Optical adapter and medical endoscope

Abstract

The utility model relates to an optical adapter and a medical endoscope. The optical adapter sequentially comprises a first cemented lens group with positive focal power from an object side to an image side along an optical axis, the first cemented lens group comprises a first lens and a second lens which are cemented, and the object side surface of the first lens is a convex surface; the third lens has negative focal power, and the image side surface of the third lens is a concave surface; the fourth lens structure has positive focal power, and the image side surface of the fourth lens structure is a convex surface; and the fifth lens has negative focal power, and the image side surface of the fifth lens is a convex surface. The optical adapter is favorable for correcting various aberrations such as chromatic aberration and distortion, improving the relative illumination of imaging and improving the imaging resolution, thereby being favorable for improving the imaging quality of the optical adapter.

Description

Technical Field

[0001] This application relates to the field of endoscope technology, and in particular to an optical adapter and a medical endoscope. Background Technology

[0002] A medical endoscope is a medical device that can be inserted into the human body for observation, diagnosis, or treatment. A typical medical endoscope includes an endoscope image processing unit, a light source, an optical adapter, and an endoscope itself. The endoscope is inserted into the body to acquire images. The optical adapter connects the endoscope and the image sensor of the endoscope image processing unit, enabling the adjustment and transmission of light to transfer the images acquired by the endoscope to the image sensor. However, current optical adapters generally suffer from poor image quality. Utility Model Content

[0003] Therefore, it is necessary to provide an optical adapter and medical endoscope to address the problem of poor imaging quality of current optical adapters.

[0004] An optical adapter, comprising, along the optical axis from the object side to the image side, the following components in sequence:

[0005] A first cemented lens assembly with positive optical power, the first cemented lens assembly including a first lens and a second lens cemented together, the object side of the first lens being convex.

[0006] A third lens with negative optical power, wherein the image-side surface of the third lens is concave;

[0007] A fourth lens structure with positive optical power, wherein the image-side surface of the fourth lens structure is convex;

[0008] A fifth lens with negative optical power, wherein the image-side surface of the fifth lens is convex.

[0009] The optical adapter described above features a well-designed lens with appropriate power and surface shape. Combined with the design of the cemented lens assembly, this design helps correct various aberrations such as chromatic aberration and distortion, improves relative illumination, and enhances image resolution. This, in turn, improves the image quality of the optical adapter. When used in medical endoscopes, this design enhances the accuracy of diagnosis and treatment. It also helps to reduce the size of the optical adapter, making it easier for users to operate and hold the medical endoscope for extended periods.

[0010] In one embodiment, the fourth lens structure has one lens with optical power, and the object side of the fourth lens structure is convex.

[0011] In one embodiment, the optical adapter satisfies the following condition:

[0012] 6.5 ≤ f2 / (f2+f3) ≤ 7.5;

[0013] 31.5≤f4 / CT4≤32.5;

[0014] 0.2≤CT5 / T45≤0.8;

[0015] Wherein, f2 is the focal length of the first cemented lens group, f3 is the focal length of the third lens, f4 is the focal length of the fourth lens structure, CT4 is the thickness of the fourth lens structure on the optical axis, CT5 is the thickness of the fifth lens on the optical axis, and T45 is the distance on the optical axis from the image side of the fourth lens structure to the object side of the fifth lens.

[0016] In one embodiment, the optical adapter further includes a light-transmitting protection element disposed on the object side of the first cemented lens assembly, and the optical adapter satisfies the following condition:

[0017] 0.6≤CT2 / (T12+T23)≤0.8;

[0018] Wherein, CT2 is the thickness of the first cemented lens assembly on the optical axis, T12 is the distance on the optical axis from the image side of the light-transmitting protective element to the object side of the first cemented lens assembly, and T23 is the distance on the optical axis from the image side of the first cemented lens assembly to the object side of the third lens.

[0019] In one embodiment, the fourth lens structure includes a first sub-lens and a second sub-lens bonded together. The first sub-lens has positive optical power and both the object-side and image-side surfaces are convex, while the second sub-lens has negative optical power and the object-side surface is concave.

[0020] In one embodiment, the optical adapter satisfies the following condition:

[0021] 1.6≤f2 / (f2+f3)≤2;

[0022] 7.3≤f4 / CT4≤7.9;

[0023] 0.3≤CT5 / T45≤0.6;

[0024] Wherein, f2 is the focal length of the first cemented lens group, f3 is the focal length of the third lens, f4 is the focal length of the fourth lens structure, CT4 is the thickness of the fourth lens structure on the optical axis, CT5 is the thickness of the fifth lens on the optical axis, and T45 is the distance on the optical axis from the image side of the fourth lens structure to the object side of the fifth lens.

