Large-aperture small fixed-focus imaging lens based on Pancake structure

Abstract

The invention discloses a large-aperture small fixed-focus imaging lens based on a Pancake structure. The large-aperture small fixed-focus imaging lens comprises a first lens with positive focal power, a diaphragm STO, a second lens with negative focal power, a third lens with positive focal power, a fourth lens with negative focal power, a fifth lens with negative focal power and a sixth lens with positive focal power which are sequentially arranged from an object side to an image side along an optical axis. According to the invention, the axial length of the whole optical lens is compressed by adopting a Pancake structure under the condition of meeting the requirements of large-light-circle fixed-focus high-resolution imaging, a method for solving the problem of large overall size of optical imaging equipment is found, and the method has great significance in solving the contradiction between miniaturization and high resolution of an optical system under the condition of large aperture. And the improvement of the optical system performance is of great significance.

Description

Technical Field

[0001] This invention application relates to the field of optical lenses, specifically to a large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure. Background Technology

[0002] In the field of optical imaging, the rapid advancements in portable electronic products such as smartphones, drones, and wearable devices, as well as small imaging systems like medical endoscopes and miniature monitoring devices, have intensified the demand for miniaturized optical lenses. Traditional optical lenses often employ a refractive structure, relying on a combination of multiple lens elements to correct aberrations and achieve imaging. This design generally results in bulky and heavy lenses, with the size issue being even more pronounced in large-aperture lenses. Consequently, they struggle to meet the compactness and lightweight requirements of the aforementioned application scenarios.

[0003] Therefore, in order to meet the market's urgent demand for compact and lightweight optical imaging equipment, it is particularly urgent to develop a small and lightweight imaging objective lens, which has become a technical problem that optical designers need to overcome. Summary of the Invention

[0004] To overcome the technical problems pointed out in the background section, the present invention provides a large-aperture, small-focal-length imaging lens based on a pancake structure. The pancake reflex structure enables the lens to achieve miniaturization and high-resolution imaging while meeting the requirements of large-aperture imaging.

[0005] To achieve the objectives of this invention, the technical solution provided by this invention is as follows:

[0006] A large-aperture, small-focal-length imaging lens based on a pancake structure includes a first lens with positive optical power, an aperture stop STO, a second lens with negative optical power, a third lens with positive optical power, a fourth lens with negative optical power, a fifth lens with negative optical power, and a sixth lens with positive optical power, arranged sequentially along the optical axis from the object side to the image side.

[0007] The focal length of the first lens is f1, the focal length of the second lens is f2, the focal length of the third lens is f3, the focal length of the fourth lens is f4, the focal length of the fifth lens is f5, and the focal length of the sixth lens is f6.

[0008] Where f1 to f6 satisfy the following relationship: 60mm <f1<62mm;-279mm<f2<-277mm;67mm<f3<69mm;-482mm<f4<-480mm;-227mm<f5<-225mm;118mm<f6<120mm;

[0009] The focal length of the fixed-focus imaging lens is f = 116mm. <f<18mm。

[0010] Furthermore, the image plane height of the fixed-focus imaging lens and the total optical length of the fixed-focus imaging lens satisfy the following:

[0011] H / TTL>0.6;

[0012] Furthermore, H represents the image plane height of the fixed-focus imaging lens, and TTL represents the total optical length of the fixed-focus imaging lens.

[0013] Furthermore, the diameter of the incident pupil of the fixed-focus imaging lens and the focal length of the fixed-focus imaging lens satisfy the following:

[0014] 0.7 <D / f<0.8;

[0015] Furthermore, D is the diameter of the incident pupil of the fixed-focus imaging lens, and f is the focal length of the fixed-focus imaging lens. When the lens parameters satisfy the above relationship, the relationship between the aperture of the entire lens and the focal length of the optical system can be controlled, thereby achieving a large aperture light-gathering effect.

[0016] Furthermore, the image plane height of the fixed-focus imaging lens and the focal length of the fixed-focus imaging lens satisfy the following:

[0017] 1≤f / H≤1.2.

[0018] Furthermore, the image plane height of the fixed-focus imaging lens satisfies:

[0019] 15mm <H<17mm。

[0020] Furthermore, the first lens is a meniscus lens, the second lens is a meniscus lens, the third lens is a biconvex lens, the fourth lens is a meniscus lens, the fifth lens is a biconcave lens, and the sixth lens is a meniscus lens.

