A fixed focus lens

Abstract

The present invention discloses a fixed-focus lens, comprising a stop, a first lens, a second lens, a third lens and a fourth lens arranged in sequence from an object plane to an image plane along an optical axis, the first lens is a positive focal power lens, the second lens is a negative focal power lens, the third lens is a positive focal power lens, the fourth lens is a positive focal power lens, or the fourth lens is a negative focal power lens, the first lens, the second lens, the third lens and the fourth lens are all plastic aspheric lenses, 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, and the focal length of the fixed-focus lens is f, wherein: 0.7<|f1 / f|<1.4, 0.6<|f2 / f|<1.2, 0.6<|f3 / f|<1.5, 1.5<|f4 / f|<4.0. The fixed-focus lens provided by the present invention reduces the lens cost while meeting the lens resolution requirements.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of optical devices, and in particular to a fixed-focus lens. Background Art

[0002] Fixed-focus lenses are a very important part of optical lenses. Currently, most lenses are composed of more than four lenses and have at least one glass lens, which is relatively expensive. Therefore, it is necessary to develop a low-cost lens to solve the above problems. Summary of the invention

[0003] The present invention provides a fixed-focus lens, which meets the requirements of lens resolution and reduces the lens cost.

[0004] In a first aspect, an embodiment of the present invention provides a fixed-focus lens, comprising a stop, a first lens, a second lens, a third lens, and a fourth lens arranged in sequence from an object plane to an image plane along an optical axis;

[0005] The first lens is a positive power lens, the second lens is a negative power lens, and the third lens is a positive power lens;

[0006] The fourth lens is a positive power lens, or the fourth lens is a negative power lens;

[0007] The first lens, the second lens, the third lens and the fourth lens are all plastic aspherical lenses;

[0008] 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, and the focal length of the fixed focus lens is f, wherein:

[0009] 0.7<|f1 / f|<1.4; 0.6<|f2 / f|<1.2; 0.6<|f3 / f|<1.5; 1.5<|f4 / f|<4.0.

[0010] Optionally, the object side surface of the first lens is a convex surface, or the object side surface of the first lens is a concave surface;

[0011] The image side surface of the first lens is a convex surface;

[0012] The object side surface of the second lens is a concave surface, and the image side surface of the second lens is a convex surface;

[0013] The object side surface of the third lens is a convex surface, or the object side surface of the third lens is a concave surface;

[0014] The image side surface of the third lens is a convex surface;

[0015] The object side surface of the fourth lens is convex, and the image side surface of the fourth lens is concave.

[0016] Optionally, the sag of the image-side surface of the first lens is SAG12, and the central thickness of the first lens is CT1, wherein 0.5<|SAG12 / CT1|<0.8.

[0017] Optionally, the combined focal length of the first lens and the second lens is f12, where |f12 / f|>5.0.

[0018] Optionally, the center thickness of the third lens is CT3, and the edge thickness of the third lens is ET3, wherein 2.4 <CT3 / ET3<3.0。

[0019] Optionally, the object-side surface of the fourth lens includes at least one inflection point; and the image-side surface of the fourth lens includes at least one inflection point.

[0020] Optionally, the distance from the optical axis center of the image side surface of the fourth lens to the image plane is BFL, and the distance from the optical axis center of the object side surface of the first lens to the image plane is TTL, wherein 0.27 <BFL / TTL<0.4。

[0021] Optionally, the distance from the center of the optical axis of the object side of the first lens to the image plane is TTL, and the sum of the intervals between any two lenses from the first lens to the fourth lens on the optical axis is ∑AT, where TTL / ∑AT>5.0.

[0022] Optionally, the distance from the center of the optical axis of the object side of the first lens to the image plane is TTL, where TTL / f<1.9.

[0023] Optionally, the effective image plane diameter of the fixed-focus lens is IC, and the distance from the center of the optical axis of the object side of the first lens to the image plane is TTL, wherein IC / TTL>0.85.

