JP2011081323A - Microlens optical system having super-high depth of field - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To observe an image having a super-high depth of a field characteristic through an optical fiber by designing a microlens for an endoscope by using a high-performance compound lens including an aspherical surface having a refractive curved surface. <P>SOLUTION: An optical system is designed to form an image by forming an appropriate gap between the microlens including an aspherical surface having an outside diameter of about ϕ0.3 to 0.5 and the optical fiber without directly fixing the microlens on a surface of the optical fiber by using an adhesive as in a conventional manner. Thus, a favorable wide-angle image whose object distance is from 1 mm to infinity is observed through the optical fiber. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a microlens that enables observation of ultra-high depth of field characteristics using a compound lens including an aspheric lens in an observation system using an optical fiber such as a catheter endoscope.

  Conventional ultra-fine outer-diameter microlenses use rod lenses that change the internal refractive index distribution produced by vapor deposition including a chemical vapor deposition method so that light can be collected. Or, lenses using spherical microlenses by grinding are used, but an objective lens system with a wide-angle optical characteristic with sufficient depth of field and high resolution is impossible, so observation including medical treatment is also possible In the field, the close-up magnified image was not in focus, and a sufficient image observation effect could not be expected.

“Selfoc Lens Array” published by Nippon Sheet Glass. “Image Scanner Development Office” by iMeasure Co., Ltd., NIFTY Cocolog.

  In conventional narrow tube endoscopes using rod lenses and spherical lenses, it has been difficult to provide a wide range of focus adjustment mechanisms. Therefore, an object of the present invention is to create a fixed focal length lens that transmits an image at a long working distance from close-up and high-magnification photography to infinity in an optical system of a completely different system.

  In the method using the rod lens and the spherical microlens described above, a method in which the lens end is directly bonded to the surface of the optical fiber is generally used. However, in the present invention, in order to easily increase the magnification, the lens end and the fiber are connected to each other. We considered a mechanism that forms an image with an appropriate interval on the end face. This image observation system is characterized in that a microlens system and an optical fiber are simultaneously incorporated in one narrow tube, and their relative positions (spaces) are adjusted to match the imaging positions. The objective lens to be used is a compound lens system that includes an aspherical surface, and it is possible to transfer images with ultra-high depth of field, wide angle, and high resolution characteristics in addition to high magnification by lens design that properly considers the above space. To.

  The conventional optical system has a very shallow depth, so it cannot be focused when the close-up lens is brought close to it, and close-up enlargement photography is difficult especially in the medical department. When a lens system based on a completely new idea according to the present invention is used, high-resolution and wide-angle observation is possible from a proximity distance of 1 mm to a state close to infinity. That is, in the conventional method, for example, even if the affected part of the lesion is enlarged, the image is unclear and cannot be clearly determined, and the effect of observing the image is often not achieved. This makes it possible to make a diagnosis based on a clear image by proximity enlargement. Since such a system can be performed without excising the affected area, it is a very effective means for endoscopic imaging of a thin tube part in the body. In addition, the use of this lens can be useful for all industrial sectors that require high-magnification observation of the capillary tube.

(A) Composition of a capillary endoscope with two lenses including an aspherical surface according to the present invention (b) Composition of a conventional capillary endoscope using a rod lens Layout of the imaging system with two objective micro objective lenses Layout of the triple objective micro objective imaging system MTF transfer characteristics of two-piece micro objective lens Spot image of two-piece micro objective lens Wavefront aberration of two-piece micro objective lens Chromatic aberration, distortion, and relative illuminance characteristics of a two-lens micro objective lens MTF transfer characteristics of a three-lens micro objective lens Spot image of a 3 objective micro objective lens Wavefront aberration of three objective micro objective lens Chromatic aberration, distortion, and relative illuminance characteristics of a three-lens micro objective lens

