Zoom lens for machine vision imaging

Abstract

The invention relates to the technical field of machine vision imaging, and discloses a zoom lens for machine vision imaging, which comprises a front fixed group, a zoom group, a compensation group and a rear fixed group which are sequentially arranged from an object space to an image space along an optical axis, the front fixed group comprises at least one meniscus positive lens and a biconvex lens and is used for preliminarily correcting spherical aberration and coma aberration; the zoom group comprises at least one aspheric negative lens and at least one aspheric positive lens, and the focal length of the system is changed through linear movement. By adopting the combination of a plurality of lenses with different curvatures and refractive indexes and the design of special aspheric lenses, the lens disclosed by the invention can be used for carrying out all-directional and accurate correction on various aberrations such as spherical aberration, coma aberration, astigmatism, field curvature and distortion under the complex conditions of large view field, high-magnification zooming and the like, so that the definition and the accuracy of an image are effectively improved; especially in an image edge area, distortion is obviously reduced, and the requirements for high-precision image measurement and analysis in industrial detection are met.

Description

Technical Field

[0001] The present invention relates to the technical field of machine vision imaging, and specifically to a zoom lens for machine vision imaging. Background Art

[0002] Machine vision imaging refers to enabling a machine to imitate the visual function of humans, obtaining image information of a target object through optical devices and imaging technologies, and then analyzing, processing, and understanding these images with the aid of a computer or other processing systems, so as to achieve functions such as recognition, detection, measurement, and positioning of the target. The principle of machine vision imaging is essentially to convert optical information into digital information and perform processing and analysis.

[0003] A Chinese invention patent with the publication number CN115166956A discloses a zoom industrial lens for machine vision imaging, including a front fixed group A, a variable magnification group B, a variable aperture S, a middle fixed group C, and a rear compensation group D fixedly arranged in sequence along the incident light direction; the front fixed group A includes a meniscus negative lens A1, a biconvex positive lens A2, and a crescent positive lens A3 arranged in sequence along the light incident direction; the variable magnification group B includes a biconcave negative lens B1, a biconcave negative lens B2, and a crescent positive lens B3 arranged in sequence along the light incident direction, and the biconcave negative lens B2 and the crescent positive lens B3 are cemented together; the middle fixed group C includes a meniscus negative lens C1 and a plano-convex positive lens C2 arranged in sequence along the light incident direction, and the meniscus negative lens C1 and the plano-convex positive lens C2 are cemented together; the rear compensation group D includes a biconvex positive lens D1, a meniscus negative lens D2, a plano-convex positive lens D3, and a meniscus negative lens D4 arranged in sequence along the light incident direction. When the present invention captures image planes with different ratios, only the lens needs to be zoomed, and there is no need to switch the lens.

[0004] In addition, in terms of aberration correction, although some correction measures are taken for some lenses, in a complex industrial environment, such as in the case of a large field of view and high magnification zoom, it is difficult to perform comprehensive and accurate correction of various aberrations such as spherical aberration, coma, astigmatism, field curvature, and distortion, resulting in obvious distortion at the image edge, seriously affecting the measurement and analysis accuracy of the image. In addition, in terms of chromatic aberration control, with the continuous improvement of the requirements for color restoration degree and accuracy in industrial machine vision, the existing lenses cannot control chromatic aberration within an extremely small range in a wide spectral range, and the focusing differences of light rays with different wavelengths are obvious, causing colored edges to appear in the image, reducing the clarity and color accuracy of the image. At the same time, in terms of the balance between the relative aperture and the depth of field, when the existing lenses pursue a large relative aperture to increase the light input and adaptability to low-light environments, it often leads to a shallower depth of field, making it difficult to meet the requirement of clear imaging of objects with a certain depth in industrial inspection. Summary of the Invention

[0005] (I) Technical Problems to be Solved

[0006] In view of the deficiencies of the prior art, the present invention provides a zoom lens for machine vision imaging, which has the advantages of accurate and comprehensive aberration correction and remarkable chromatic aberration control effect, and solves the problems of easy distortion at the image edge and low image clarity.