[0025] In one embodiment, the optical adapter further includes a light-transmitting protection element disposed on the object side of the first cemented lens assembly, and the optical adapter satisfies the following condition:

[0026] 0.8≤CT2 / (T12+T23)≤1.2;

[0027] Wherein, CT2 is the thickness of the first cemented lens assembly on the optical axis, T12 is the distance on the optical axis from the image side of the light-transmitting protective element to the object side of the first cemented lens assembly, and T23 is the distance on the optical axis from the image side of the first cemented lens assembly to the object side of the third lens.

[0028] In one embodiment, the optical adapter satisfies the following condition:

[0029] 5°≤Semi-FOV≤10°;

[0030] 25mm≤f≤30mm;

[0031] Wherein, Semi-FOV is half of the maximum field of view of the optical adapter, and f is the focal length of the optical adapter.

[0032] In one embodiment, the first lens has negative optical power and its image-side surface is concave; the second lens has positive optical power and its object-side surface is convex; the third lens has a concave object-side surface; and the fifth lens has a concave object-side surface; and / or,

[0033] The optical adapter also includes an aperture stop, which is located on the object side of the first cemented lens assembly.

[0034] A medical endoscope includes an optical adapter as described in any of the above embodiments. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the optical adapter in the first embodiment.

[0036] Figure 2 This is a transfer function curve of the optical adapter in the first embodiment.

[0037] Figure 3 This is a defocus curve diagram of the optical adapter in the first embodiment.

[0038] Figure 4 This is a dot diagram of the optical adapter in the first embodiment.

[0039] Figure 5 This is a field curvature and distortion curve diagram of the optical adapter in the first embodiment.

[0040] Figure 6This is a schematic diagram of the optical adapter in the second embodiment.

[0041] Figure 7 This is a transfer function curve of the optical adapter in the second embodiment.

[0042] Figure 8 This is a defocus curve diagram of the optical adapter in the second embodiment.

[0043] Figure 9 This is a dot diagram of the optical adapter in the second embodiment.

[0044] Figure 10 This is a field curvature and distortion curve diagram of the optical adapter in the second embodiment.

[0045] Figure label:

[0046] 10. Optical adapter; E1. Light transmission protection element; S3. Aperture stop; E2. First cemented lens group; E21. First lens; E22. Second lens; E3. Third lens; E4. Fourth lens structure; E41. First sub-lens; E42. Second sub-lens; E5. Fifth lens; S13. Imaging plane. Detailed Implementation

[0047] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0048] In the description of this application, it should be understood that if terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" appear, these terms indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0049] Furthermore, where the terms "first" and "second" appear, these terms are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, where the term "multiple" appears, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0050] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0051] In this application, unless otherwise expressly specified and limited, the use of descriptions such as "above" or "below" the second feature indicates that the first and second features are in direct contact or indirect contact via an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. Similarly, "below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0052] It should be noted that if an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. If an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. If so, the terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in this application are for illustrative purposes only and do not represent the only possible implementation.

[0053] Please see Figure 1 , Figure 1This is a schematic diagram of the optical adapter 10 in the first embodiment. The optical adapter 10 provided in this application can be used in medical devices, such as in medical endoscopes. In some embodiments, the medical endoscope may further include an endoscope mirror and an image sensor. The optical adapter 10 is disposed between the endoscope mirror and the image sensor. The endoscope mirror can at least partially extend into the human body to collect images of the lesion area. The light collected by the endoscope mirror can be adjusted and transmitted by the optical adapter 10 and then transmitted to the image sensor. The optical adapter 10 plays a connecting and adapting role between the endoscope mirror and the image sensor to improve the imaging quality of the medical endoscope.