[0021] Furthermore, the front and rear surfaces of the first lens are both made of even-order aspherical surfaces, and the rear surface also includes a polarizer; the front surface of the second lens is made of even-order aspherical surfaces, the rear surface is a standard sphere, the front surface includes a quarter-wave plate that serves as a phase delay, and the rear surface includes a circularly polarized semi-transparent and semi-reflective film.

[0022] The third lens is a positive power biconvex lens with an even-order aspherical front surface and a spherical rear surface. The front surface includes a quarter-wave plate that acts as a phase delay. The fourth lens is a negative power meniscus lens with spherical front and rear surfaces. The fifth lens is a negative power biconcave lens with spherical front and rear surfaces. The sixth lens is a positive power meniscus lens with even-order aspherical front and rear surfaces, and the front surface also includes a unidirectional reflective polarizer.

[0023] Furthermore, the third lens, the fourth lens, and the fifth lens are cemented together to form a cemented triplet lens.

[0024] Furthermore, the aperture stop STO is an aperture stop.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The fixed-focus imaging lens provided by this invention employs six lenses and utilizes a pancake polarization-reflecting structure to overcome the problem of miniaturization of the optical system under large aperture; at the same time, through surface shape control, it achieves the optical imaging requirements under large aperture.

[0027] In addition, the fixed-focus imaging lens of the present invention can effectively correct the aberrations of the optical system by reasonably configuring the refractive index and Abbe number, ensuring that the contrast of the lens image reaches 0.5 at 160lp / mm, and achieving higher image quality on the basis of miniaturized lens. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structural layout of the large-aperture, small-focus imaging lens based on the Pancake structure of the present invention.

[0029] Figure 2 This is a schematic diagram of the modulation transfer function (MTF) curve of the large-aperture, small-focus imaging lens based on the Pancake structure of this invention under visible light.

[0030] Figure 3 This is a schematic diagram of the field curvature and distortion curves of the large-aperture, small-focus imaging lens based on the Pancake structure of this invention.

[0031] Figure 4 This is a schematic diagram of the coating on each surface of the large-aperture, small-focus imaging lens based on the Pancake structure of the present invention. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] To achieve the miniaturization objective of the present invention, the present invention compresses the lens size by means of a Pancake catadioptric structure. This method can significantly compress the axial optical path while ensuring the same size of the imaging surface, forcing the lens design to break through the length limitation of the traditional structure and develop towards miniaturization, so as to adapt to application scenarios with strict requirements for thinness such as mobile phones, AR, and VR devices. At the same time, aspherical lenses and high-refractive-index materials are used to correct the aberrations introduced by the folded optical path.

[0034] Figure 1 The following is a schematic structural diagram of a fixed-focus imaging lens provided by an embodiment of the present invention. Referring to Figure 1 , the fixed-focus imaging lens includes a first lens 10 with positive optical power, an aperture STO, a second lens 20 with negative optical power, a third lens 30 with positive optical power, a fourth lens 40 with negative optical power, a fifth lens 50 with negative optical power, and a sixth lens 60 with positive optical power, which are arranged in sequence from the object side to the image side along the optical axis. Among them, the first lens 10, the second lens 20, the third lens 30, and the sixth lens 60 are glass aspherical lenses; the fourth lens 40 and the fifth lens 50 are both glass spherical lenses; the optical total length of the fixed-focus lens satisfies the following relationship: H / TTL>0.6; where H is the image height of the fixed-focus lens, and TTL is the optical total length of the fixed-focus imaging lens.

[0035] Among them, the aspherical surface satisfies the following formula

[0036]

[0037] In the formula, z is the axial distance from the vertex of the curved surface at the position where the height perpendicular to the optical axis along the optical axis is y; c represents the curvature at the vertex of the aspherical surface; k is the conic coefficient; A2, A4, A6, A8, A10, A12, A14... are the aspherical coefficients of the second order, fourth order, sixth order, eighth order, tenth order, twelfth order, fourteenth order... respectively.

[0038] By reasonably configuring spherical and aspherical lenses, various aberrations of the system are effectively corrected, thereby improving the resolution of the lens. By reasonably configuring the refractive index and Abbe number of each lens, the spherical aberration and chromatic aberration of the optical system are effectively corrected, thereby improving the imaging quality of the fixed-focus lens.

[0039] In addition, in the embodiment of the present invention, the entrance pupil diameter D and the effective focal length f of the fixed-focus imaging lens satisfy the following relationship: 0.7<D / f<0.8; the effective focal length f of the fixed-focus lens and the image height H of the image plane of the fixed-focus lens satisfy the following relationship: 1≤f / H≤1.2, which is beneficial to ensuring a smaller volume while achieving a larger field of view and higher image quality. The image height H of the image plane of the fixed-focus lens satisfies the following relationship: 15mm<H<17mm, which is adapted to a 1-inch CMOS chip.

