 8 represents the refractive index, with the refractive index increasing upward on the vertical axis.

[0024] As shown in the refractive index distribution diagram in Figure 8, the refractive index of the image transmission POF 1 decreases continuously from the center C of the diameter toward the outside. This type of refractive index distribution is generally called a GI type, and light is less likely to leak to the outside and is less likely to be scattered. Therefore, the image transmission POF 1 is a POF with low transmission loss and a wide transmission band.

[0025] In this embodiment, a refractive index grading agent (dopant) is used to achieve the refractive index profile shown in the refractive index profile diagram of FIG. 8 . The dopant will be described later. Furthermore, in this embodiment, the image transmission POF 1 is a single-layer fiber to minimize variations in the refractive index profile in the radial direction of the fiber. During the manufacturing process of the image transmission POF 1 to obtain this single-layer fiber, an outer layer is provided on the outer peripheral surface of the core portion that will ultimately correspond to the image transmission POF 1. Forming the core portion together with the outer layer controls (adjusts) the refractive index profile of the core portion. FIG. 9 is a cross-sectional view showing a linear intermediate product during the manufacturing process of the image transmission POF 1, cut in a direction perpendicular to its axis. In this embodiment, this linear intermediate product is referred to as a preform 100. The preform 100 has a core portion 101 and an outer layer 102 provided on the outer peripheral surface of the core portion.

[0026] 10 shows an example of a method for manufacturing the POF lens 7. The manufacturing process S10 of the POF lens 7 includes the manufacturing process of the POF 1 for image transmission (S13 to S16) and a protection process S17, which is a lens manufacturing process.

[0027] [Manufacturing Process of Image Transmission POF1] The manufacturing process of the image transmission POF1 includes a preparation step S13, a pressure smoothing step S14, a heat drawing step S15, and a cutting step S16, as shown in FIG.

[0028] The manufacturing method of the POF 1 for image transmission exemplified below may include a process for manufacturing the above-described preform. Here, the preform is a cylindrical structure having thermoplastic properties. Specifically, it includes a cylindrical core portion 101 ( FIG. 9 ) located toward the center in the radial direction and a cylindrical outer layer portion 102 ( FIG. 9 ) located toward the outer periphery in the radial direction, and the core portion and the outer layer portion are integrated. Therefore, there is no interface between the core portion and the outer layer portion. The core portion and the outer layer portion can be identified by detecting the molecular structure (e.g., molecular weight or substituent) or physical properties (e.g., strength) of the polymer of each base material using a known method.

[0029] [Core portion 101] The core portion 101 is made of a thermoplastic and transparent polymer (hereinafter also referred to as "core material polymer"). The core portion 101 is a core material portion in an integrated product in which the core material and the sheath material in a prepreform described below are integrated. The composition of the core portion 101 will be described later in the explanation of the core material.

[0030] The core 101 has a refractive index that is distributed in a convex curve shape, with the refractive index increasing toward the center in the radial direction. The refractive index distribution in the core 101 in the radial direction is represented by a continuous, gentle convex curve.

[0031] For example, the curvature at any position on a convex curve showing the refractive index distribution in the radial direction of the preform is 0.35 to 4.00 in the core portion. The refractive index distribution curve of the preform has a maximum curvature at the radial center of the preform (the radial center of the core portion) and tends to decrease radially outward. The curvature of the distribution curve also changes continuously. The curvature of the refractive index distribution curve is determined by setting the refractive index difference Δn (the difference between the maximum and minimum values) of the distribution curve in the core portion and the radius of the core portion to "1." The curvature can be appropriately set within the above range depending on the desired optical characteristics of the POF, which is the final product.

[0032] Furthermore, for example, the refractive index difference Δn in the preform is 0.0003 to 0.030 in the core portion. The refractive index difference is the difference between the maximum and minimum values ​​of the refractive index in the core portion in the refractive index distribution curve of the preform. The minimum value of the refractive index may be the average value of the refractive indexes at both ends of the core portion in the distribution curve, or it may be one of the refractive index values ​​at both ends, e.g., the smaller value. The refractive index difference may be determined depending on the application and desired optical characteristics of the POF, which is the final product. For example, a large refractive index difference is preferable for use as a tip lens with a wide angle of view, while a small refractive index difference in the core portion (e.g., around 0.002) is preferable for use as a relay lens that transmits image signals. The refractive index difference can be adjusted by changing the base material (polymer for the core material) or the type of dopant.

[0033] In the preform, the refractive index of the central portion of the core is sufficiently larger than the refractive index of the peripheral portion. The refractive index difference varies depending on the subsequent use of the POF and the required optical properties. However, from the viewpoint of realizing, for example, use in transmitting images inside a living body, the maximum value of the refractive index difference in the distribution is preferably, for example, 0.0003 or more. The larger the refractive index difference in image transmission use, the wider the field of view. However, the upper limit may be set appropriately from the viewpoint of feasibility by controlling the dopant distribution, and may be, for example, 0.030 or less from the viewpoint of use of the POF.

[0034] The refractive index distribution of the core region 101 is realized by the distribution of dopants in the core region. The dopants will be described later in the explanation of the core material. The distribution of the amount (concentration) of dopants in the axial direction of the core region can be substantially represented by the convex curve described above.

[0035] The core portion of a preform can be identified based on the dimensions of the preform if the dimensions of the core portion are known. Furthermore, if the core portion and the outer layer portion of the preform are optically distinguishable (for example, if the outer layer portion contains a colorant and the optical properties of the outer layer portion are clearly different from those of the core portion), the core portion can be identified based on the optical properties. Alternatively, the refractive index distribution of the preform can be determined and the core portion can be identified as a portion within a specific range from the center in the radial direction (for example, a range from the center of the cross section to 3 mm (a circular region with a diameter of 6 mm)) that has an appropriate refractive index distribution including the above-mentioned curvature and refractive index difference.

[0036] [Outer layer portion 102] The outer layer portion 102 is made of a thermoplastic polymer (hereinafter also referred to as a "sheath material polymer"). The outer layer portion is the sheath material portion of the integrated product in which the core material and sheath material in the prepreform described below are integrated. The composition of the outer layer portion 102 will be described later in the explanation of the sheath material.

[0037] The outer layer part 102 preferably has transparency from the viewpoint of enhancing the visibility of the core part 101. Furthermore, the outer layer part 102 preferably has higher physical properties than the core part 101, such as mechanical strength and heat resistance, from the viewpoint of protecting the core part 101 in the subsequent manufacturing process of the POF. Furthermore, the outer layer part 102 may contain the same dopant as that contained in the core part 101. These physical properties can be achieved by a sheath material, which will be described later.

[0038] The preform having the above configuration is manufactured in a manufacturing process including a preparation step S13 and a pressure smoothing step S14, which will be described below.

[0039] [Preparation Step S13] In preparation step S13 of Fig. 10, a preform is manufactured. The preform is heated in a vacuum atmosphere while being pressurized along its axial direction. The preform may be composed of a cylindrical core material and a cylindrical sheath material into which the core material is inserted.

[0040] The core material can be inserted into the sheath material, and a clearance is provided between the core material and the sheath material that allows the two to be integrated by subsequent heating. The clearance can be appropriately determined within a range that allows the core material to be inserted into the sheath material and allows the core material to be integrated by subsequent heating for smoothing during POF manufacturing. If the clearance is too small, it may be difficult to insert the core material into the sheath material. If the clearance is too large, the core material and the sheath material may not be sufficiently integrated during subsequent heating for smoothing, resulting in insufficient smoothing. From the above perspective, the clearance is preferably a gap that allows the core material to fit inside the sheath material (without rattle). For example, the difference between the diameter of the core material and the inner diameter of the sheath material may be 50 μm or more and 1 μm or less.

[0041] The thickness of the sheath material relative to the diameter of the core material can be determined appropriately from the viewpoints of achieving the desired dopant distribution in the core material during smoothing in the subsequent POF manufacturing process and protecting the core material with the sheath material during subsequent drawing. If the sheath material is too thin, the desired dopant distribution in the core material during smoothing may not be achieved. On the other hand, since the sheath material in this embodiment does not directly contribute to the POF, an excessively thick sheath material is undesirable from the viewpoints of productivity and cost. From the above viewpoints, the ratio t2 / r1 of the sheath material thickness t2 to the core material radius r1 may be, for example, 0.425 or more and 0.5 or less.