  Embodiments will be described with reference to the drawings. FIG. 1A is a composition of a catheter endoscope according to the present invention. Light beams from an object shown on the left are condensed on an optical fiber 2 by an objective lens system 1 using a plurality of microlenses, and an image is transmitted. . These elements are housed inside a fixed tube 3 that is particularly structurally devised in order to accurately maintain the distance between the lenses according to the lens design and the distance from the lens end face to the optical fiber. Although the surroundings are not shown in the figure, there are many cases where a light guide fiber or a washing tube is provided. Further, the outside has a mechanism protected by the clothing tube 4. In the present invention, by providing a relatively long space between the rear end of the lens and the image plane in this way, it is possible to design a lens for increasing the magnification and the depth of field by taking advantage of the characteristics of the aspherical lens. To do. On the other hand, FIG. 1B shows a composition using a rod lens 5 mainly used in a conventional catheter endoscope. The rod lens is characterized by a finer outer dimension, and a chemical vapor phase. It is a product made by vapor deposition including the method. By changing the internal refractive index distribution, light is transmitted by drawing a periodic arc as shown in the figure. In this case, since the end face of 5 is usually the image plane, a method of sticking to the entire surface of the optical fiber 2 with the adhesive 6 has been employed. The characteristic of the rod lens is that it has a low resolution and is about 8 lenses / mm at the maximum, and the depth of field at that time is said to be about ± 0.4 mm or less. In this method, for example, with a catheter designed for an object position of 20 mm, a detailed image of the affected area cannot be captured by close-up imaging of 2 to 3 mm, and accurate diagnosis is impossible. In some optical systems processed with spherical lenses, the depth is improved to about ± 2 mm. However, it is inevitable that the peripheral resolution deteriorates without correcting aberrations for wide-angle incidence. In order to improve flexibility and operability, these methods take a method in which the entire length is shortened without much consideration of optical aberrations, and the lens is closely attached to the optical fiber.

  FIG. 2 is a layout diagram of the lens design according to the present invention, which is based on a micro objective lens having two lenses, and FIG. 3 is a micro objective lens having three lenses. The unit of length of the lens system used below is all mm. In FIG. 2, the front lens 7 is designed so that the lens outer diameter is small and within φ0.38. When a meniscus concave lens such as that found in a normal wide-angle camera is used, the front lens becomes extremely large in shape and is not suitable for the outer diameter of a thin tube catheter. Therefore, the front lens designed in the present invention is a biconcave lens having a small curvature consisting of R1 = −0.39, R2 = 0.55, and a center thickness of 0.3, and has a large negative power at a focal length f1 = −0.33. In addition, the lens distance d1 is 0.5 and the back focus distance dN is 1.0, which does not use a telecentric parallel light imaging system in which dN is almost zero as seen in the conventional design. The condensing lens 8 has a center thickness of 0.4, curvature is R3 = 0.890, R4 = -0.781, has a curvature suitable for cutting tools for mass production of microlenses, and has an aspherical surface considering up to the 16th order higher order term. It has a superior optical aberration using a lens. The focal length of the lens 8 is f2 = 0.53. The focal length of the entire two-lens system is f = 0.33, the maximum viewing angle is 90 degrees in all angles, and it has excellent resolution and optics with depth of field. In the figure, the distance between lenses is d1 = 0.51, and the back focus is dN = 1.0. The characteristic diagrams are described in detail in FIGS. 4, 5, 6 and 7, which will be described later. However, the condition according to the present invention is similar to that of the two-lens microlens having the same imaging performance, and −0.4 ≦ f1 ≦. It is defined in the range of −0.2, 0.3 ≦ f 2 ≦ 0.7, 0.5 ≦ d N ≦ 1.5, and the lens diaphragm is placed immediately before the condenser lens 8 (the distance between the diaphragm and 8 is zero). The overall focal length satisfies the condition of 0.3 ≦ f ≦ 0.5.