[0054] Further, in some embodiments, the optical adapter 10 includes, along the optical axis from the object side to the image side, a first cemented lens group E2, a third lens E3, a fourth lens structure E4, and a fifth lens E5. The optical adapter 10 has an imaging surface S13, and light from the object side can be projected onto the imaging surface S13 after being adjusted by each lens in sequence. The first cemented lens group E2 includes a first lens E21 and a second lens E22 cemented together. The second lens E22 is disposed on the image side of the first lens E21. The first cemented lens group E2 has positive optical power, and the object side is convex. In other words, the combined optical power of the first lens E21 and the second lens E22 is positive, and the object side of the first lens E21 is convex. The third lens E3 has negative optical power, and the image side of the third lens E3 is concave. The fourth lens structure E4 contains one or two lenses with optical power; in other words, the optical adapter 10 contains five or six lenses with optical power. The fourth lens structure E4 has positive optical power, and its image-side surface is convex. The fifth lens E5 has negative optical power, and its image-side surface is also convex.

[0055] The optical adapter 10 described above features a rationally designed lens power and surface shape. Combined with the cemented lens assembly design, this facilitates the correction of various aberrations such as chromatic aberration and distortion, improves relative illumination, and enhances image resolution. This, in turn, improves the image quality of the optical adapter 10, enhancing the accuracy of diagnosis and treatment when used in medical endoscopes. Furthermore, it allows for a reduction in the size of the optical adapter 10, making it easier for users to operate and hold the medical endoscope for extended periods. Additionally, it increases the depth of field of the optical adapter 10, improving its imaging capabilities and adapting to the data acquisition requirements of medical endoscopes.

[0056] Specifically, the positive optical power and convex object-side design of the first cemented lens group E2 effectively converge incident light rays, thereby capturing more light, improving light energy collection efficiency, and enhancing image brightness. It also helps to reduce the axial dimensions of the optical adapter 10. The cemented arrangement of the first lens E21 and the second lens E22 also helps to balance chromatic aberration and improve image quality. The optical power and surface shape of the third lens E3, combined with the first cemented lens group E2, help to balance the optical power distribution at the front end of the optical adapter 10 and rationally diverge light, thus helping to correct and balance various aberrations and improve the image quality of the optical adapter 10. The optical power and surface shape design of the fourth lens, in conjunction with the other lenses, can rationally control the direction of light between the third lens E3 and the fifth lens E5, resulting in a smooth transition of light. This helps to reduce the aberration sensitivity and tolerance sensitivity of the optical adapter 10, reduces the surface complexity of each lens, thereby improving the forming yield of each lens and reducing the risk of ghosting. The optical power and surface shape of the fifth lens E5 are combined to reasonably disperse light onto the imaging surface S13, improving the matching degree between the incident angle of light on the imaging surface S13 and the image sensor, thereby helping to improve the relative illumination and resolution of the image.

[0057] In some embodiments, the first lens E21 has negative optical power and a concave image-side surface; the second lens E22 has positive optical power and a convex object-side surface and a flat image-side surface; the third lens E3 has a concave object-side surface; and the fifth lens E5 has a concave object-side surface. The combined optical power and surface design of these lenses helps to make the light path more rational, suppresses various aberrations such as distortion, chromatic aberration, and astigmatism, improves imaging illumination and resolution, and also helps to reduce the axial dimensions of the optical adapter 10 and increase the depth of field of the optical adapter 10.

[0058] In some embodiments, the optical adapter 10 further includes an aperture stop S3, which is disposed on the object side of the first cemented lens group E2. The front-positioned design of the aperture stop S3, combined with the optical power and surface design of each lens in the optical adapter 10, helps to improve the structural compactness of the optical adapter 10, thereby further reducing the axial dimensions of the optical adapter 10 and facilitating long-term operation and handling of the medical endoscope by the user. In some embodiments, the optical adapter 10 further includes a light-transmitting protection element E1, which is disposed on the object side of the first cemented lens group E2. The light-transmitting protection element E1 includes, but is not limited to, flat glass, used to protect each lens in the optical adapter 10. In some embodiments, the material of each lens in the optical adapter 10 includes, but is not limited to, any suitable glass or plastic. Using common and readily available materials helps to reduce processing difficulty and manufacturing costs.

[0059] In some embodiments, the optical adapter 10 satisfies the following conditions: 5° ≤ Semi-FOV ≤ 10°; 25mm ≤ f ≤ 30mm; where Semi-FOV is half of the maximum field of view of the optical adapter 10, and f is the focal length of the optical adapter 10. When the above conditions are met, the optical power and surface design of each lens can balance the field of view and imaging quality of the optical adapter 10, achieving good imaging quality while meeting the imaging requirements of medical endoscopes, thereby improving the accuracy of diagnosis and treatment.