[0024] The fixed-focus lens provided in the embodiment of the present invention adopts a structure of four plastic aspherical lenses, which greatly reduces the lens cost. By reasonably setting the aperture position in the fixed-focus lens, the focal length of each lens, and the relative relationship between the focal lengths of each lens, the resolution requirements are met, and it can be used in application places such as access control. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic structural diagram of a fixed-focus lens provided by an embodiment of the present invention;

[0026] Figure 2 A distortion curve diagram of the fixed-focus lens provided in Embodiment 1 of the present invention;

[0027] Figure 3 A vertical axis chromatic aberration diagram of the fixed-focus lens provided in Embodiment 1 of the present invention;

[0028] Figure 4 A schematic diagram of the structure of a fixed-focus lens provided in Embodiment 2 of the present invention;

[0029] Figure 5 A distortion curve diagram of the fixed-focus lens provided in Embodiment 2 of the present invention;

[0030] Figure 6 A vertical axis chromatic aberration diagram of the fixed-focus lens provided in Embodiment 2 of the present invention;

[0031] Figure 7 A schematic diagram of the structure of a fixed-focus lens provided in Embodiment 3 of the present invention;

[0032] Figure 8 A distortion curve diagram of the fixed-focus lens provided in Embodiment 3 of the present invention;

[0033] Fig. 9 This is a vertical axis chromatic aberration diagram of the fixed-focus lens provided in Example 3 of the present invention. DETAILED DESCRIPTION

[0034] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0035] Figure 1 A structural schematic diagram of a fixed-focus lens provided by an embodiment of the present invention is shown in FIG. Figure 1 As shown, the embodiment of the present invention provides a fixed focus lens comprising an aperture 100, a first lens 110, a second lens 120, a third lens 130 and a fourth lens 140 which are arranged in sequence from the object plane to the image plane along the optical axis; the first lens 110 is a positive power lens, the second lens 120 is a negative power lens, the third lens 130 is a positive power lens; the fourth lens 140 is a positive power lens, or the fourth lens 140 is a negative power lens; the first lens 110, the second lens 120, the third lens 130 and the fourth lens 140 are arranged in sequence from the object plane to the image plane along the optical axis; the first lens 110 is a positive power lens, the second lens 120 is a negative power lens, the third lens 130 is a positive power lens; the fourth lens 140 is a positive power lens, or the fourth lens 140 is a negative power lens; The first lens 120, the third lens 130 and the fourth lens 140 are all plastic aspherical lenses; the focal length of the first lens 110 is f1, the focal length of the second lens 120 is f2, the focal length of the third lens 130 is f3, the focal length of the fourth lens 140 is f4, and the focal length of the fixed-focus lens is f, wherein: 0.7<|f1 / f|<1.4; 0.6<|f2 / f|<1.2; 0.6<|f3 / f|<1.5; 1.5<|f4 / f|<4.0.

[0036] Among them, by setting the aperture 100 in the light path between the first lens 110 and the object plane, the propagation direction of the light beam can be adjusted, and the incident angle of the light can be adjusted, which is beneficial to improving the imaging quality.

[0037] Exemplarily, the focal length is equal to the difference between the convergence of the image-side light beam and the convergence of the object-side light beam, and it characterizes the ability of the optical system to deflect light. The larger the absolute value of the focal length, the stronger the ability to bend light, and the smaller the absolute value of the focal length, the weaker the ability to bend light. When the focal length is a positive number, the refraction of light is convergent; when the focal length is a negative number, the refraction of light is divergent. The focal length can be used to characterize a certain refractive surface of a lens (i.e., a surface of a lens), can be used to characterize a certain lens, and can also be used to characterize a system formed by multiple lenses (i.e., a lens group). In the fixed-focus lens provided in this embodiment, each lens can be fixed to a lens barrel ( Figure 1 In the embodiment of the present invention, the second lens 120 is a negative power lens; the first lens 110 and the third lens 130 are positive power lenses; the fourth lens 140 is a positive power lens, or the fourth lens 140 is a negative power lens, so that the power of the entire fixed-focus lens is distributed according to a certain proportion, ensuring the balance of the incident angle of the lens, thereby reducing the sensitivity of the lens and improving the possibility of production.

[0038] Optionally, the first lens 110, the second lens 120, the third lens 130 and the fourth lens 140 are all plastic aspheric lenses. Among them, the aspheric lenses play a role in correcting all high-order aberrations. Since the cost of plastic lenses is much lower than that of glass lenses, the fixed-focus lens provided in the embodiment of the present invention has good image quality and low cost by setting 4 plastic aspheric lenses.

[0039] Furthermore, the focal length f1 of the first lens 110, the focal length f2 of the second lens 120, the focal length f3 of the third lens 130, the focal length f4 of the fourth lens 140 and the focal length f of the fixed focus lens are set to meet the following conditions: 0.7<|f1 / f|<1.4; 0.6<|f2 / f|<1.2; 0.6<|f3 / f|<1.5; 1.5<|f4 / f|<4.0. By rationally allocating the focal lengths of the lenses, it is beneficial to correct aberrations and ensure that the lens has a high resolution.