[0042] The core material and the sheath material are used as POF materials in a subsequent POF manufacturing process, with the core material inserted into the sheath material. The lengths of the core material and the sheath material may be substantially the same as long as they are long enough to be used as POF materials. The lengths of the core material and the sheath material may be long enough to be used as POF materials as they are, or may be long enough to be cut to an appropriate length when used as POF materials.

[0043] In the prepreform, a specific core material is inserted into a corresponding specific sheath material. Therefore, the core material and the sheath material may have additional configurations indicating the specific combination, such as a printed portion displaying a mark or description such as a model number indicating the specific combination, and a textured shape such as an engraved mark or description indicating the specific combination, on the surface of each end.

[0044] The core material is composed of a polymer matrix for the core material and a dopant dispersed in the matrix.

[0045] The core material polymer has thermoplastic properties and transparency. The core material polymer can be selected from known transparent thermoplastic resins. The core material polymer is preferably amorphous from the viewpoints of improving transparency and suppressing birefringence. The core material polymer may be one or more types. Examples of the core material polymer include polymethyl methacrylate (PMMA), polystyrene (PS), polytrifluoromethacrylate (P3FEMA), and polycarbonate (PC). Examples of the core material polymer also include polymers and copolymers of various monomers. Examples of such monomers include styrene, methyl methacrylate, 2,2,2-trifluoroethyl methacrylate, 4-methylcyclohexyl methacrylate, cyclohexyl methacrylate, furfuryl methacrylate, 1-phenylethyl methacrylate, 1-phenylcyclohexyl methacrylate, benzyl methacrylate, 1,2-diphenylethyl methacrylate, o-chlorobenzyl methacrylate, p-chlorobenzyl methacrylate, diphenylmethyl methacrylate, pentachlorophenyl methacrylate, pentabromophenyl methacrylate, isobornyl methacrylate, 2-hydroxyethyl methacrylate, and heptadecafluorodecyl methacrylate. Examples of the polymers include polymers of the above monomers. Examples of the copolymers include copolymers of methyl methacrylate and a monomer copolymerizable with methyl methacrylate among the above monomers, other than methyl methacrylate.

[0046] The dopant is a component that can impart a specific refractive index to the core material by dispersing it in the matrix. The dopant only needs to be mobile when the core polymer is in a flow region, and therefore has a sufficiently low molecular weight relative to the core polymer. One or more dopants may be used. Examples of dopants include various carboxylic acid esters, more specifically, phthalic acid esters, benzoic acid esters, phenylacetic acid esters, adipic acid esters, and sebacate esters.

[0047] Examples of phthalate esters include dimethyl phthalate, diethyl phthalate, diallyl phthalate, dibutyl phthalate, diisobutyl phthalate, di-n-hexyl phthalate, bis(2-ethylhexyl) phthalate, dioctyl phthalate, di-n-octyl phthalate, diisononyl phthalate, dinonyl phthalate, diisodecyl phthalate, bis-butylbenzyl phthalate, and butylbenzyl phthalate. Examples of benzoate esters include ethyl benzoate, propyl benzoate, benzyl benzoate, 4-biphenylyl benzoate, and phenyl benzoate. Examples of phenylacetic acid esters include methyl phenylacetate, ethyl phenylacetate, and phenylacetic acid chloride. Examples of adipate esters include bis(2-ethylhexyl) adipate, diisononyl adipate, diisodecyl adipate, and bis(2-butoxyethyl) adipate. Examples of sebacate esters include diisopropyl sebacate, diethyl sebacate, dioctyl sebacate, sebacoyl dichloride, dibutyl sebacate and bis-2-ethylhexyl sebacate.

[0048] The core material has a specific refractive index determined by the dopant. The core material will then become the light guide (core) in the POF. Therefore, the core material only needs to have a refractive index high enough to achieve the desired function of the POF. The refractive index of the core material can be appropriately determined taking into account the refractive index of the base material, which varies depending on the type. For example, for the D line of sodium, the refractive index is 1.400 or more from the above viewpoint, and 1.600 or less from the viewpoint of feasibility with the combination of the core material polymer and the dopant.

[0049] The refractive index of the core material can be determined by a known technique capable of measuring the refractive index of plastic products. For example, the refractive index of the core material can be measured by the refraction angle distribution polarimetry using an index profiler ("IP-5500" manufactured by Seiko E&G Corporation) at measurement wavelengths of 589.3 nm (D-line), 486.0 nm (F-line), and 656.3 nm (C-line). The refractive index of the core material can also be adjusted by the type of dopant or the content of the dopant in the core material.

[0050] The sheath material is integrated with the core material in the subsequent manufacturing process of the POF to form the outer layer of the integrated product, and is a component that can adjust the refractive index distribution in the radial direction of the core material by adjusting the distribution of dopants in the core material during integration. The sheath material is made of a sheath material polymer having thermoplasticity. The sheath material polymer may be one type or two or more types, and may be the same as or different from the core material polymer. Examples of the sheath material polymer are the same as the examples of the core material polymer described above.

[0051] The sheath material may be composed of a resin composition having a polymer for the sheath material as a matrix. For example, the above-mentioned dopant may be dispersed in the sheath material in order to adjust the refractive index of the sheath material or the absorption or emission of the dopant in the sheath material. It is preferable that the dopant in the sheath material also diffuses into the core material in order to form a refractive index distribution in the core material that enables image transmission. The dopant in the sheath material may be one or more types, and may be the same as or different from that in the core material. Examples of dopants for the sheath material are the same as the examples of dopants for the core material described above.

[0052] It is preferable that the refractive index of the sheath material is lower than that of the core material from the viewpoint of appropriately distributing the refractive index of the core material in the radial direction in the subsequent manufacturing process of the POF.

[0053] The difference (n1-n2) between the refractive index n2 of the sheath material and the refractive index n1 of the core material may be determined appropriately from the viewpoint of realizing a suitable radial refractive index distribution when the POF is formed. The method for determining the refractive index of the sheath material is the same as that for the core material, and the method for adjusting the refractive index of the sheath material is also the same as that for the core material.

[0054] The sheath material does not have to be transparent, but is preferably transparent from the viewpoint of enabling visual or optical confirmation of the state of the core material or the portion derived from the core material in the subsequent process of manufacturing the POF.

[0055] The sheath material preferably has higher physical properties than the core material in order to protect the core material during the subsequent POF manufacturing process. For example, the sheath material preferably has a higher mechanical strength than the core material in order to protect the core material during drawing in the subsequent POF manufacturing process. Furthermore, the sheath material preferably has high heat resistance due to the sheath material polymer in order to protect the core material during heating in the subsequent POF manufacturing process. The mechanical strength and heat resistance of the sheath material can be improved by the type of sheath material polymer, increasing the polymerization degree of the sheath material polymer, or introducing a crosslinked structure.

[0056] In this embodiment, it is preferable that the temperature of the flow region of the core polymer and the temperature of the flow region of the sheath polymer overlap at least partially, from the viewpoint of realizing a desired refractive index in the radial direction of the core material in the subsequent POF manufacturing process. Furthermore, from the viewpoint of suppressing changes in the core material and the sheath material due to heating in the POF manufacturing process, it is preferable that the temperature of the flow region of the core polymer and the sheath polymer overlap at least partially at a temperature below the thermal decomposition reaction temperature of these polymers. Note that, when the above polymer is a polymer that depolymerizes, the above thermal decomposition reaction temperature is the depolymerization temperature of the polymer. The depolymerization temperature is the temperature at which depolymerization begins.

[0057] (Production of Core Material and Sheath Material) The core material and sheath material can be produced as described below. The core material and sheath material may be commercially available products or processed products thereof as long as they have the above-mentioned characteristics.

[0058] (Production of Core Material) The core material can be produced by generating the core polymer in a cylindrical closed vessel containing the thermoplastic and transparent core polymer material and the dopant. The vessel can be configured to include a circular tube with caps sealing both ends.

[0059] The core material polymer may be a polymerizable composition that produces a core material polymer. Such a core material polymerizable composition contains a monomer capable of constituting the core material polymer and a polymerization initiator. The monomer is selected appropriately depending on the core material polymer, and the polymerization initiator is selected appropriately depending on the monomer. The core material polymerizable composition may further contain additives that affect the structure of the resulting polymer, such as a chain transfer agent and a crosslinking agent, as needed. The chain transfer agent and crosslinking agent can be selected appropriately depending on the monomer and the desired physical properties of the resulting core material polymer.