[0007] (II) Technical Solution

[0008] To achieve the above object, the present invention provides the following technical solution: A zoom lens for machine vision imaging, comprising a front fixed group, a variable magnification group, a compensation group, and a rear fixed group arranged in sequence along the optical axis from the object side to the image side;

[0009] The front fixed group includes at least one meniscus positive lens and a double convex lens, which are used for preliminary correction of spherical aberration and coma;

[0010] The variable magnification group includes at least one aspherical negative lens and an aspherical positive lens, and the system focal length is changed by linear movement;

[0011] The compensation group includes at least one aspherical lens, which is linked with the variable magnification group to compensate for image plane displacement;

[0012] The rear fixed group includes at least one doublet made of low-dispersion glass (νd≥80), which is used for correcting axial chromatic aberration and lateral chromatic aberration;

[0013] The lens satisfies the following conditional equations:

[0014] 0.5 < fG2 / f < 1.2, where fG2 is the focal length of the variable magnification group and f is the total focal length of the lens;

[0015] 0.3 < |fG3 / fG2| < 0.8, where fG3 is the focal length of the compensation group.

[0016] Preferably, the lens further includes an adjustable aperture stop, which is located between the variable magnification group and the compensation group, and realizes stepless aperture adjustment through an electronic control unit, and satisfies the following conditional equations:

[0017] Fmin ≤ 2.8 and Fmax ≥ 8.0, where Fmin is the minimum aperture value and Fmax is the maximum aperture value;

[0018] The aperture stop (ST) maintains a relative position unchanged during the zooming process, and reduces the diffraction effect by optimizing the shape of the aperture blades.

[0019] Preferably, the meniscus positive lens of the front fixed group is made of high-refractive-index glass (nd≥1.8), and the double convex lens is made of low-dispersion glass (νd≥60), and the two are combined to correct primary spherical aberration and chromatic aberration.

[0020] Preferably, the aspherical negative lens of the variable magnification group adopts an even-order aspherical equation:

[0021] Z = (r 2 / R) / [1 + √(1 - (1 + k)(r 2 / R 2 ))] + ∑Ai·r^i (i = 4, 6, 8, 10)

[0022] Where R is the radius of curvature, k is the conic coefficient, and Ai is the high-order aspheric coefficient. The field curvature and astigmatism are corrected through this aspheric design.

[0023] Preferably, the doublet lens of the rear fixed group is made of crown glass (nd ≈ 1.5, νd ≈ 64) and flint glass (nd ≈ 1.7, νd ≈ 29) and satisfies the following conditional formula:

[0024] 0.9 < |fL6 / fL7| < 1.1, where fL6 and fL7 are the focal lengths of the positive and negative lenses in the doublet lens, respectively.

[0025] Preferably, the lens satisfies the following aberration correction conditions within the full zoom range:

[0026] Axial chromatic aberration: Δλ ≤ 0.002 mm (435 nm - 656 nm)

[0027] Distortion: |Distortion| ≤ 0.5% (full field of view)

[0028] Field curvature: |Sagittal / Tangential Field Curvature| ≤ 0.1 mm.

[0029] Preferably, the electronic control unit automatically adjusts the aperture size based on the real-time feedback of the machine vision system to balance the depth of field and resolution. The specific algorithm includes:

[0030] When the detected depth difference of the target object > 5 mm, automatically reduce the aperture to F ≥ 5.6;

[0031] When the ambient light intensity < 50 lux, automatically increase the aperture to F ≤ 3.5.

[0032] Preferably, the lens further includes a mechanical compensation mechanism to realize the linkage between the zoom group and the compensation group through a cam curve, and the cam curve satisfies the polynomial equation:

[0033] X = a0 + a1·t + a2·t 2 + a3·t 3 + a4·t 4

[0034] Where X is the displacement of the lens group, t is the rotation angle of the zoom ring, and a0 - a4 are the polynomial coefficients.