[0040] In an embodiment of the present invention, the focal length of the first lens 10 is f1, the focal length of the second lens 20 is f2, the focal length of the third lens 30 is f3, the focal length of the fourth lens 40 is f4, the focal length of the fifth lens 50 is f5, and the focal length of the sixth lens 60 is f6;

[0041] f1 to f6 satisfy the following relationship: 60 mm < f1 < 62 mm; -279 mm < f2 < -277 mm; 67 mm < f3 < 69 mm; -482 mm < f4 < -480 mm; -227 mm < f5 < -225 mm; 118 mm < f6 < 120 mm; the focal length of the fixed-focus imaging lens is f, 116 mm < f < 18 mm.

[0042] The optical length of the lens body is 24 to 26 mm, the F number is 0.6 to 0.8, and the maximum optical aperture is less than 30 mm, meeting the requirements of miniaturization.

[0043] In addition, the first lens 10 and the sixth lens 60 are made of low softening point glass materials, which is beneficial to the processing of aspherical surfaces; the second lens 20, the third lens 30, the fourth lens 40, and the fifth lens 50 are made of environmentally friendly glass materials.

[0044] In addition, in an embodiment of the present invention, the lens body further includes an aperture stop, which is disposed between the first lens group 10 and the second lens group 20. The aperture stop is a pupil aperture stop that can limit the imaging light beam; the position of the aperture stop and the size of the light passing hole have a direct relationship with the brightness, clarity, and the size of some aberrations of the image formed by the lens body. Setting the aperture stop between the first lens group 10 and the second lens group 20 can balance the aberrations of the front and rear lenses to achieve a more appropriate imaging brightness and clarity.

[0045] It should be noted that the aperture stop can be an independent hole or a surface of a certain lens. In this embodiment, the aperture stop is a hole on the rear side of the first lens 10.

[0046] Table 1 shows the parameter design values of a specific embodiment of a fixed-focus lens provided by an embodiment of the present invention:

[0047] Table 1

[0048] Face number face shape R(mm) D(mm) nd k S1 aspherical 30.6 7.00 1.57 0 S2 aspherical 252.7 1.97 0 Aperture flat PL 0.04 S4 aspherical 41.3 1.00 1.95 0 S5 spherical 35.3 2.90 S6 aspherical 31.1 6.45 1.50 -0.17 S7 spherical -76.5 1.00 1.59 S8 spherical -499.7 1.00 2.00 S9 spherical 279.0 0.05 S10 aspherical 108.2 1.32 1.62 0 S11 aspherical -231.8 1.50 0

[0049] In this context, surface number S3 represents the virtual surface in the lens design, surface number S1 represents the front surface of the first lens 10 near the object side, and so on, PL indicates that the surface is a plane; R represents the radius of the sphere, with a positive value indicating that the center of the sphere is closer to the image plane and a negative value indicating that the center of the sphere is closer to the object plane; D represents the distance on the optical axis from the current surface to the next surface; nd represents the refractive index of the lens; and k represents the conicity of the aspherical surface.

[0050] In this embodiment, the aspherical data are shown in Table 2 below:

[0051] Table 2

[0052] Face number A4 A6 A8 A10 S1 -5.560539E-06 -2.283188E-08 6.104238E-11 0 S2 -2.655112E-06 7.132776E-09 5.234747E-11 0 S4 -1.211853E-06 1.699161E-08 -4.098935E-11 0 S6 9.888801E-08 1.375240E-08 -1.037587E-10 5.305612E-13 S10 3.565917E-06 -5.093519E-09 7.410421E-11 0 S11 4.287528E-05 -2.962882E-07 2.235117E-09 0

[0053] In Table 2, the surface numbers correspond to those in Table 1. A4, A6, A8, and A10 are the fourth, sixth, eighth, and tenth order aspherical coefficients, respectively.

[0054] Figure 2 The figure shown is a schematic diagram of the modulation transfer function (MTF) curve of the fixed-focus imaging lens provided in this embodiment under visible light. Figure 3 The diagram shows the field curvature and distortion curves of the fixed-focus imaging lens provided in this embodiment. The fixed-focus lens provided in this embodiment meets the resolution requirement of 0.5 for visible light with a contrast ratio of 160 lp / mm.