[0060] Please see again. Figure 1 In the first embodiment, the fourth lens structure E4 has one lens with optical power. The object side of the fourth lens structure E4 is convex, which is beneficial for smoothing the transition of light, suppressing various aberrations, and reducing the aberration sensitivity and tolerance sensitivity of the optical adapter 10.

[0061] In the first embodiment, the optical adapter 10 satisfies the condition: 6.5 ≤ f2 / (f2+f3) ≤ 7.5; where f2 is the focal length of the first cemented lens group E2 and f3 is the focal length of the third lens E3. For example, f2 / (f2+f3) can be 6.5, 6.7, 6.9, 7, 7.1, 7.3, or 7.5. When the above condition is satisfied, the optical power distribution of the first cemented lens group E2 and the third lens E3 can be reasonably configured, which is beneficial to suppressing the generation of various aberrations such as distortion, improving the imaging quality of the optical adapter 10, and also helps to compress the axial dimension of the optical adapter 10.

[0062] In the first embodiment, the optical adapter 10 satisfies the condition: 31.5 ≤ f4 / CT4 ≤ 32.5; where f4 is the focal length of the fourth lens structure E4, and CT4 is the thickness of the fourth lens structure E4 along the optical axis. For example, f4 / CT4 can be 31.5, 31.7, 31.8, 32, 32.2, or 32.5. Satisfying the above condition allows for a reasonable configuration of the ratio of the focal length to the center thickness of the fourth lens, which facilitates a smooth transition of light between the third lens E3 and the fifth lens E5, suppresses spherical aberration and other aberrations, improves the imaging quality of the optical adapter 10, and also helps to reduce the axial dimensions of the optical adapter 10.

[0063] In the first embodiment, the optical adapter 10 satisfies the condition: 0.2 ≤ CT5 / T45 ≤ 0.8; CT5 is the thickness of the fifth lens E5 on the optical axis, and T45 is the distance on the optical axis from the image side of the fourth lens structure E4 to the object side of the fifth lens E5. For example, CT5 / T45 can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or 0.8. Satisfying the above condition is beneficial to improving the structural compactness of the optical adapter 10, thereby reducing the axial dimensions of the optical adapter 10. It also helps to rationally control the light path between the fourth lens structure E4 and the fifth lens E5, balancing various aberrations such as chromatic aberration, astigmatism, and field curvature, thereby improving the imaging quality of the optical adapter 10.

[0064] In the first embodiment, the optical adapter 10 satisfies the condition: 0.6 ≤ CT2 / (T12+T23) ≤ 0.8; where CT2 is the thickness of the first cemented lens group E2 on the optical axis, T12 is the distance on the optical axis from the image side of the light-transmitting protective element E1 to the object side of the first cemented lens group E2, and T23 is the distance on the optical axis from the image side of the first cemented lens group E2 to the object side of the third lens E3. For example, CT2 / (T12+T23) can be 0.6, 0.7, or 0.8. When the above condition is satisfied, the structural compactness of the optical adapter 10 is improved, thereby reducing the axial dimension of the optical adapter 10. It also helps to reserve sufficient space for the propagation of light on the object side of the first cemented lens group E2, optimize the light path, and thus help to suppress various aberrations such as spherical aberration, astigmatism, and field curvature. At the same time, it also helps to increase the focusing range of the optical adapter 10 and increase the depth of field of the optical adapter 10.

[0065] Please see Figures 2-5 As shown, Figure 2 This is a graph showing the transfer function (MTF) of the optical adapter 10 in the first embodiment. Figure 3 This is a defocus curve diagram of the optical adapter 10 in the first embodiment. Figure 4 This is a dot diagram of the optical adapter 10 in the first embodiment. Figure 5 The figures show the field curvature and distortion curves of the optical adapter 10 in the first embodiment. Figures 2-5 As can be seen, in the first embodiment, when the resolution of the optical adapter 10 meets 250 lp / mm, the MTF value across the entire field of view is greater than 0.2 and close to the diffraction limit. In the dot plot, the diffuse spots are all close to the Airy disk, and the spot diameter is mostly contained within the Airy disk, essentially at the diffraction limit. The distortion across the entire field of view is less than 1%. Therefore, the optical adapter 10 has good imaging quality, balancing clear imaging and small size requirements, and meets the needs of 4K optical mounts.