[0035] Preferably, at least one glass aspheric lens manufactured by a molding process is included in the optical material of the lens, with a surface roughness Ra ≤ 0.5 nm and a curvature accuracy PV ≤ λ / 10 (λ = 632.8 nm).

[0036] Preferably, within the temperature range of -20°C to +60°C, the focal length change rate of the lens is ≤ 0.05% / °C. By manufacturing the lens barrel with a metal material having a low coefficient of thermal expansion (such as invar) and setting a thermal compensation structure at key positions.

[0037] (III) Beneficial Effects

[0038] Compared with the prior art, the present invention provides a zoom lens for machine vision imaging, having the following beneficial effects:

[0039] 1. By adopting a combination of multiple lenses with different curvatures and refractive indices and a special aspheric lens design, the lens of the present invention can comprehensively and accurately correct various aberrations such as spherical aberration, coma, astigmatism, field curvature, and distortion under complex conditions such as a large field of view and high magnification zoom, effectively improving the clarity and accuracy of the image. Especially in the edge region of the image, the distortion is significantly reduced, meeting the requirements for high-precision image measurement and analysis in industrial inspection.

[0040] 2. By using a special optical glass material with low dispersion and a double-glued lens structure for the rear fixed group, the focusing difference of light rays with different wavelengths can be significantly reduced, and the chromatic aberration can be controlled within a very small range within a wide spectral range, making the color restoration degree of the image higher, avoiding the appearance of color edge phenomena, and improving the quality and reliability of the image.

[0041] 3. Through the optimization of the optical structure and the design of an adjustable aperture, while ensuring a large relative aperture to meet the imaging requirements in a low-light environment, the lens of the present invention can flexibly adjust the depth of field range to adapt to the imaging requirements of objects with different depths, improving the versatility and applicability of the lens in industrial inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic structural diagram of a zoom lens for machine vision imaging proposed by the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0044] Embodiment:

[0045] Referring to the attached Figure 1 As shown, a zoom lens for machine vision imaging includes a front fixed group G1, a varifocal group G2, a compensating group G3, and a rear fixed group G4 arranged in sequence from the object side to the image side along the optical axis;

[0046] The front fixed group G1 includes at least one meniscus positive lens L1 and a biconvex lens L2 for initially correcting spherical aberration and coma;

[0047] The varifocal group G2 includes at least one aspherical negative lens L3 and an aspherical positive lens L4 to change the system focal length by linear movement;

[0048] The compensating group G3 includes at least one aspherical lens L5, which is linked with the varifocal group G2 to compensate for image plane displacement;

[0049] The rear fixed group G4 includes at least one doublet lens (L6 / L7) made of low-dispersion glass (νd≥80) for correcting axial chromatic aberration and lateral chromatic aberration;

[0050] The lens satisfies the following conditional equations:

[0051] 0.5 < fG2 / f < 1.2, where fG2 is the focal length of the varifocal group and f is the total focal length of the lens;

[0052] 0.3 < |fG3 / fG2| < 0.8, where fG3 is the focal length of the compensating group.

[0053] Furthermore, the lens further includes an adjustable diaphragm ST. The diaphragm is located between the varifocal group G2 and the compensating group G3, and stepless aperture adjustment is achieved through an electronic control unit, satisfying the following conditional equations:

[0054] Fmin ≤ 2.8 and Fmax ≥ 8.0, where Fmin is the minimum aperture value and Fmax is the maximum aperture value;

[0055] The diaphragm ST maintains a constant relative position during zooming, and the diffraction effect is reduced by optimizing the shape of the diaphragm blades.

[0056] Furthermore, the meniscus positive lens L1 of the front fixed group G1 is made of high-refractive-index glass (nd≥1.8), and the biconvex lens L2 is made of low-dispersion glass (νd≥60). The two are combined to correct primary spherical aberration and chromatic aberration.