[0060] Examples of the monomer include methyl methacrylate (MMA), styrene, 2,2,2-trifluoromethacrylate, 4-methylcyclohexyl methacrylate, cyclohexyl methacrylate, furfuryl methacrylate, 1-phenylethyl methacrylate, 1-phenylcyclohexyl methacrylate, benzyl methacrylate, 1,2-diphenylethyl methacrylate, o-chlorobenzyl methacrylate, p-chlorobenzyl methacrylate, diphenylmethyl methacrylate, pentachlorophenyl methacrylate, pentabromophenyl methacrylate, isobornyl methacrylate, 2-hydroxyethyl methacrylate, and heptadecafluorodecyl methacrylate.

[0061] Examples of the polymerization initiator include benzoyl peroxide (BPO), n-butyl-4,4-bis(t-butylperoxy)valerate, di-t-butyl peroxide, 1,1-bis(t-butylperoxybutane), 1,1-bis(t-hexylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-hexylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and t-butylperoxy 2-ethylhexyl monocarbonate. The amount of the polymerization initiator in the polymerizable composition for the core material cannot be generally determined, but may be, for example, 0.1 to 10×10 in terms of the amount of polymerization initiator per mole of monomer. －３ moles, 1 to 3 × 10 －３ It can be molar.

[0062] Examples of the chain transfer agent include n-butyl mercaptan (n-BM), n-octyl mercaptan, n-lauryl mercaptan, furfuryl mercaptan, n-decyl mercaptan, and undecyl mercaptan. The amount of the chain transfer agent in the polymerizable composition for the core material cannot be generally determined, but for example, the amount of the chain transfer agent per mole of the monomer is 0.01 to 10×10 －３ moles, and 0.1 to 3 × 10 －３ It can be molar.

[0063] Examples of the crosslinking agent include ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, polybutylene glycol dimethacrylate, and trimethylolpropane trimethacrylate. The amount of the crosslinking agent in the polymerizable composition for the core material cannot be generally determined, but for example, the amount of the crosslinking agent relative to 1 mole of the monomer is 0.1 to 10×10 －５ moles, 1 to 3 × 10 －５ It can be molar.

[0064] A dopant is mixed with a polymerizable composition for the core material to obtain a raw material composition for the core material, which is then placed in the container and heated to a polymerization temperature while the container is held upright, thereby producing a cylindrical core material. The amount of dopant in the raw material composition for the core material cannot be generally determined, but is, for example, 1 to 10 × 10 in terms of the amount of dopant per mole of monomer. －２ It can be molar.

[0065] [Production of Sheath Material] The sheath material can be produced by rotating a cylindrical closed vessel containing a thermoplastic polymer material for the sheath material around its axis to produce the polymer for the sheath material. The vessel for producing the sheath material can have a configuration similar to that of the vessel for producing the core material.

[0066] Like the core material polymer, the sheath material polymer material may be a polymerizable composition that produces a sheath material polymer. Like the core material polymerizable composition described above, this sheath material polymerizable composition may contain a monomer and a polymerization initiator, and, if necessary, a chain transfer agent and a crosslinking agent. Examples of these components in the sheath material polymerizable composition include the same components as those exemplified in the core material polymerizable composition.

[0067] In the production of the sheath material, a dopant can be further added to the polymerizable composition for the sheath material to obtain a raw material composition for the sheath material, similar to the production of the core material. The inclusion of a dopant in the raw material composition for the sheath material is preferable from the viewpoint of appropriately controlling the radial distribution of the dopant in the core material in the subsequent POF production process. As mentioned above, it is preferable that the mechanical properties or thermal properties of the sheath material are higher than those of the core material, and from this viewpoint, the types and amounts of the monomer, chain transfer agent, and crosslinking agent used in the polymerizable composition for the sheath material can be appropriately determined. The amount of dopant in the raw material composition for the sheath material cannot be generalized, but it is preferably smaller than that of the core material, for example, 0 to 5 × 10 in terms of the amount of dopant per 1 mole of monomer. －２ It can be molar.

[0068] In the production of sheath material, a dopant is optionally mixed with the polymerizable composition for the sheath material to obtain a raw material composition for the sheath material, which is then placed in the container. The container is then laid down (with the axis of the container horizontal) and rotated around its axis at a sufficiently high speed so that the raw material composition for the sheath material is unevenly distributed around the periphery of the container and not around the axis. The raw material composition for the sheath material is then heated to a polymerization temperature, producing a cylindrical sheath material. Rotation of the container around its axis can be achieved by attaching the container to a rotating device that grips both ends of the container and rotates it around its axis, placing the rotating device in a heating chamber, and heating the container in the heating chamber while operating the rotating device.

[0069] [Remixing during viscosity increase] In this embodiment, in the production of both the core material and the sheath material, it is preferable to carry out the steps of polymerizing the monomer at a low polymerization temperature to obtain a fluid intermediate product, and inverting the container containing the intermediate product to cause the intermediate product to flow in the axial direction of the container, from the viewpoint of uniformly distributing the dopant in the core material or the sheath material.

[0070] The step of obtaining an intermediate product is a step of slowly reacting the raw material composition by mild heating to increase viscosity. The intermediate product is a liquid composition in which a portion of the monomers in the polymerizable composition described above has polymerized, resulting in a higher viscosity. The reaction temperature and reaction time in the step of obtaining an intermediate product can be appropriately determined depending on the types of monomer and polymerization initiator, within a range in which the intermediate product flows when the container is inverted.

[0071] The step of flowing the intermediate product in the axial direction of the container is a step of flowing the intermediate product widely to homogenize the composition of the intermediate product. The axial flow of the intermediate product may be performed once or multiple times. If the flow is performed multiple times, the flow may be performed continuously or intermittently. By flowing the intermediate product in the axial direction of the container by inverting the container, settling or floating of the dopant in the intermediate product is eliminated, and the dopant is uniformly distributed in the intermediate product.

[0072] In the production of the sheath material, it is preferable to polymerize the raw material composition for the sheath material while rotating the container at a sufficiently high speed around the axis of the container at the polymerization temperature as described above, from the viewpoint of enhancing the dispersibility of the dopant in the sheath material.

[0073] In both the core material and the sheath material, after the reaction of the polymer material is completed, the resulting polymer composition is removed from the container to obtain the core material and the sheath material. Because the internal composition of each of these core materials and sheath materials is substantially uniform, it is possible to obtain core materials and sheath materials that exhibit the specific refractive index set for each material.

[0074] The cylindrical core material is inserted into a cylindrical sheath material to obtain a pre-preform, which is a precursor of the preform.

[0075] [Pressure Smoothing Step S14] In the pressure smoothing step S14, the prepreform obtained in the preparation step S13 is heated in a vacuum atmosphere while being pressurized along its axial direction, thereby obtaining a preform in which the core material (core portion) and sheath material (outer layer portion) constituting the prepreform are integrated.

[0076] The prepreform can be compressed in the axial direction during heating by a configuration that allows the prepreform to be compressed. For example, the prepreform can be compressed in the axial direction by placing the prepreform in a metal tube such as stainless steel, inserting a spring into the tube, and pressing both ends of the tube with clamps or other tools to bring the compressed spring into contact with the prepreform. In this case, the strength of the pressure can be adjusted by adjusting the strength of the spring's elastic force (such as the spring constant, the length of the coil spring, or the number of coil springs). By applying a sufficiently strong pressure, the axial position of the prepreform or integrated object within the heating tube can be properly maintained before, during, and after heating.

[0077] The prepreform that integrates the core material and sheath material is heated in a vacuum atmosphere. This allows gas components to be removed from the core material and sheath material during heating, preventing the generation of bubbles from the core material or sheath material during heating. In addition, since air and moisture in the atmosphere are sufficiently removed, this is suitable for achieving a desired distribution of the refractive index in the radial direction in the core material (core portion). The degree of vacuum in the atmosphere during heating can be appropriately determined within a range in which the above-mentioned degassing effect can be obtained. From this perspective, it is preferable to use a vacuum of 2 × 10 －１ Pa or less. Generally, the higher the degree of vacuum, the higher the equipment costs tend to be. Therefore, the degree of vacuum of the atmosphere during heating may be appropriately set by further considering the viewpoint of suppressing the increase in equipment costs.

[0078] When the prepreform is heated to integrate the core material and sheath material, the core material and sheath material thermally expand and integrate. Further heating causes the dopant in the core material to diffuse into the core material's matrix (the core material polymer) and from the core material to the sheath material. Similarly, if the sheath material contains a dopant, the dopant in the sheath material diffuses into the sheath material and from the sheath material to the core material. This diffusion of dopants causes the refractive index distribution in the radial direction of the core material to approach the desired distribution.