  FIG. 3 shows an example of image formation by a three-lens microlens, which is based on the design concept shown in FIG. 2, but by adding another lens at an intermediate position, resolution characteristics including MTF can be obtained. It can be improved further. In the example of FIG. 3, the total viewing angle is 90 degrees and the lens outer diameter is φ0.38 as in FIG. If the outer diameter is φ6, the light beam is not kicked at a wide angle, and a high depth of field and high resolution can be maintained even when the total viewing angle is 100 degrees. In the example of FIG. 3, the front lens 7 is a biconcave lens having a small curvature of R1 = −0.371 and R2 = 0.273, the center thickness of the lens is 0.4, and the focal length is f1 = −0.204, which is the same as the two-lens system. The condition of −0.4 ≦ f1 ≦ −0.2 is satisfied. The intermediate lens 9 is a convex lens made of a spherical surface, R3 = 1.03, R4 = -0.443, and the center thickness is 0.3. The condensing lens 8 in FIG. 3 is an aspherical lens having a curvature of R5 = 2.404 R6 = -2.263 and including a 16th-order high-order term, and its center thickness is 0.4. Assuming that the combined focal length of the intermediate lens 9 and the condenser lens 8 is f23, f23 = 0.578, which is substantially equivalent to the focal length f2 of the condenser lens in the case of two lenses. The distance between lenses d1 is 0.3, d2 is 0.3, and the back focus distance dN is 1.0. Further, the focal length of the entire three-lens system in the figure is f = 0.34. The characteristics of this lens are specified in detail in FIGS. 8, 9, 10, and 11 to be described later. The conditions according to the present invention having similar imaging performance are 0.3 ≦ f23 ≦ 0.9, 0.5 ≦ dN. Defined within the range of ≦ 1.5. The lens diaphragm is characterized by being placed immediately before the lens 8 (the distance between the diaphragm and 8 is zero), and the overall focal length satisfies 0.3 ≦ f ≦ 0.5.

  FIG. 4 shows the resolution characteristics of the two-lens microlens optical system according to the MTF transfer function. The upper part of the figure shows the characteristics at the time of close-up close-up photography, and the distance from the lens tip to the observation object is a The upper row shows the characteristics when a = 1, the lower row shows the characteristics when separated from the observation object, and the time when a = 1000. Although not shown in this figure, when the distance exceeds a = 15, the characteristics are almost the same as at infinity. The straight line marked at the top of the figure shows the diffraction limit curve of the aberration-free lens. Here, the portions written in T and S fine letters are identification symbols when ray tracing is performed for Sagital (spherical image plane) and Tangential (meridian image plane), respectively. From this figure, the characteristics and the characteristics are remarkably improved as compared with the case of using the conventional rod lens and spherical lens. In the lens system according to the present invention, the MTF curve hardly changes from the proximity of a = 1 to infinity. I understand. Even with a normal wide-angle lens, several lens configurations are required to give a full viewing angle of 90 degrees or more. In the present invention, an image is clearly observed up to a wide-angle field of view with only two lenses. Is possible. The vertical axis indicates the MTF value, the maximum value is 1.0, and the horizontal axis indicates the number of resolutions per mm. Normally, when the point where the MTF reaches about 30% is used as a reference, the horizontal axis in this figure is displayed with a maximum of 120 lines / mm.

FIG. 5 shows the spot shape focused on the image plane. The upper part shows the shape when the distance to the object is a = 2, and the lower part shows the shape when a = 20. In each figure, spot images are shown when the viewing angles are 0, 10, 20, 30, 35, 40, and 45 degrees in order from the upper left to the lower right. One block is 20 μm, and in these figures, the spot diameter is very small and sufficient convergence of the light beam is obtained. It can be said that the coma aberration is small even at a wide angle.
Figure 6 shows the wavefront aberration shape. The top row is when the distance from the observation object is a = 2 and the incident angle is 0 °. The wavefront aberration RMS is 0.045λ (λ = 0.587μm), and the bottom row is when a = 20. When the incident angle is 0 °, the RMS of the wavefront aberration is 0.120λ. Note that. Even at an incident angle of 45 degrees, at a = 2, the RMS is 0.146λ, and the wavefront aberration is within the Rayleigh quarter wavelength rule. That is, the image degradation is extremely small even in the vicinity.
FIG. 7 shows other aberrations. The left side of the upper diagram shows field curvature and chromatic aberration when a = 2, and the right side shows distortion aberration, which is a maximum of 44.2% with respect to an incident angle of 45 degrees. Even if a is almost infinite, the degree of distortion hardly changes. The lower diagram of FIG. 7 shows the relative illuminance with respect to the incident angle when a = 20. Even at the edge of the screen at an incident angle of 45 degrees, 96.5% of the illuminance at the center can be obtained. This illuminance has almost no change in characteristics from close to infinity.