[0066] Please see Figure 6 As shown, Figure 6 This is a schematic diagram of the optical adapter 10 in the second embodiment. In the second embodiment, the optical power and surface design of the lenses in the optical adapter 10, except for the fourth lens structure E4, can be the same as in the first embodiment. In the second embodiment, the fourth lens structure E4 has two lenses with optical power. The fourth lens structure E4 includes a first sub-lens E41 and a second sub-lens E42 cemented together. The second sub-lens E42 is located on the image side of the first sub-lens E41. The first sub-lens E41 has positive optical power, and both its object side and image side are convex. The second sub-lens E42 has negative optical power, and its object side is concave, while its image side is convex. The optical power and surface design of the first sub-lens E41 and the second sub-lens E42, in conjunction with the optical power and surface design of the other lenses, enable the fourth lens structure E4 to smoothly transition light, which is beneficial for balancing chromatic aberration, suppressing various aberrations, and improving the imaging quality of the optical adapter 10.

[0067] In the second embodiment, the optical adapter 10 satisfies the condition: 1.6 ≤ f2 / (f2+f3) ≤ 2; where f2 is the focal length of the first cemented lens group E2 and f3 is the focal length of the third lens E3. For example, f2 / (f2+f3) can be 1.6, 1.7, 1.8, 1.9, or 2. When the above condition is satisfied, the optical power distribution of the first cemented lens group E2 and the third lens E3 can be synthesized, which is beneficial to suppressing the generation of various aberrations such as distortion, improving the imaging quality of the optical adapter 10, and also beneficial to compressing the axial dimension of the optical adapter 10.

[0068] In the second embodiment, the optical adapter 10 satisfies the condition: 7.3 ≤ f4 / CT4 ≤ 7.9; where f4 is the focal length of the fourth lens structure E4, i.e., the combined focal length of the first sub-lens E41 and the second sub-lens E42, and CT4 is the thickness of the fourth lens structure E4 on the optical axis. For example, f4 / CT4 can be 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, or 7.9. When the above condition is satisfied, the ratio of the focal length and center thickness of the fourth lens can be reasonably configured, which is beneficial to make the light transition smoothly between the third lens E3 and the fifth lens E5, suppressing the generation of various aberrations such as spherical aberration, improving the imaging quality of the optical adapter 10, and also helping to compress the axial dimension of the optical adapter 10.

[0069] In the second embodiment, the optical adapter 10 satisfies the condition: 0.3 ≤ CT5 / T45 ≤ 0.6; CT5 is the thickness of the fifth lens E5 on the optical axis, and T45 is the distance on the optical axis from the image side of the fourth lens structure E4 to the object side of the fifth lens E5. For example, CT5 / T45 can be 0.3, 0.4, 0.5, or 0.6. Satisfying the above condition is beneficial to improving the structural compactness of the optical adapter 10, thereby reducing the axial dimensions of the optical adapter 10. It also helps to rationally control the light path between the fourth lens structure and the fifth lens E5, balancing various aberrations such as chromatic aberration, astigmatism, and field curvature, thereby improving the imaging quality of the optical adapter 10.

[0070] In the second embodiment, the optical adapter 10 satisfies the condition: 0.8 ≤ CT2 / (T12+T23) ≤ 1.2; where CT2 is the thickness of the first cemented lens group E2 on the optical axis, T12 is the distance on the optical axis from the image side of the light-transmitting protective element E1 to the object side of the first cemented lens group E2, and T23 is the distance on the optical axis from the image side of the first cemented lens group E2 to the object side of the third lens E3. For example, CT2 / (T12+T23) can be 0.8, 0.9, 1.0, 1.1, or 1.2. When the above condition is satisfied, the structural compactness of the optical adapter 10 is improved, thereby reducing the axial dimension of the optical adapter 10. It also helps to reserve sufficient space for the propagation of light on the object side of the first cemented lens group E2, optimizing the light path and thus helping to suppress various aberrations such as spherical aberration, astigmatism, and field curvature. At the same time, it also helps to increase the focusing range of the optical adapter 10 and increase the depth of field of the optical adapter 10.