[0079] If the temperature at which the prepreform for integrating the core material and sheath material is heated is too low, the dopant will not migrate sufficiently, and the desired refractive index distribution in the radial direction of the core portion may not be achieved. From the viewpoint of sufficient dopant migration in the core material and sheath material, it is preferable to heat the prepreform in a temperature environment that includes both the core material flow region and the sheath material flow region. Therefore, in this embodiment, it is preferable that the temperature of the core material polymer flow region and the temperature of the sheath material polymer flow region at least partially overlap. Furthermore, from the viewpoint of improving the stability of the core material and sheath material during heating, it is preferable that the temperature at which the prepreform for integrating the core material and sheath material is heated be the temperature of the core material polymer flow region that is equal to or lower than the thermal decomposition reaction temperature of the core material polymer, and the temperature of the sheath material polymer flow region that is equal to or lower than the thermal decomposition reaction temperature of the sheath material polymer. Note that, if the polymer is a depolymerizable polymer, the thermal decomposition reaction temperature is the depolymerization temperature of the polymer. The depolymerization temperature is the temperature at which depolymerization begins.

[0080] The heating time of the prepreform that integrates the core material and the sheath material is determined appropriately from the perspective of achieving the desired refractive index distribution in the radial direction of the core material. If the heating time is short, the desired refractive index distribution may not be achieved due to insufficient dopant migration, while if the heating time is long, the desired refractive index difference may not be achieved due to excessive dopant migration. The appropriate heating time varies depending on various conditions, such as the prepreform material, the radial size of the prepreform, the axial pressure applied during heating, and the desired refractive index distribution to be achieved. While it cannot be generalized, it can be determined experimentally based on these conditions. The heating time may be determined by further referring to the results of a computer simulation of the diffusion of low-molecular-weight compounds in a resin matrix. Furthermore, since the appropriate heating time tends to be proportional to the radial size of the prepreform, if the appropriate heating time for a certain size of prepreform is known, it can also be calculated from this known heating time.

[0081] After heating the prepreform, it is preferable to slowly cool the integrated core material and sheath material obtained by the heating at a specific cooling rate slower than that obtained by natural cooling at room temperature, from the viewpoint of realizing the desired refractive index distribution. The temperature gradient during cooling may also affect the refractive index distribution. Cooling at a sufficiently slow rate is preferable from the viewpoint of realizing the desired refractive index distribution, and cooling with a specific temperature gradient is preferable from the viewpoint of stably realizing the refractive index distribution. It is also preferable to perform this cooling in a vacuum atmosphere, from the viewpoint of suppressing the fluctuation of the refractive index that has appeared.

[0082] The cooling of the integrated product is preferably gradual in consideration of the final density of the core polymer matrix. The cooling rate of the integrated product should be determined appropriately depending on factors other than the final density of the matrix, such as the desired refractive index distribution or the type of polymer material, and is not generally determined; however, it may be, for example, 5 to 15°C / hour. This cooling rate is sufficiently slower than the cooling rate when the integrated product is left at room temperature (e.g., 20°C). The cooling rate of natural cooling may vary depending on the room temperature. Therefore, the cooling of the integrated product is preferably programmed cooling, in which the cooling rate is controlled. Furthermore, while the integrated product is still hot (in the initial stage of cooling), this tends to have a greater impact on the formation of the refractive index distribution in the core portion. Therefore, programmed cooling is preferable from the viewpoint of stably achieving the desired refractive index distribution in the core portion. The end temperature of the cooling of the integrated product may be the temperature of the integrated product at which the refractive index distribution in the core portion substantially stabilizes, and may be, for example, 70 to 80°C. For example, the final temperature of the aforementioned programmed cooling may be set to 77°C. Such cooling can be achieved by controlling the internal temperature of a temperature-controlled chamber in which the heating is performed. Thereafter, the integrated product may be slowly cooled to room temperature by natural cooling in the temperature-controlled chamber. For example, the integrated product may be cooled by programmed natural cooling at a room temperature of 25°C and a cooling rate of 10°C / hour. Note that, once the temperature of the integrated product drops to about 87°C, it tends to take about one hour to cool the integrated product from 87°C to 77°C even by natural cooling. Therefore, the final temperature of the programmed natural cooling may be set to 87°C.

[0083] In the case of programmed cooling using a temperature-controlled chamber, the cooling rate may vary depending on the temperature-controlled chamber. In this case, the cooling rate to be set may be an appropriate value depending on the temperature-controlled chamber used, based on the difference in the cooling rate from room temperature or the cooling time at the end of the cooling period (cooling from 87°C to 77°C).

[0084] The prepreform is heated and cooled while it is housed in the heating tube and pressurized from both ends. It is preferable that the integrated product be pressed from the outside in the circumferential direction when it is cooled and thermally shrinking, in order to prevent deformation of the integrated product, such as sink marks, or the generation of bubbles in the integrated product. From the above perspective, it is preferable that the heating tube have a thermal expansion coefficient equal to or greater than (for example, 1 to 1.2 times) the thermal expansion coefficients of the core material polymer and the sheath material polymer. From this perspective, it is preferable that the heating tube be made of metal.

[0085] Alternatively, from the viewpoint of preventing the above-mentioned deformation or the generation of bubbles, it is preferable to cover the prepreform inside the heating tube from its outer periphery with a member having a thermal expansion coefficient equal to or greater than (e.g., 1 to 1.2 times) the thermal expansion coefficients of the core material polymer and the sheath material polymer. Examples of such thermally expandable members include tubes and pipes made of thermally expandable resins, and examples of thermally expandable resins include fluororesins. In this case, the prepreform is housed in the heating tube while inserted into the thermally expandable tube or pipe. Since the thermally expandable member thermally contracts more than the integrated body upon cooling, the thermally expandable member presses the integrated body from its outer periphery and follows the thermally contracting integrated body from the outer periphery. In this case, the thermal expansion of the heating tube is not limited, and the heating tube may be made of a material, such as heat-resistant glass, having a thermal expansion coefficient sufficiently smaller than that of the core material polymer and the sheath material polymer.

[0086] The preform obtained by the pressure smoothing step S14 exhibits a desired refractive index distribution in the radial direction. Because the preform is composed of a core portion and an outer layer portion, the desired refractive index distribution is exhibited not only in the core portion but also in the outer layer portion. The preform is then stretched in the subsequent heat-stretching step S15, and the outer layer portion is further removed in the cutting step S16, resulting in a POF essentially consisting of only the core portion. Because the core portion of this POF has a single-phase base material and exhibits the desired refractive index distribution in this state, it has a refractive index distribution closer to the ideal distribution than conventional POFs. Therefore, the preform of this embodiment is useful as a material for manufacturing POF that can be used to transmit high-resolution images.

[0087] [Heat-Stretching Step S15] In the subsequent heat-stretching step S15, the preform that has been pressurized and smoothed in the pressurizing and smoothing step S14 is heat-stretched. In the heat-stretching step S15, a heat-stretching device having at least a heating furnace, a feeding section, and a pulling section is used to stretch the preform thinly while maintaining the image transmission ability of the preform. The heating furnace heats a portion of the preform within the furnace. The feeding section feeds the held preform toward the entrance of the heating furnace. The pulling section pulls the preform heated by the heating furnace out of the furnace through the exit of the heating furnace, thereby stretching the preform. The feeding speed by the feeding section and the pulling speed by the pulling section are controlled by a control section. The control section controls the feeding speed and the pulling speed based on the heating conditions of the heating furnace and the temperature and humidity of the ambient environment of the heating furnace.

[0088] Here, the refractive index distribution of the core portion 101 is adjusted stepwise in a preparation step S13 and a pressure-smoothing step S14. In the preparation step S13, the refractive index distribution is controlled within the layer of the rod-shaped core portion 101. Thereafter, in the pressure-smoothing step S14, the rod-shaped core portion 101, whose outer peripheral surface is covered with the outer layer portion 102, is heated in a vacuum environment, thereby achieving a desired refractive index distribution from the center of the core portion 101 to its outer peripheral surface. Then, in the subsequent heat-stretching step S15, heat-stretching is performed with the outer peripheral surface protected by the outer layer portion 102, thereby preventing the refractive index distribution of the core portion 101 from being disturbed by moisture absorption, and thereby allowing the core portion 101 to be stretched to a desired small diameter.