[0040] The fixed-focus lens provided in the embodiment of the present invention adopts a structure of four plastic aspherical lenses, which greatly reduces the lens cost. By reasonably setting the aperture position in the fixed-focus lens, the focal length of each lens, and the relative relationship between the focal lengths of each lens, the resolution requirements are met, and it can be used in application places such as access control.

[0041] As a feasible implementation, optionally, the object side of the first lens 110 is convex, or the object side of the first lens 110 is concave; the image side of the first lens 110 is convex; the object side of the second lens 120 is concave, and the image side of the second lens 120 is convex; the object side of the third lens 130 is convex, or the object side of the third lens 130 is concave; the image side of the third lens 130 is convex; the object side of the fourth lens 140 is convex, and the image side of the fourth lens 140 is concave.

[0042] Exemplarily, as Figure 1 shown, by reasonably setting the surface types of the respective lenses, while ensuring that the optical powers and focal lengths of the respective lenses meet the requirements for optical power and focal length in the above embodiments, it is also possible to ensure that the entire fixed-focus lens has a compact structure and a high integration degree.

[0043] Optionally, the sagittal height of the image side of the first lens 110 is SAG12, and the central thickness of the first lens 110 is CT1, where 0.5 < |SAG12 / CT1| < 0.8.

[0044] Among them, the sagittal height represents the vertical distance from the highest point to the lowest point on the lens surface. By setting the sagittal height SAG12 of the image side of the first lens 110 and the central thickness CT1 of the first lens 110 to satisfy 0.5 < |SAG12 / CT1| < 0.8, while meeting the imaging requirements, the processing difficulty is reduced, which is conducive to mass production.

[0045] Optionally, the combined focal length of the first lens 110 and the second lens 120 is f12, where |f12 / f| > 5.0.

[0046] Among them, by setting the combined focal length f12 of the first lens 110 and the second lens 120 to satisfy |f12 / f| > 5.0, the focal length of the fixed-focus lens is determined by the third lens 130 and the fourth lens 140, which helps to correct the aberration of the fixed-focus lens.

[0047] Optionally, the central thickness of the third lens 130 is CT3, and the edge thickness of the third lens 130 is ET3, where 2.4 < CT3 / ET3 < 3.0.

[0048] Among them, by setting the central thickness CT3 of the third lens 130 and the edge thickness ET3 of the third lens 130 to satisfy 2.4 < CT3 / ET3 < 3.0, while ensuring the imaging quality of the fixed-focus lens, the processing difficulty is reduced, which is conducive to mass production.

[0049] Optionally, the object side of the fourth lens 140 includes at least one inflection point; the image side of the fourth lens 140 includes at least one inflection point.

[0050] Among them, by setting the object side surface of the fourth lens 140 to include at least one inflection point and the image side surface to include at least one inflection point, it is beneficial to correct the peripheral aberration of the lens and compress the total length of the lens, thereby achieving a good balance between imaging quality and lens volume.

[0051] Optionally, the distance from the optical axis center of the image side surface of the fourth lens 140 to the image plane is BFL, and the distance from the optical axis center of the object side surface of the first lens 110 to the image plane is TTL, where 0.27 <BFL / TTL<0.4。

[0052] Among them, the distance BFL from the optical axis center of the image side surface of the fourth lens 140 to the image plane can be understood as the back focus of the fixed-focus lens, and the distance TTL from the optical axis center of the object side surface of the first lens 110 to the image plane can be understood as the total length of the fixed-focus lens. By reasonably setting the relationship between the back focus of the fixed-focus lens and the total length of the fixed-focus lens, it can be ensured that the entire fixed-focus lens has a compact structure and a high degree of integration, thereby meeting the back focus requirements of the lens during assembly.

[0053] Optionally, the distance from the center of the optical axis of the object side surface of the first lens 110 to the image plane is TTL, and the sum of the intervals between any two lenses from the first lens 110 to the fourth lens 140 on the optical axis is ∑AT, where TTL / ∑AT>5.0.

[0054] Among them, by reasonably controlling the distance TTL from the optical axis center of the object side surface of the first lens 110 to the image plane and the sum ∑AT of the intervals on the optical axis between any two lenses from the first lens 110 to the fourth lens 140 to satisfy TTL / ∑AT>5.0, it is beneficial to ensure the miniaturization and good processing characteristics of the lens.

[0055] Optionally, the distance from the center of the optical axis of the object-side surface of the first lens 110 to the image plane is TTL, where TTL / f<1.9.

[0056] The ratio of the distance TTL from the center of the optical axis of the object side surface of the first lens 110 to the image plane and the focal length f of the fixed-focus lens is reasonably controlled to be TTL / f<1.9, thereby ensuring that the fixed-focus lens structure is miniaturized.