  FIG. 8 shows the characteristics of the resolution by the MTF transfer function of the three-lens microlens optical system. The upper part of FIG. The lower diagram shows the characteristics when a = 20 from the observation object. The horizontal axis in the figure shows the maximum resolution of 150 lines / mm. Compared to the MTF in FIG. 3, it can be seen that the optical characteristics are improved by the effect of the three lenses, and the resolution is closer to the diffraction limit. In particular, when a = 2, the diffraction limit is approached most, but almost the same performance is maintained until a = 3, which is normally used. Above a = 20, it is considered close to infinity with respect to these proximity.

Fig. 9 shows the spot shape that is focused on the image plane by the same three-lens microlens optical system as above. The upper diagram shows the shape when the distance to the object is a = 2 and the lower diagram shows the shape when a = 20. Indicates. Each block shows a spot image when the viewing angle is 0, 10, 20, 30, 40, and 45 degrees, respectively.
FIG. 10 shows the wavefront aberration shape in the case of the same three-lens configuration as above, and the upper diagram shows the wavefront obtained when the angle of incidence is 0 ° when a = 2 and the RMS of the wavefront aberration is 0.023λ, and the lower diagram shows a When the angle is 20 and the incident angle is 0 °, the wavefront obtained by the wavefront has an RMS of 0.101λ. Also in this figure, the improvement in aberration is seen compared to the case of two lenses.
FIG. 11 shows other aberrations in the case of the same three-lens configuration as above, the upper left figure shows field curvature and chromatic aberration when a = 20, the right figure shows distortion aberration, and a maximum of 46.4 for an incident angle of 45 degrees. %. Distortion is almost unchanged even from close to infinity. The chromatic aberration is improved compared to the two-lens system. Although the field curvature aberration is deteriorated, it can be improved by changing the curvature of the lens, and hardly causes a problem. The lower row shows the relative illuminance with respect to the incident angle when a = 20. Even at the edge of the screen with an incident angle of 45 degrees, an illuminance of 97.6% in the center can be obtained. In these lens systems, the front lens and the intermediate lens use spherical shapes, so that cutting is possible even with an outer diameter of about φ0.3, and commonly used glass materials such as BK7 and SF66 are used. However, since the condensing lens uses an aspherical surface considering the 16th order higher order term, it is impossible to cut the microlens, and the molding process is performed by using the latest low melting point and high refractive index glass material.

  In the case of a two-lens system, the total magnification m is m = -0.148 when a = 2, and the resolution of the image plane is 18 to 18 when the resolution on the object plane in FIG. A resolution of about 22 lines / mm can be obtained. In the case of a three-lens system, when a = 2, m = −0.153 and the resolution on the object plane is 150 to 200 lines / mm, the image plane resolution is 23 to 30 lines / mm. This means that a resolution about three times as high as the actual measurement value of 9 lines / mm obtained at the best position with a shallow depth of field using a rod lens is obtained. Actually, it is said that the image formation by the conventional lens has a depth of field of about ± 0.4 mm or less at most. In this state, it is difficult to compare with the depth characteristics from 1 mm to infinity according to the present invention, and the conventional microlens has the spot focusing as shown in FIGS. 5 and 9 and the wavefront aberration of FIGS. It's hard to argue, and if you move it a little away from the focal point, the image will soon become blurry. Further, the conventional method has a mechanism for adhering the lens to the end face of the optical fiber, including the use of a spherical microlens, and therefore an optical system considering the back focus of the lens according to the present invention is not considered. . This is considered to be a convenient method for shortening the length of the lens system and improving the flexibility inside the narrow tube including the inside of the body. In the present invention, the total length of the two-lens and three-lens configurations is in the range of about 2.2 to 2.7 mm. On the other hand, in the case of commercially available wide-angle lenses for video, there are many lens systems having several lens configurations and at least a total length of 30 to 40 mm and an outer diameter of φ20 to 30. In the present invention, the optical system has a very small micro shape compared to these and an excellent optical system with an extremely high depth of field.