[0055] The fixed-focus imaging lens provided in this embodiment utilizes a pancake structure to achieve lens miniaturization. This is achieved by coating different surfaces of the lens, causing light to become P-polarized after passing through the polarizer on surface S2, then circularly polarized after passing through the quarter-wave plate on surface S4, then semi-reflective coating on surface S5 without changing its polarization state, and finally becoming S-polarized after passing through the quarter-wave plate on surface S6. At this point, the polarization direction of the light is perpendicular to the polarization direction of the reflective polarizer on surface S10, so it cannot be transmitted but is reflected back into the lens assembly, maintaining its polarization state. After passing through the quarter-wave plate on surface S6 again, it becomes circularly polarized again, reaches the semi-reflective coating on surface S5, is reflected, passes through the quarter-wave plate on surface S6 again, becomes P-polarized again, and finally is transmitted through the reflective polarizer on surface S10. Figure 4 The diagram shows the coatings on various surfaces of the fixed-focus imaging lens provided in an embodiment of the present invention.

[0056] Finally, it should be noted that the above embodiments are merely illustrative and explanatory of the present invention, and are not intended to limit the present invention to the scope of the described embodiments. Furthermore, those skilled in the art will understand that the present invention is not limited to the above embodiments, and many more variations and modifications can be made based on the teachings of the present invention, all of which fall within the scope of protection claimed by the present invention.

Claims

1. A large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure, characterized in that, It includes a first lens (10) with positive optical power, an aperture stop (STO), a second lens (20) with negative optical power, a third lens (30) with positive optical power, a fourth lens (40) with negative optical power, a fifth lens (50) with negative optical power, and a sixth lens (60) with positive optical power, arranged sequentially from the object side to the image side along the optical axis. The focal length of the first lens (10) is f1, the focal length of the second lens (20) is f2, the focal length of the third lens (30) is f3, the focal length of the fourth lens (40) is f4, the focal length of the fifth lens (50) is f5, and the focal length of the sixth lens (60) is f6. The focal lengths of f1 to f6 satisfy the following relationship: 60mm <f1<62mm;-279mm<f2<-277mm;67mm<f3<69mm;-482mm<f4<-480mm;-227mm<f5<-225mm;118mm<f6<120mm; The focal length of the fixed-focus imaging lens is f = 116mm. <f<18mm。 2. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 1, characterized in that, The image plane height of the fixed-focus imaging lens and the total optical length of the fixed-focus imaging lens satisfy the following: H / TTL>0.6; Wherein, H is the image plane height of the fixed-focus imaging lens, and TTL is the total optical length of the fixed-focus imaging lens.

3. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 2, characterized in that, The diameter of the entrance pupil of the fixed-focus imaging lens and the focal length of the fixed-focus imaging lens satisfy the following: 0.7 <D / f<0.8; Where D is the diameter of the entrance pupil of the fixed-focus imaging lens, and f is the focal length of the fixed-focus imaging lens.

4. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 3, characterized in that, The image plane height of the fixed-focus imaging lens and the focal length of the fixed-focus imaging lens satisfy the following: 1≤f / H≤1.

2.

5. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 4, characterized in that, The image plane height of the fixed-focus imaging lens satisfies: 15mm <H<17mm。 6. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 1, characterized in that, The first lens is a meniscus lens, the second lens is a meniscus lens, the third lens is a biconvex lens, the fourth lens is a meniscus lens, the fifth lens is a biconcave lens, and the sixth lens is a meniscus lens.

7. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 6, characterized in that, The front and rear surfaces of the first lens (10) are both made of even-order aspherical surfaces, and the rear surface also includes a polarizer; the front surface of the second lens (20) is made of even-order aspherical surfaces, the rear surface is a standard sphere, the front surface includes a quarter-wave plate that acts as a phase delay, and the rear surface includes a circularly polarized semi-transparent and semi-reflective film. The third lens (30) is a positive optical power biconvex lens, with an even-order aspherical surface on the front surface and a spherical surface on the rear surface. The front surface includes a quarter-wave plate that serves as a phase delay. The fourth lens (40) is a negative optical power meniscus lens, with spherical surfaces on both the front and rear surfaces. The fifth lens (50) is a negative optical power biconcave lens, with spherical surfaces on both the front and rear surfaces. The sixth lens (60) is a positive optical power meniscus lens, with even-order aspherical surfaces on both the front and rear surfaces, and the front surface also includes a unidirectional reflective polarizer.

8. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 6, characterized in that, The third, fourth, and fifth lenses are cemented together to form a cemented triplet lens.

9. The large-aperture, small-focal-length fixed-focus imaging lens based on a pancake structure according to claim 1, characterized in that, The aperture stop STO is an aperture stop.

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