[0089] The heated and stretched preform is wound up on a winding device (not shown). In one example, the total length of the heated and stretched preform is 64 times the total length of the preform before being heated and stretched. The diameter (diameter) of the heated and stretched preform is, for example, 0.75 mm. Note that, assuming a target diameter of 0.75 mm, the diameter (thickness) may vary within an allowable range while being pulled by the tensioning unit 40. The allowable range refers to a range in which image transmission capability can be maintained. For example, the allowable range of diameter variation is ±2.6% of the target diameter, ±3.8% of the target diameter when there is a large variation, and ±1% during mass production.

[0090] [Cutting Step S16] In the subsequent cutting step S16, the outer layer portion is cut (removed) from the preform that has been heated and drawn in the heating and drawing step S15 and has a desired diameter, and an uneven portion 101 ( FIG. 3 ) having a fine uneven shape is formed on the outer peripheral surface of the core portion. As described above, the outer layer portion, which is the outer layer of the preform, exists to control the refractive index distribution of the core portion and to structurally protect the core portion, and is therefore not necessary for the image transmission POF 1. Therefore, this cutting step S16 is performed.

[0091] In the cutting step S16, for example, a file is used to cut the outer layer portion from the preform and form the uneven portion 101k (FIG. 3) on the outer peripheral surface of the core portion. Sandpaper can be used as an example of a file. The sandpaper is laid on a horizontal surface, and the preform is placed on top of it with its central axis parallel to the surface of the sandpaper, and the preform is rolled. This causes the outer layer portion to be scraped off by sanding, and further rolling can form the uneven portion on the outer peripheral surface of the core portion. There are no particular limitations on the rolling method, but for example, a method can be used in which a worker at the manufacturing site manually presses the preform lightly against the sandpaper and rolls it.

[0092] The uneven portion 101k is realized by a surface roughness that can absorb light incident at an angle larger than the numerical aperture (NA) when the light reaches the outer peripheral surface of the core portion, among light incident from the tip end 1a of the POF 1 for image transmission, which is comprised of only the core portion as shown in Fig. 6. The surface roughness that can absorb light is, for example, the size of the unevenness formed when the outer peripheral surface of the core portion 101 is filed with sandpaper having an abrasive grain size of 57 µm.

[0093] In this embodiment, the outer layer portion is removed by physical friction with sandpaper and the uneven portion 101k is formed on the outer peripheral surface of the core portion. However, this is not limited to this method, and for example, chemical mechanical polishing (CMP) can also be used. It is also possible to use different methods for removing the outer layer portion and for forming the uneven portion on the outer peripheral surface of the core portion. One example of such a method is to remove the outer layer portion by some method, and then spray a material that can become the uneven portion onto the exposed outer peripheral surface of the core portion to form the uneven portion 101k.

[0094] In manufacturing the POF1 for image transmission, within the range where the desired distribution of refractive index in the radial direction is expressed, as described above, only the portion substantially corresponding to the outer layer portion may be removed, but not only the outer layer portion but also the portion corresponding to the peripheral portion of the core portion may be removed.

[0095] Through the above steps, the POF 1 for image transmission is manufactured.

[0096] 10 , the POF lens 7 is manufactured by covering the outer circumferential surface of the image transmission POF 1, on which the uneven portion 101k is formed, with a protective layer in a protecting step S17 performed subsequent to the cutting step S16. The protective layer functions as a protective pipe 2. There are no particular limitations on the covering method, and as an example, the POF lens 7 can be manufactured by inserting the image transmission POF 1 into the hollow portion of a cylindrical protective layer.

[0097] As described above, the first image transmission lens assembly 8A of this embodiment includes the image transmission POF 1 having a single layer with a predetermined refractive index distribution and having fine unevenness 101k formed on its outer peripheral surface, the protective pipe 2 covering the outer peripheral surface of the image transmission POF 1 and made of a moisture-proof material, and the outer shell pipe 3 accommodating the image transmission POF 1 covered by the protective pipe 2 from its proximal end to its distal end. This makes it possible to realize an image transmission lens assembly equipped with a plastic optical fiber for image transmission.

[0098] Furthermore, the first image transmission lens assembly 8A allows the image transmission POF 1 to be manufactured from inexpensive plastic materials. Therefore, it can be realized at a lower cost than a configuration including glass lenses. Furthermore, the first image transmission lens assembly 8A, which is configured using inexpensive plastic materials, can be realized in a single-use, so-called disposable form. This allows for a hygienic image transmission lens assembly to be provided. Furthermore, unlike glass lenses, the image transmission POF 1 made of plastic materials is easy to handle because it does not crack or break.

[0099] In this embodiment, the first image transmission lens assembly 8A has been described, but a configuration that includes the POF lens 7 obtained by removing the outer shell pipe 3 from the first image transmission lens assembly 8A, i.e., the image transmission POF 1, and the protective pipe 2 that covers the outer surface of the image transmission POF 1, can also be realized as a configuration suitable for image transmission and is within the scope of the present invention.

[0100] [Embodiment 2] Another image transmission lens assembly according to one aspect of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and their descriptions will not be repeated.

[0101] An external side view of the second image transmission lens assembly 8B of this embodiment is shown in Fig. 11. Also, Fig. 12 shows a cross section of the framed portion P including the distal end of the image transmission lens assembly 8B shown in Fig. 11, cut along the central axis of the outer shell pipe 3.

[0102] The second image transmission lens assembly 8B differs from the image transmission lens assembly 8A of the first embodiment in that it is in the form of a needle tube of a medical needle, as shown in Fig. 11. Note that although Fig. 12 does not show the distal end of the outer shell pipe 3 as being needle-shaped, the outer shell pipe 3 is in the form of a needle toward the left side of the page.

[0103] The outer shell pipe 3 realized as the needle tube of a medical needle can be a needle of a predetermined gauge. As an example, the needle tube of an 18-gauge medical needle (injection needle) can be used, but is not limited to this.

[0104] The second image transmission lens assembly 8B is provided with an illumination hollow fiber 4 (light guide) in the hollow portion of the outer shell pipe 3 whose distal end is needle-shaped, i.e., between the outer peripheral surface of the POF lens 7 and the inner peripheral surface of the outer shell pipe 3. In this respect as well, it differs from the first image transmission lens assembly 8A of the first embodiment described above.

[0105] The illumination hollow fiber 4 shown in Figure 12 is composed of multiple fibers, which are bundled and arranged between the outer peripheral surface of the POF lens 7 and the inner peripheral surface of the outer shell pipe 3. The illumination hollow fiber 4 is a light guide that guides light to be irradiated onto an illumination object, and a lighting device is connected to the base (not shown) of the illumination hollow fiber 4. Light emitted from the lighting device propagates through the illumination hollow fiber 4 toward the needle-shaped tip 8a of the second image transmission lens assembly 8B, and illumination light can be irradiated from the tip 8a toward the illumination object, i.e., the subject. In other words, the end of the light guide that emits light to be irradiated onto the illumination object is located at the needle tip of the needle tube.

[0106] In this embodiment, only the cover glass 5, which is located at the distal end of the POF lens 7, is disposed at the tip 8a of the second image transmission lens assembly 8B. However, instead of this, the cover glass 5 may be configured to be larger in the radial direction, and in that case, it may be configured so that a portion of it is open so as not to block the light emitted from the tip of the illumination hollow fiber 4.

[0107] The outer pipe 3 accommodates the POF lens 7 and the illumination hollow fiber 4 therein. In the state shown in Fig. 12, the central axis of the outer pipe 3 and the central axis C of the POF lens 7 are arranged concentrically and generally coincident with each other. However, the present invention is not limited to this, and the POF lens 7 may be eccentric with respect to the outer pipe 3 as shown in Fig. 13.

[0108] The outer shell pipe 3 will be further described with reference to Fig. 14. Fig. 14 is a side view of the outer peripheral surface of the outer shell pipe 3 in the framed portion Q including the proximal end of the second image-transmitting lens assembly 8B shown in Fig. 11. Fig. 13 shows only the outer shell pipe 3 in an extracted state.

[0109] 14, an opening 130 is provided on the circumferential surface of the outer pipe 3 near the proximal end. A portion of the illumination hollow fiber 4 (light guide) housed inside the outer pipe 3 is inserted into the opening 130. This allows the base (end of the light guide, not shown) of the illumination hollow fiber 4 described above to be pulled out to the outside of the outer pipe 3. A lighting device is connected to the pulled-out base.

[0110] Figure 15 shows a cross section of the framed portion Q including the proximal end of the image transmission lens assembly 8B shown in Figure 11, cut along the central axis of the outer shell pipe 3. In Figure 15, at the proximal end of the image transmission lens assembly 8B, an attachment part 9 which is a needle hub is fixed to the proximal end of the outer shell pipe 3 which is a needle tube.