[0057] Furthermore, the aspherical negative lens L3 of the varifocal group G2 adopts an even-order aspherical equation:

[0058] Z = (r 2 / R) / [1 + √(1 - (1 + k)(r 2 / R 2 ))] + ∑Ai·r^i (i = 4, 6, 8, 10)

[0059] Where R is the radius of curvature, k is the conic coefficient, and Ai is the high-order aspheric coefficient. The field curvature and astigmatism are corrected through this aspheric design.

[0060] Furthermore, the doublet lens (L6 / L7) of the rear fixed group G4 is made of crown glass (nd≈1.5, νd≈64) and flint glass (nd≈1.7, νd≈29) and glued together, satisfying the following conditional formula:

[0061] 0.9 < |fL6 / fL7| < 1.1, where fL6 and fL7 are the focal lengths of the positive and negative lenses in the doublet lens respectively.

[0062] Furthermore, the lens satisfies the following aberration correction conditions within the full zoom range:

[0063] Axial chromatic aberration: Δλ ≤ 0.002 mm (435 nm - 656 nm)

[0064] Distortion: |Distortion| ≤ 0.5% (full field of view)

[0065] Field curvature: |Sagittal / Tangential Field Curvature| ≤ 0.1 mm.

[0066] Furthermore, the electronic control unit automatically adjusts the aperture size based on the real-time feedback of the machine vision system to balance the depth of field and resolution. The specific algorithm includes:

[0067] When the detected depth difference of the target object > 5 mm, automatically reduce the aperture to F ≥ 5.6;

[0068] When the ambient light intensity < 50 lux, automatically increase the aperture to F ≤ 3.5.

[0069] Furthermore, the lens further includes a mechanical compensation mechanism to realize the linkage between the variable magnification group G2 and the compensation group G3 through a cam curve. The cam curve satisfies the polynomial equation:

[0070] X = a0 + a1·t + a2·t 2 + a3·t 3 + a4·t 4

[0071] Where X is the displacement of the lens group, t is the rotation angle of the zoom ring, and a0 - a4 are the polynomial coefficients.

[0072] Furthermore, at least one glass aspheric lens manufactured by the molding process is included in the optical material of the lens, with a surface roughness Ra ≤ 0.5 nm and a curvature accuracy PV ≤ λ / 10 (λ = 632.8 nm).

[0073] Furthermore, within the temperature range of -20°C to +60°C, the focal length change rate of the lens is ≤0.05% / °C. By using a metal material with a low coefficient of thermal expansion (such as Invar) to manufacture the lens barrel and setting a thermal compensation structure at key positions.

[0074] I. Optical Structure Design

[0075] Front Fixed Group G1: It consists of a combination of multiple lenses with different curvatures and refractive indices. Its main function is to initially converge light and perform preliminary aberration correction on the light. For example, a meniscus lens with a convex surface facing the object side and a double convex lens are used in combination. The meniscus lens can reduce the scattering of light, and the double convex lens enhances the light converging ability. At the same time, the two cooperate to correct some spherical aberration and coma.

[0076] Zoom Group G2: A variable focal length component composed of multiple lenses. The focal length of the lens is changed by changing the relative positions of the lenses. A special aspherical lens design is adopted in the Zoom Group G2. The surface shape of the aspherical lens can accurately adjust the refraction path of light according to the incident angle and position of the light, so as to better correct aberrations during the zoom process, especially field curvature and distortion. For example, the surface equation of the aspherical lens can be optimized according to the actual optical design requirements to achieve the best aberration correction effect.

[0077] Compensation Group G3: Its function is to compensate for the movement of the image plane during the zoom process to ensure that the image always remains clear. The Compensation Group G3 also uses aspherical lenses and works in cooperation with the Zoom Group G2. Through an accurate mechanical drive system and optical design, it is ensured that the Compensation Group G3 can accurately move to the corresponding position during the zoom process, thereby effectively correcting the change of the image plane and reducing the generation of aberrations.