[0057] Optionally, the effective image diameter of the fixed-focus lens is IC (Image circle), and the distance from the center of the optical axis of the object side of the first lens 110 to the image plane is TTL, wherein IC / TTL>0.85.

[0058] Among them, by reasonably controlling the effective image plane diameter IC of the fixed-focus lens and the distance TTL from the optical axis center of the object side of the first lens 110 to the image plane to satisfy IC / TTL>0.85, it is ensured that the fixed-focus lens has the characteristics of short length and large image plane, thereby meeting the miniaturization and large image plane requirements of the access control system.

[0059] The fixed-focus lens provided in the embodiment of the present invention adopts a structure of four plastic aspherical lenses, which greatly reduces the lens cost. The fixed-focus lens has the characteristics of short system length and small effective image distortion, which meets the requirements of miniaturization and large image surface of the access control system, and is therefore suitable for the access control system.

[0060] Specific embodiments of the fixed-focus lens applicable to the above-mentioned embodiments are further described below with reference to the accompanying drawings.

[0061] Embodiment 1

[0062] Continue to refer Figure 1 The fixed-focus lens provided in the first embodiment of the present invention includes an aperture 100, a first lens 110, a second lens 120, a third lens 130, a fourth lens 140 and a protective glass plate 150 which are sequentially arranged along the optical axis from the object plane to the image plane.

[0063] As a feasible implementation, Table 1 shows the surface shape, curvature radius, thickness, K value, refractive index and Abbe number of each lens in the fixed-focus lens provided in Example 1, wherein the units of the curvature radius and thickness are both millimeters (mm).

[0064] Table 1 Design values ​​of fixed-focus lenses

[0065]

[0066] Among them, the surface numbers are numbered according to the surface order of each lens, where "S2" represents the object side surface of the first lens 110, "S3" represents the image side surface of the first lens 110, and so on; "S10" and "S11" are the object side surface and image side surface of the protective glass plate 150 of the fixed-focus lens; the radius of curvature represents the curvature of the lens surface, a positive value represents that the surface is bent toward the image side, and a negative value represents that the surface is bent toward the object side, where "PL" represents that the surface is a plane and the radius of curvature is infinite; the thickness represents the central axial distance from the current surface to the next surface, the refractive index represents the ability of the material between the current surface and the next surface to deflect light, and a blank space represents that the current position is air and the refractive index is 1; the Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light, and a blank space represents that the current position is air; the K value represents the numerical value of the best fitting cone coefficient of the aspheric surface, and a blank space represents that the K value of the current position is 0.

[0067] The aspheric cone coefficient can be defined by the following aspheric formula, but is not limited to the following representation method:

[0068]

[0069] Wherein, r is the height of the aspheric surface; z is the distance vector height from the vertex of the aspheric surface when the aspheric surface is at a height of r along the optical axis; c is the curvature of the fitted sphere, which is the inverse of the radius of curvature; k is the fitted cone coefficient; AI is the coefficient of the 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, and 20th order terms of the aspheric polynomial, and the units of z and r are both mm.

[0070] Exemplarily, Table 2 describes in detail the aspheric coefficients of each lens in this embodiment in a feasible implementation manner.

[0071] Table 2 Design values ​​of aspheric coefficients in fixed-focus lenses

[0072] Surface number A B C D E F G H I S2 -5.10E-002 9.10E-002 -2.69E-001 3.89E-001 -2.74E-001 -2.27E-002 1.74E-001 -1.09E-001 2.24E-002 S3 -2.22E-002 -3.24E-002 5.02E-002 -5.16E-002 2.65E-002 -3.16E-003 -3.11E-003 1.23E-003 -9.89E-005 S4 -2.65E-002 5.34E-002 4.24E-004 -8.99E-003 6.40E-003 -2.13E-003 3.17E-004 -1.09E-005 2.12E-006 S5 -1.35E-002 -8.40E-003 1.51E-002 -7.91E-003 2.89E-003 -6.67E-004 6.30E-005 5.93E-006 -1.30E-006 S6 6.28E-002 -5.51E-002 2.49E-002 -7.04E-003 8.80E-004 5.53E-005 -2.03E-005 1.08E-006 -4.33E-008 S7 -3.38E-002 2.84E-002 -1.37E-002 2.50E-003 1.30E-004 -8.60E-005 -7.18E-006 5.02E-006 -4.50E-007 S8 2.32E-003 -6.84E-003 5.71E-004 1.21E-004 -2.09E-005 -1.11E-006 4.96E-007 -4.56E-008 1.52E-009 S9 5.88E-003 -8.62E-003 1.76E-003 -1.59E-004 3.81E-007 8.89E-007 -1.49E-008 -5.40E-009 2.72E-010

[0073] Among them, -5.10E-002 means that the coefficient A of the surface number S2 is -5.10*10 -2 , and so on.