[0111] The attachment part 9, which is the needle base, has an opening 90 at a location facing the opening 130 of the outer shell pipe 3, which is the needle tube. This allows the base (not shown) of the illumination hollow fiber 4 (the end of the light guide) to be drawn out from the opening 130 of the outer shell pipe 3 and the opening 90 of the attachment part 9.

[0112] Furthermore, the openings 130 and 90 have gaps formed between them and the illumination hollow fiber 4 sealed with a light-shielding material 95. Specifically, the light-shielding material 95 covers a portion of the illumination hollow fiber 4, including the portion through which the illumination hollow fiber 4 is inserted, while sealing the gaps formed between the openings 130 and 90 and the illumination hollow fiber 4. This blocks external light from entering the interior of the outer shell pipe 3 through the openings 130 and 90. The light-shielding material 95 may be, for example, a paint containing a black pigment, an adhesive, or other gap-filling material. Alternatively, light-shielding tape may be wrapped around the openings 130 and 90. Sealing the openings 130 and 90 with the light-shielding material 95 blocks external light from entering the interior of the outer shell pipe 3 through the openings 130 and 90. This prevents external light from affecting image transmission on the image-transmitting plastic optical fiber 1, providing an environment in which image transmission can be performed satisfactorily. As described above, the second image transmission lens assembly 8B of this embodiment also makes it possible to realize an image transmission lens assembly equipped with plastic optical fibers for image transmission, similar to the first image transmission lens assembly 8A of Embodiment 1. Furthermore, according to this embodiment, even if there is insufficient brightness in the location where the image to be transmitted can be acquired, i.e., the location where the subject (illumination target) is located, light can be irradiated by the illumination hollow fiber 4, thereby realizing a good imaging environment and contributing to the acquisition of a clear image.

[0113] Furthermore, with the second image transmission lens assembly 8B, like the first image transmission lens assembly 8A of embodiment 1, the image transmission POF 1 can be manufactured from inexpensive plastic materials. Therefore, it can be realized at a lower cost than a configuration including glass lenses. Furthermore, the first image transmission lens assembly 8B, which is constructed using inexpensive plastic materials, can be realized in a single-use, so-called disposable form. This allows for a hygienic image transmission lens assembly to be provided. For example, when attached to the tip of a rigid endoscope (described below), a new second image transmission lens assembly 8B can be attached after each use, providing a hygienic diagnostic environment. [Embodiment 3] A rigid endoscope and an image transmission system including the rigid endoscope according to one aspect of the present invention are described below. For convenience of explanation, components having the same functions as those described in the above embodiments are designated by the same reference numerals, and their descriptions will not be repeated.

[0114] The rigid endoscope of this embodiment includes, for example, the second image transmission lens assembly 8B described in the second embodiment at its tip. Fig. 16 is a diagram schematically illustrating the configuration of the rigid endoscope of this embodiment. The rigid endoscope 150 shown in Fig. 16 includes an endoscopic camera 160 and, for example, the second image transmission lens assembly 8B shown in Fig. 10 of the second embodiment. The second image transmission lens assembly 8B is removably attached to the light incident side end portion 160a of the endoscopic camera 160, and Fig. 16 shows the detached state. Note that the second image transmission lens assembly 8B can be replaced with the first image transmission lens assembly 8A, which does not include the illumination hollow fiber 4. The first image transmission lens assembly 8A can be applied as a medical needle by providing an attachment portion 9 on the proximal end of the outer shell pipe 3, similar to the second image transmission lens assembly 8B.

[0115] As described above, the second image transmission lens assembly 8B can be realized in a disposable form, and therefore, after being attached to the light incident side end 160a of the endoscopic camera 160 and used in a manner described below, it can be detached from the light incident side end 160a and discarded. This is the same even when the first image transmission lens assembly 8A is attached to the tip of the endoscopic camera 160.

[0116] The endoscopic camera 160 is equipped with a magnifying lens at the tip of the light incident end 160a, and light passing through the magnifying lens forms an image on a CMOS sensor (not shown). The endoscopic camera is not limited to this configuration, and any known endoscopic camera can be used. Figure 15 shows the light incident end 160a of the endoscopic camera 160 inserted into the end of the attachment part 9 of the second image transmission lens assembly 8B opposite the end connected to the outer shell pipe 3.

[0117] Fig. 17 illustrates the configuration of an image transmission system according to this embodiment. The image transmission system according to this embodiment is a system that transmits images of the inside of a patient's body to the outside of the body using the rigid endoscope described above. The image transmission system 200 shown in Fig. 17 includes the rigid endoscope 150 described above and a display unit 202.

[0118] 17 shows an example of use of the image transmission system 200, in which the inside of a knee joint 600n of a patient 600 is observed with an endoscope. In the example of Fig. 17, the tip of the needle of the second image transmission lens assembly 8B attached to the light incident end portion 160a of the endoscopic camera 160 is inserted into the gap between the patella and the patellar ligament of the knee joint 600n. In this state, the display unit 202 displays the state between the patella and the patellar ligament of the knee joint 600n via the rigid endoscope 150.

[0119] Based on the endoscopic image of the area between the patella and the patellar ligament displayed on the display unit 202, a doctor or technician can assess the condition of the area between the patella and the patellar ligament and, if necessary, perform some kind of treatment on the area between the patella and the patellar ligament. The rigid endoscope 150 illustrated in this embodiment does not include an instrument for performing the treatment. However, the instrument may be inserted into the outer pipe 3 of the second image transmission lens assembly 8B of the rigid endoscope 150. In this case, the instrument inserted into the outer pipe 3 may be pulled out to the outside through the opening 130 of the outer pipe 3 and the opening 90 of the attachment unit 9.

[0120] As described above, the rigid endoscope of this embodiment and the image transmission system including it can achieve good image transmission because it includes the first image transmission lens assembly 8A or the second image transmission lens assembly 8B illustrated in the above-described embodiments. The image transmission lens assembly 8B (8A) included in the rigid endoscope of this embodiment can form an image of an object on the opposite side in the same way as a relay lens, and by making the refractive index distribution of the image transmission POF 1 closer to the ideal distribution as described above, it is possible to transmit high-definition images.

[0121] In this embodiment, the rigid endoscope is used as a knee joint endoscope, but the present invention is not limited to this. It can be used as a rigid endoscope for other arthroscopy procedures, as well as a rigid endoscope for laparoscopy. It can also be used for endoscopy, where the endoscope is inserted percutaneously through an incision to enable viewing of the surgical site.

[0122] Furthermore, with the above-mentioned configuration, the aforementioned needle tube of the medical needle can be realized, thereby reducing the burden on the patient and improving diagnostic accuracy. Such effects also contribute to the achievement of Goal 3 of the Sustainable Development Goals (SDGs) advocated by the United Nations, "Ensure healthy lives and promote well-being for all at all ages."

[0123] [Additional Note 1] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0124] [Additional Note 2] The image transmission lens assembly in aspect 1 of the present invention comprises: an image transmission plastic optical fiber comprising a transparent light-guiding section having a refractive index distributed in a convex curve that is larger radially toward the center, and an uneven section formed on the surface of the light-guiding section to absorb light that reaches the surface from the inside of the light-guiding section; a protective pipe covering the outer surface of the image-transmitting plastic optical fiber, the protective pipe being made of a moisture-proof material; and an outer shell pipe that houses the image-transmitting plastic optical fiber covered by the protective pipe from the proximal end to the distal end.

[0125] According to the above-mentioned configuration, it is possible to realize an image transmission lens assembly equipped with a plastic optical fiber that can perform good image transmission.

[0126] The image transmission lens assembly of aspect 2 of the present invention is the same as that of aspect 1, further comprising a light guide housed inside the outer shell pipe for guiding light to be irradiated onto an object to be illuminated, and the outer shell pipe has an opening on the circumferential surface of the pipe near the proximal end of the image transmission plastic optical fiber, through which a portion of the light guide housed inside the outer shell pipe is inserted.

[0127] According to the above configuration, even if there is insufficient brightness in the location where the illumination object is located, the light guide can guide light to the location where the illumination object is located and irradiate the illumination object, thereby creating a good imaging environment and contributing to the acquisition of clear images.

[0128] In the image transmission lens assembly of Aspect 3 of the present invention, in the above-mentioned Aspect 2, a gap formed between the opening and the light guide is sealed with a light-shielding material.