[0078] Rear Fixed Group G4: It is responsible for further correcting and converging the light after zooming and compensation, so that the light can be accurately focused on the image sensor. The lenses in the Rear Fixed Group G4 are made of special optical glass materials with low dispersion, such as fluorite glass or special lanthanide optical glass. These materials have a low dispersion coefficient, which can significantly reduce the generation of chromatic aberration. At the same time, the Rear Fixed Group G4 also adopts a double cemented lens structure, which is composed of two lenses with different refractive indices glued together, and can further correct aberrations and chromatic aberration, improving the clarity and contrast of imaging.

[0079] II. Relative Aperture and Depth of Field Optimization Design

[0080] Optical Structure Optimization: By reasonably designing the lens parameters and spacings of the front fixed group G1, variable magnification group G2, compensation group G3, and rear fixed group G4, while ensuring a large relative aperture, the depth of field of the lens is increased. For example, optimizing the lens curvature and refractive index of the front fixed group G1 enables more reasonable light distribution when light enters the lens, reducing the divergence of light, thereby increasing the depth of field to a certain extent.

[0081] Diaphragm Design: An adjustable diaphragm is set in the lens, and the size of the diaphragm is adjusted according to the actual imaging requirements. When a large depth of field is required, the diaphragm aperture is reduced to increase the diffraction effect of light, thereby expanding the depth of field range; when high-brightness imaging is required, the diaphragm aperture is increased to improve the light input. The adjustment of the diaphragm can be achieved through mechanical transmission or electronic control methods, facilitating flexible adjustment by users according to different industrial inspection scenarios.

[0082] Specific Implementation Steps

[0083] Lens Manufacturing: According to the design requirements, suitable optical glass materials are selected for lens manufacturing. For aspherical lenses, a precision molding process is used to ensure that the surface accuracy of the lens meets the design requirements. The manufactured lenses are strictly inspected, including the detection of parameters such as the radius of curvature, surface finish, refractive index, and dispersion coefficient, to ensure that the lens quality meets the standards.

[0084] Lens Assembly: The manufactured lenses are assembled according to the requirements of the optical design. During the assembly process, high-precision jigs and positioning devices are used to ensure that the optical axes of the lenses coincide, and the spacing and positions between the lenses are accurate. At the same time, an adjustable diaphragm is installed, and the adjustment mechanism of the diaphragm is connected to ensure that the diaphragm can work properly.

[0085] Debugging and Calibration: The assembled lens is debugged and calibrated. By adjusting the position and angle of the lenses, the aberration correction effect is further optimized. Professional optical testing equipment, such as interferometers and chromatic aberration meters, is used to detect and evaluate the optical performance of the lens. According to the test results, the lens is finely adjusted to ensure that all performance indicators of the lens meet the design requirements.

[0086] Performance Testing: Comprehensive performance testing is carried out on the debugged and calibrated lens. This includes aberration testing, chromatic aberration testing, relative aperture and depth of field testing at different focal lengths, and imaging testing under different lighting conditions and within different field of view ranges to evaluate the imaging quality and applicability of the lens. According to the test results, the lens is further optimized and improved to ensure that the lens can meet the requirements of actual industrial applications.

[0087] It should be noted that the term "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or apparatus comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or apparatus. Without further limitation, an element defined by the phrase "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or apparatus comprising said element.

[0088] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A zoom lens for machine vision imaging, characterized in that, It includes a front fixed group (G1), a variable magnification group (G2), a compensating group (G3), and a rear fixed group (G4) arranged in sequence from the object side to the image side along the optical axis; The front fixed group (G1) includes at least one meniscus positive lens (L1) and a biconvex lens (L2), which are used for initially correcting spherical aberration and coma; The variable magnification group (G2) includes at least one aspherical negative lens (L3) and an aspherical positive lens (L4), and changes the system focal length by linear movement; The compensating group (G3) includes at least one aspherical lens (L5), which is linked with the variable magnification group (G2) to compensate for image plane displacement; The rear fixed group (G4) includes at least one doublet lens (L6 / L7) made of low-dispersion glass (νd≥80), which is used for correcting axial chromatic aberration and lateral chromatic aberration; The lens satisfies the following conditional expressions: 0.5<fG2 / f<1.2, where fG2 is the focal length of the variable magnification group and f is the total focal length of the lens; 0.3<|fG3 / fG2|<0.8, where fG3 is the focal length of the compensating group.