  Regarding the actual application of the present invention, even if the total length is about 2 to 3 mm as described above, the optical fiber imaging system of FIG. 1 and the catheter endoscope have sufficient specifications, and high performance images can be obtained at any position. It is possible to observe. (For example, the length of a normal direct endoscope is on the order of 10 to 20 cm). In the case of FIG. 1, in order to fix the optical system, the fixing tube 3 may be made of stainless steel. However, in the aspheric lens system according to the present invention, even if there is an influence on the tilt and tilt between the lenses, it is considered that the performance may be maintained as compared with the case of using a rod lens. It is also possible to mechanically devise a lens mounting structure having a slight flexibility by using a resin tube without using the metal tube as described above. Further, the feature of the present invention is that the resolution can be maintained up to a wide angle even with only two lens configurations. Usually, if the number is small, sufficient MTF characteristics cannot be obtained at a wide angle, and an MTF curve due to peripheral blur, chromatic aberration, etc. The phenomenon of sudden drop in response is often seen. Further, when a wider angle is required, the entire viewing angle of 110 degrees or more is possible by using a lens having a large outer diameter and a diameter of about 0.7 mm in order to eliminate kicking of the light beam. In addition, the distortion aberration can be grasped from the characteristic diagrams shown in FIGS. 7 and 11 and an accurate shape can be grasped with less distortion of about 10% in the image of the region where the total viewing angle is within 40 degrees. This performance can greatly contribute not only to the medical field but also to many other industrial fields that require observation, for miniaturization of lenses and microlenses.

1 Objective Lens System 2 Optical Fiber 3 Fixed Tube 4 Clothing Tube 5 Rod Lens 6 Adhesive 7 Front Lens 8 Condensing Lens 9 Intermediate Lens

Claims (4)

  The distance from the rear end of the condenser lens closest to the imaging plane to the imaging plane (back focus) in an endoscope objective lens having a plurality of aspherical lenses and a lens having a diaphragm with an outer diameter of 1 mm or less The dN satisfies the range of 0.5 mm ≦ dN ≦ 1.5 mm, and the front lens first arranged on the object side is a double-sided concave lens, and its focal length f1 is −0.4 mm ≦ f1 ≦ −. A microlens optical system having characteristics in a range of 0.2 mm and a combined focal length f of the entire lens system in a range of 0.3 mm ≦ f ≦ 0.5 mm. The endoscope objective lens according to claim 1 is composed of two lenses, the condenser lens is an aspheric lens, the focal length f2 satisfies 0.3 mm ≦ f2 ≦ 0.7 mm, and the aperture is An optical system characterized by being provided immediately before the condenser lens.   The objective lens for an endoscope according to claim 1, which is composed of three lenses, is provided with an imaging intermediate lens in front of the condenser lens, and a combined focal length f23 of this lens and the condenser lens is 0.3 mm. ≦ f23 ≦ 0.9 mm is satisfied, and an aperture is provided immediately before the condenser lens.   In an endoscope objective lens composed of two to three lenses, the focal length of the lens satisfies the conditions of f1, f2, f23, and f shown in the above claims, and the outer diameter of the lens is 0.5 mm or less. In addition, it has a wide-angle characteristic with a total viewing angle of 90 degrees or more, and a wavefront aberration of ¼ wavelength or less until the working distance from the lens tip to the object surface is 1 mm to infinity. An optical system characterized by high depth-of-field characteristics that preserve the characteristics.

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