[0129] According to the above configuration, the gap is sealed with a light-blocking material, thereby blocking the intrusion of external light from the outside to the inside of the outer shell pipe through the opening, thereby providing an environment in which image transmission can be carried out smoothly without affecting image transmission through the plastic optical fiber for image transmission due to the intrusion of external light.

[0130] In the image transmission lens assembly of Aspect 4 of the present invention, in Aspects 2 or 3, the light guide is configured by a hollow fiber arranged on the outer periphery of the image transmission plastic optical fiber.

[0131] According to the above configuration, the light guide made of the hollow fiber can guide the light to the location where the illumination object is located and irradiate the illumination object.

[0132] The image transmission lens assembly in Aspect 5 of the present invention is any one of Aspects 2 to 4, wherein the outer shell pipe is a needle tube of a predetermined gauge, and an end of the light guide that emits light to be irradiated onto the illumination object is disposed at the needle tip of the needle tube.

[0133] According to the above configuration, even if the illumination object is not located in a place exposed to the outside but in a place that can be reached by piercing with a needle tube (for example, inside the patient's body), the end of the light guide that emits light to be irradiated onto the illumination object is located at the tip of the needle tube, so that light can be irradiated onto the illumination object at the reached place, and an image of the illumination object can be captured well via the image transmission plastic optical fiber.

[0134] A rigid endoscope according to a sixth aspect of the present invention is provided with the image-transmitting lens assembly according to any one of the first to fifth aspects at its tip.

[0135] According to the above configuration, an image of an object to be observed by the endoscope can be transmitted satisfactorily, thereby realizing a highly accurate rigid endoscope.

[0136] An image transmission system according to a seventh aspect of the present invention is an image transmission system that transmits an image of the inside of a patient's body to the outside of the body using the rigid endoscope according to the sixth aspect, and includes a display unit that displays the image transmitted to the outside of the body.

[0137] According to the above configuration, endoscopic observation can be performed satisfactorily by using a high-precision rigid endoscope that can satisfactorily transmit an image of an object to be observed with the endoscope.

[0138] An embodiment of the present invention will be described below.

[0139] [Core Material Production Example 1] The following components were mixed in the amounts shown below and filtered through a membrane filter to prepare a raw material composition (1) for a core material.

[0140] Methyl methacrylate 3350.0 mg (3.346 x 10 －２ mol) Benzyl benzoate 502.5 mg (2.2367 × 10 －３ mol) 1,1-bis(t-hexylperoxy)cyclohexane 33.5 mg (1.058 × 10 －４ mol) n-octyl mercaptan 12.8975 mg (8.816 × 10 －５ mol) Trimethylolpropane trimethacrylate 0.1675 mg (4.9496 × 10 －７ mol) Next, the raw material composition (1) was placed in a cylindrical container having an inner diameter of 4 mm and a length of 350 mm, both ends of which could be sealed with caps, and caps were attached to both ends of the container.

[0141] Next, one end of the container was held, and the other end of the container was immersed in a water bath at 70 to 75°C, and heated for 1 to 1.5 hours.

[0142] Next, the container was removed from the hot water bath, and the thickened reaction liquid in the container was caused to flow in the axial direction of the container by inverting one end of the container and the other end of the container several times.

[0143] Next, one end of the container was grasped and the container was again submerged in the hot water bath to complete the reaction in the raw material composition (1). The cylindrical reaction product was then observed, and a portion substantially free of bubbles generated by polymerization shrinkage was cut out. In this way, a cylindrical core material (1) made of transparent acrylic resin was obtained. The core material (1) had a diameter of 4 mm and a length of 30 to 200 mm.

[0144] [Production Example 1 of Sheath Material] The following components were mixed in the amounts shown below and filtered through a membrane filter to prepare a raw material composition (2) for sheath material.

[0145] Methyl methacrylate 8548.0 mg (8.5378 x 10 －２ mol) Benzyl benzoate 410.3 mg (1.933 × 10 －３ mol) 1,1-bis(t-hexylperoxy)cyclohexane 85.5 mg (2.7017 × 10 －４ mol) n-octyl mercaptan 32.9 mg (2.249 × 10 －４ mol) Trimethylolpropane trimethacrylate 0.4274 mg (1.263 × 10 －６ mol) Next, the raw material composition (2) was placed in a cylindrical container having an inner diameter of 6 mm and a length of 460 mm, both ends of which could be sealed with caps, and both ends of the container were fitted with caps.

[0146] Next, one end of the container was held, and the other end of the container was immersed in a water bath at 70 to 75°C, and heated for 1 to 1.5 hours.

[0147] Next, the container was removed from the hot water bath, and the thickened reaction liquid in the container was caused to flow in the axial direction of the container by inverting one end of the container and the other end of the container several times.

[0148] Next, the container was placed on a rotating device in a heating chamber heated to 70 to 75° C. The rotating device was configured to hold both ends of the container to support the container horizontally and to be able to rotate the container around its axis.

[0149] Next, the container was heated in a heating chamber while rotating at a speed of 2000 rpm using a rotating device. Due to this rotation, the raw material composition (2) in the container was unevenly distributed on the inner circumferential surface of the container, forming a cylindrical cavity in the center of the container, and the reaction in the raw material composition (2) was completed in this state. Thus, a transparent acrylic cylindrical sheath material (1) was obtained. The inner diameter of the sheath material (1) was 4 mm, the thickness was 1 mm, and the length of the sheath material (1) was 450 mm.

[0150] [Measurement of refractive index] The refractive indexes of the core material (1) and the sheath material (1) were each measured. An index profiler ("IP-5500" manufactured by Seiko E&G Corporation) was used as the refractive index measurement device, and measurements were performed using the refraction angle distribution polarization function method under measurement wavelengths of 589.3 nm (D line), 486.0 nm (F line), and 656.3 nm (C line). As a result, the refractive index of the core material (1) was 1.5025 at the C line, 1.506 at the D line, and 1.513 at the F line, and the refractive index of the sheath material (1) was 1.4925 at the C line, 1.496 at the D line, and 1.502 at the F line.

[0151] [Preform Manufacturing Example 1] A core material (1) was inserted into a sheath material (1), thus obtaining a preform (1).

[0152] Next, the prepreform (1) was inserted into a metal circular tube having a length of 500 mm and an inner diameter of 6.5 mm. Furthermore, a coil spring was inserted into the circular tube, and caps were attached to both ends of the circular tube while the coil spring was pressed into the circular tube. The caps on both ends were clamped along the axial direction of the circular tube. In this way, the prepreform (1) was compressed and held in its axial direction.

[0153] Next, the circular tube held by the clamp is placed in a vacuum oven, and the inside of the oven is heated to 2 × 10 －１ The pressure was reduced to 100 Pa, the temperature was raised to 201°C, and this state was maintained for 7 hours.

[0154] Next, the temperature inside the oven was lowered to 77°C at a cooling rate of 10°C / hour, and then allowed to cool naturally to room temperature. In this way, a preform (1) was obtained in which the core material (1) and sheath material (1) of the prepreform (1) were integrated.

[0155] [Evaluation of Preform (1)] The radial refractive index distribution in the core of Preform (1) was measured. The aforementioned index profiler was used as the measuring device for this refractive index measurement. In addition, the refractive index was measured for each of three wavelengths of light: 486 nm, 589 nm, and 656 nm. The measurement temperature was 23.1 to 23.5°C.

[0156] As a result, for each wavelength in the core portion, the maximum value of the refractive index was the refractive index at the center in the radial direction, and the minimum value was the refractive index at the end in the radial direction of the core portion. The maximum value of the refractive index of the core portion at a wavelength of 486 nm was 1.510 to 1.512, and the minimum value was 1.5075 to 1.508. The maximum value of the refractive index of the core portion at a wavelength of 589 nm was 1.510 to 1.512, and the minimum value was 1.5075 to 1.508. The maximum value of the refractive index of the core portion at a wavelength of 656 nm was 1.5075 to 1.508, and the minimum value was 1.498 to 1.497.

[0157] The refractive index curves of the core portion at each wavelength are set to a curve where the refractive index difference Δn for each curve is 1 and the radius of the core portion is 1. Then, due to the influence of chromatic aberration of the dopant, Δn becomes slightly larger in the order of C line < D line < F line, but the shape of all curves is essentially the same. In other words, the distribution curves of all wavelengths are the same in that they are ideal distribution curves that enable image transmission. In all refractive index distribution curves, the curvature changes continuously and is within the range of 0.350 to 4.000.