2. The zoom lens for machine vision imaging according to claim 1, wherein: The lens further includes an adjustable diaphragm (ST), which is located between the variable magnification group (G2) and the compensating group (G3), and realizes stepless aperture adjustment through an electronic control unit, and satisfies the following conditional expressions: Fmin≤2.8 and Fmax≥8.0, where Fmin is the minimum aperture value and Fmax is the maximum aperture value; The diaphragm (ST) keeps its relative position unchanged during the zooming process, and reduces the diffraction effect by optimizing the shape of the diaphragm blades.

3. The zoom lens for machine vision imaging according to claim 1, wherein: The meniscus positive lens (L1) of the front fixed group (G1) uses high-refractive-index glass (nd≥1.8), and the biconvex lens (L2) uses low-dispersion glass (νd≥60), and the two are combined to correct primary spherical aberration and chromatic aberration.

4. A zoom lens for machine vision imaging according to claim 1, characterized in that: The aspherical negative lens (L3) of the variable magnification group (G2) uses an even-order aspherical equation: Z = (r 2 / R) / [1 + √(1 - (1 + k)(r 2 / R 2 ))] + ∑Ai·r^i (i = 4, 6, 8, 10) where R is the radius of curvature, k is the conic coefficient, and Ai is the high-order aspherical coefficient, and field curvature and astigmatism are corrected through this aspherical design.

5. A zoom lens for machine vision imaging according to claim 1, characterized in that: The doublet lens (L6 / L7) of the rear fixed group (G4) is made of crown glass (nd≈1.5, νd≈64) and flint glass (nd≈1.7, νd≈29) glued together, and satisfies the following conditional expressions: 0.9<|fL6 / fL7|<1.1, where fL6 and fL7 are the focal lengths of the positive and negative lenses in the doublet lens respectively.

6. The zoom lens for machine vision imaging according to claim 1, wherein: The lens satisfies the following aberration correction conditions within the full zoom range: Axial chromatic aberration: Δλ≤0.002mm (435nm - 656nm) Distortion: |Distortion|≤0.5% (full field of view) Field curvature: |Sagittal / Tangential Field Curvature|≤0.1mm.

7. A zoom lens for machine vision imaging according to claim 2, characterized in that: The electronic control unit automatically adjusts the diaphragm size based on the real-time feedback of the machine vision system to balance depth of field and resolution, and the specific algorithm includes: When the detected depth difference of the target object > 5mm, automatically reduce the aperture to F≥5.6; When the ambient light intensity < 50lux, automatically increase the aperture to F≤3.

5.

8. A zoom lens for machine vision imaging according to claim 1, characterized in that: The lens further includes a mechanical compensation mechanism, which realizes the linkage between the variable magnification group (G2) and the compensation group (G3) through a cam curve, and the cam curve satisfies the polynomial equation: X = a0 + a1·t + a2·t 2 + a3·t 3 + a4·t 4 where X is the displacement of the lens group, t is the rotation angle of the zoom ring, and a0 - a4 are the polynomial coefficients.

9. A zoom lens for machine vision imaging according to claim 1, characterized in that: At least one glass aspherical lens manufactured by a molding process is included in the optical materials of the lens, with a surface roughness Ra ≤ 0.5 nm and a curvature accuracy PV ≤ λ / 10 (λ = 632.8 nm).

10. The zoom lens for machine vision imaging according to claim 1, characterized in that: In the temperature range of -20°C to +60°C, the focal length change rate of the lens ≤ 0.05% / °C. By using a metal material with a low expansion coefficient (such as Invar) to manufacture the lens barrel and setting a thermal compensation structure at key positions.

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