[0071] Please see Figures 7-10 As shown, Figure 7 This is a graph showing the transfer function (MTF) of the optical adapter 10 in the second embodiment. Figure 8 This is a defocus curve diagram of the optical adapter 10 in the second embodiment. Figure 9 This is a dot diagram of the optical adapter 10 in the second embodiment. Figure 10 The figures show the field curvature and distortion curves of the optical adapter 10 in the second embodiment. Figures 7-10 As can be seen, in the second embodiment, when the resolution of the optical adapter 10 meets 250 lp / mm, the MTF value across the entire field of view is greater than 0.2 and close to the diffraction limit. In the dot plot, the diffuse spots are all close to the Airy disk, and the spot diameter is mostly contained within the Airy disk, essentially at the diffraction limit. The distortion across the entire field of view is less than 1%. Therefore, the optical adapter 10 possesses good imaging quality, balancing clear imaging with small size requirements, and meets the needs of a 4K optical mount.

[0072] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0073] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. An optical adapter, characterized by, Along the optical axis from the object side to the image side, the following are included in sequence: A first cemented lens assembly with positive optical power, the first cemented lens assembly including a first lens and a second lens cemented together, the object side of the first lens being convex. A third lens with negative optical power, wherein the image-side surface of the third lens is concave; A fourth lens structure with positive optical power, wherein the image-side surface of the fourth lens structure is convex; A fifth lens with negative optical power, wherein the image-side surface of the fifth lens is convex.

2. The optical adapter of claim 1, wherein, The fourth lens structure has one lens with optical power, and the object side of the fourth lens structure is convex.

3. The optical adapter of claim 2, wherein, The optical adapter satisfies the following condition: 6.5 ≤ f2 / (f2+f3) ≤ 7.5; 31.5≤f4 / CT4≤32.5; 0.2≤CT5 / T45≤0.8; Wherein, f2 is the focal length of the first cemented lens group, f3 is the focal length of the third lens, f4 is the focal length of the fourth lens structure, CT4 is the thickness of the fourth lens structure on the optical axis, CT5 is the thickness of the fifth lens on the optical axis, and T45 is the distance on the optical axis from the image side of the fourth lens structure to the object side of the fifth lens.

4. The optical adapter of claim 2, wherein, The optical adapter further includes a light-transmitting protection element disposed on the object side of the first cemented lens assembly, and the optical adapter satisfies the following condition: 0.6≤CT2 / (T12+T23)≤0.8; Wherein, CT2 is the thickness of the first cemented lens assembly on the optical axis, T12 is the distance on the optical axis from the image side of the light-transmitting protective element to the object side of the first cemented lens assembly, and T23 is the distance on the optical axis from the image side of the first cemented lens assembly to the object side of the third lens.

5. The optical adapter of claim 1, wherein, The fourth lens structure includes a first sub-lens and a second sub-lens bonded together. The first sub-lens has positive optical power and both the object-side and image-side surfaces are convex. The second sub-lens has negative optical power and the object-side surface is concave.

6. The optical adapter of claim 5, wherein, The optical adapter satisfies the following condition: 1.6≤f2 / (f2+f3)≤2; 7.3≤f4 / CT4≤7.9; 0.3≤CT5 / T45≤0.6; Wherein, f2 is the focal length of the first cemented lens group, f3 is the focal length of the third lens, f4 is the focal length of the fourth lens structure, CT4 is the thickness of the fourth lens structure on the optical axis, CT5 is the thickness of the fifth lens on the optical axis, and T45 is the distance on the optical axis from the image side of the fourth lens structure to the object side of the fifth lens.

7. The optical adapter of claim 5, wherein, The optical adapter further includes a light-transmitting protection element disposed on the object side of the first cemented lens assembly, and the optical adapter satisfies the following condition: 0.8≤CT2 / (T12+T23)≤1.2; Wherein, CT2 is the thickness of the first cemented lens assembly on the optical axis, T12 is the distance on the optical axis from the image side of the light-transmitting protective element to the object side of the first cemented lens assembly, and T23 is the distance on the optical axis from the image side of the first cemented lens assembly to the object side of the third lens.

8. The optical adapter of any of claims 1-7, wherein, The optical adapter satisfies the following condition: 5°≤Semi-FOV≤10°; 25mm≤f≤30mm; Wherein, Semi-FOV is half of the maximum field of view of the optical adapter, and f is the focal length of the optical adapter.

9. The optical adapter according to any one of claims 1-7, characterized in that, The first lens has negative optical power and its image-side surface is concave; the second lens has positive optical power and its object-side surface is convex; the third lens has a concave object-side surface; and the fifth lens has a concave object-side surface; and / or, The optical adapter also includes an aperture stop, which is located on the object side of the first cemented lens assembly.

10. A medical endoscope characterized by comprising: Includes the optical adapter as described in any one of claims 1-9.

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