[0158] Thus, for light of any wavelength, the core region had a refractive index that was distributed in a convex curve, with the refractive index increasing radially toward the center, and the outer layer region had a lower refractive index located on the outer periphery. Furthermore, the radial refractive index distribution in the core region showed a larger overall difference as the wavelength increased, and a refractive index that changed smoothly toward the edge.

[0159] Example 2: Example of production of stretched product The preform (1) produced in the above-mentioned Example 1 was heated and stretched using a heating and stretching apparatus 10 shown in FIG.

[0160] The preform (1) was fed into the heating furnace at a speed of 4.5 mm / min by the feeding section 30 of the heating and stretching apparatus 10. At this time, the heating furnace 20 was set so as to heat the preform to 300° C. The drawing section 40 clamped the excess section located within a length of 3 cm from the tip of the preform, and while gradually increasing the drawing speed, the drawn preform was finally drawn at a speed of 400 mm / min, thereby obtaining the drawn product (1).

[0161] The diameter (total diameter) of the drawn product (1) was measured to be 0.75 mm by the measuring unit 60 of the heating and drawing device 10.

[0162] As mentioned above, the refractive index distribution in the radial direction of the preform 1 is represented by a series of convex curves that change continuously from the center to not only the core portion but also the outer layer portion. The optical characteristics of the refractive index distribution represented by this convex curve are substantially the same optical characteristics (refractive index distribution) in the elongated product 1.

[0163] [Example 3: Example of manufacturing a plastic optical fiber for image transmission] The elongated product (1) manufactured in Example 2 above was cut to a length of 9 cm. In this example, the length of 9 cm corresponds to the length over which light incident on the tip of the plastic optical fiber for image transmission (core portion 101) is transmitted through the fiber for three pitches while tracing a sine curve. A well-known cutter for cutting fibers was used for cutting.

[0164] The drawn product (1) cut to a predetermined length was placed on sandpaper (waterproof paper DC-120 C13K (NCA product)) with abrasive grains of 150 μm diameter, and the drawn product (1) was lightly pressed against the sandpaper and rolled so that the central axis C of the drawn product (1) was parallel to the surface of the sandpaper, thereby scraping off the outer layer from the drawn product (1) and forming an uneven portion on the outer circumferential surface of the core portion exposed by the scraping off of the outer layer. In this way, a plastic optical fiber (1) for image transmission was obtained.

[0165] As described above, the radial refractive index distribution of the preform (1) and the elongated material (1) is represented by a series of convex curves that change continuously from the center to not only the core portion but also the outer layer portion. The optical characteristics of the refractive index distribution represented by this convex curve are substantially the same as those of the core portion of the preform (1) and the elongated material (1), even for the POF (1) that is composed only of the portion originating from the core portion by removing the portion originating from the outer layer portion.

[0166] [Example 4: Manufacturing example of image transmission lens assembly] This shows a manufacturing example of an image transmission lens assembly (1) including the image transmission plastic optical fiber (1) manufactured in the above-mentioned Example 3. In this example, the image transmission lens assembly (1) was realized in the form of the needle tube of a medical needle described above as the second image transmission lens assembly 8B.

[0167] First, a 9 cm long plastic optical fiber (1) for image transmission was inserted into a protective pipe made of stainless steel (SUS) to prepare a POF lens (1). The length of the protective pipe was 9 cm, the same as that of the plastic optical fiber (1) for image transmission. The inner diameter of the protective pipe was large enough to allow the plastic optical fiber (1) for image transmission to be inserted without difficulty. The outer diameter of the POF lens (1) was 0.5 mm.

[0168] Next, the POF lens (1) was inserted into the hollow portion of the illumination hollow fiber, and the POF lens (1) inserted into the illumination hollow fiber was then inserted into an outer shell pipe (outer diameter 1.25 mm) which was the needle tube of an 18-gauge medical needle (injection needle), to produce an image transmission lens assembly (1). That is, the illumination hollow fiber was arranged between the POF lens (1) and the outer shell pipe.

[0169] Here, the aforementioned opening 130 ( FIG. 15 ) is provided in the outer shell pipe, which is the needle tube of an 18-gauge medical needle (injection needle). An opening 90 ( FIG. 15 ) is also provided in the attachment part 9 provided on the needle hub of the outer shell pipe, which is the needle tube. The openings 130 and 90 are provided in overlapping positions, and the illumination hollow fiber inserted inside the outer shell pipe is pulled out through the openings 130 and 90, and the gaps between the openings 130 and 90 are filled with a light-blocking material made of an adhesive containing a black pigment. This resulted in an image transmission lens assembly (1).

[0170] Since the POF (1) shown in Example 3 has substantially the same optical characteristics (refractive index distribution) as the core portion of the preform (1) and the elongated material (1), the POF lens (1) provided with the POF (1) also has the same optical characteristics (refractive index distribution) as the POF (1). The optical characteristics of the POF lens (1) were measured as follows.

[0171] Measurement of refractive index of POF lens (1) An index profiler ("IP-5500" manufactured by Seiko E&G Corporation) was used as the refractive index measurement device, and measurements were performed by the refraction angle distribution polarimetric method under measurement wavelengths of 486.0 nm (F-line), 589.3 nm (D-line), and 656.3 nm (C-line). Fig. 18 is a graph showing the measurement results at a measurement wavelength of 486.0 nm (F-line), Fig. 19 is a graph showing the measurement results at a measurement wavelength of 589.3 nm (D-line), and Fig. 20 is a graph showing the measurement results at a measurement wavelength of 656.3 nm (C-line).

[0172] 18 to 20, the refractive index is distributed in an upwardly convex curve within the range of the core portion of the POF (1) of the POF lens (1) (within a radius of 2 mm), and the value at ±2 on the horizontal axis of the graft (i.e., the outer periphery of the core portion with a radius of 2 mm) is the lower limit of the refractive index. In each graph, the range of the refractive index of the curved distribution was approximately 1.5075 to 1.5102 at a wavelength of 486 nm, approximately 1.5009 to 1.5033 at a wavelength of 589 nm, and approximately 1.4972 to 1.5002 at a wavelength of 656 nm.

[0173] In the POF (1) of the POF lens (1) of this example, the refractive index of the center is sufficiently larger than the refractive index of the peripheral edge. By providing such a refractive index difference, an image transmission lens assembly (1) including the POF lens (1) can be provided in a rigid endoscope, thereby enabling good transmission of images inside a living body.

[0174] In addition, within the range of the curved distribution, the curve (change in refractive index) was generally gentle. The curved distribution of refractive index in the radial direction can be designed by a known method for POF.

[0175] REFERENCE SIGNS LIST 1 Image transmission plastic optical fiber 2 Protection pipe 3 Outer shell pipe 4 Illumination hollow fiber 7 POF lens 7a Light incident surface 7b Light exit surface 8A First image transmission lens assembly 8B Second image transmission lens assembly 9 Attachment section 90 Opening 95 Light blocking material 101 Core section (light guiding section) 101k Concave and convex section 130 Opening 150 Rigid endoscope 160 Endoscope camera 160a Light incident side end section 200 Image transmission system 202 Display section

Claims

1. A lens assembly for image transmission comprising: a plastic optical fiber for image transmission, the plastic optical fiber comprising a transparent light guiding section having a refractive index distributed in a convex curve shape that is larger toward the center in the radial direction, and a concave-convex section formed on the surface of the light guiding section to absorb light that reaches the surface from inside the light guiding section; a protective pipe covering the outer surface of the plastic optical fiber for image transmission, the protective pipe being made of a moisture-proof material; and an outer shell pipe that houses the plastic optical fiber for image transmission covered by the protective pipe from the proximal end to the distal end.

2. The image transmission lens assembly according to claim 1, further comprising a light guide housed inside said outer shell pipe for guiding light to an illumination object, said outer shell pipe being provided with an opening on the circumferential surface of the pipe on the side of the proximal end of said plastic optical fiber for image transmission, through which a portion of said light guide housed inside said outer shell pipe is inserted.

3. The image transmission lens assembly according to claim 2, wherein a gap formed between the opening and the light guide is sealed with a light-shielding material.

4. An image transmission lens assembly according to claim 2 or 3, wherein the light guide is composed of a hollow fiber arranged on the outer periphery of the image transmission plastic optical fiber.

5. An image transmission lens assembly as claimed in claim 2 or 3, wherein the outer shell pipe is a needle tube of a specified gauge, and an end of the light guide that emits light to be illuminated on the object to be illuminated is disposed at the tip of the needle tube.

6. A rigid endoscope having an image transmitting lens assembly according to any one of claims 1 to 5 at its tip.

Images