[0074] Table 3 shows the focal lengths f1 to f4 of each lens in the fixed-focus lens provided in Example 1, the focal length f of the fixed-focus lens, the total optical length TTL, the reciprocal of the relative aperture F#, the field of view FOV and the image height ImgH.

[0075] Table 3 Related parameters of fixed-focus lens

[0076] f1=4.56mm f2=-4.25mm f3=3.28mm f4=-8.37mm f=4.46mm TTL=7.65mm F#＝2.0 FOV = 75° ImgH=7.00mm

[0077] Furthermore, Figure 2 This is a distortion curve diagram of the fixed-focus lens provided in Embodiment 1 of the present invention, such as Figure 2 As shown, the horizontal coordinate represents the size of the distortion, the unit is %; the vertical coordinate represents the normalized image height, no unit; Figure 2 It can be seen that the optical distortion of the fixed-focus lens provided in this embodiment is less than 2% in the full field of view, and the distortion is well corrected, meeting the requirement of low distortion.

[0078] Figure 3 The vertical axis chromatic aberration diagram of the fixed focus lens provided in the first embodiment of the present invention is as follows: Figure 3 As shown in the figure, the vertical direction represents the normalized field of view angle, 0 represents the optical axis, and the vertical vertex represents the maximum field of view radius; the horizontal direction represents the offset of the meridian range based on 0.587μm, in μm. The numbers on the curve in the figure represent the wavelength represented by the curve, in μm, which is Figure 3 It can be seen that the vertical axis chromatic aberration can be controlled within the range of (-5μm, 3μm).

[0079] Embodiment 2

[0080] Figure 4This is a schematic diagram of the structure of a fixed-focus lens provided in Embodiment 2 of the present invention, as shown in FIG. Figure 4 As shown, the fixed-focus lens provided in the second embodiment of the present invention includes an aperture 100, a first lens 110, a second lens 120, a third lens 130, a fourth lens 140 and a protective glass plate 150 which are sequentially arranged along the optical axis from the object plane to the image plane.

[0081] As a feasible implementation, Table 4 shows the surface shape, radius of curvature, thickness, K value, refractive index and Abbe number of each lens in the fixed-focus lens provided in Example 2, wherein the units of the radius of curvature and thickness are both millimeters (mm).

[0082] Table 4 Design values ​​of fixed focus lenses

[0083]

[0084]

[0085] Among them, the surface numbers are numbered according to the surface order of each lens, where "S2" represents the object side surface of the first lens 110, "S3" represents the image side surface of the first lens 110, and so on; "S10" and "S11" are the object side surface and image side surface of the protective glass plate 150 of the fixed-focus lens; the radius of curvature represents the curvature of the lens surface, a positive value represents that the surface is bent toward the image side, and a negative value represents that the surface is bent toward the object side, where "PL" represents that the surface is a plane and the radius of curvature is infinite; the thickness represents the central axial distance from the current surface to the next surface, the refractive index represents the ability of the material between the current surface and the next surface to deflect light, and a blank space represents that the current position is air and the refractive index is 1; the Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light, and a blank space represents that the current position is air; the K value represents the numerical value of the best fitting cone coefficient of the aspheric surface, and a blank space represents that the K value of the current position is 0.

[0086] The aspheric cone coefficient can be defined by the following aspheric formula, but is not limited to the following representation method:

[0087]

[0088] Wherein, r is the height of the aspheric surface; z is the distance vector height from the vertex of the aspheric surface when the aspheric surface is at a height of r along the optical axis; c is the curvature of the fitted sphere, which is the inverse of the radius of curvature; k is the fitted cone coefficient; AI is the coefficient of the 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, and 20th order terms of the aspheric polynomial, and the units of z and r are both mm.

[0089] Exemplarily, Table 5 describes in detail the aspheric coefficients of each lens in this embodiment in a feasible implementation manner.

[0090] Table 5 Design values ​​of aspheric coefficients in fixed-focus lenses

[0091] Surface number A B C D E F G H I S2 -4.92E-002 9.04E-002 -2.69E-001 3.90E-001 -2.73E-001 -2.24E-002 1.74E-001 -1.09E-001 2.27E-002 S3 -2.26E-002 -3.37E-002 5.00E-002 -5.14E-002 2.67E-002 -3.09E-003 -3.12E-003 1.21E-003 -1.03E-004 S4 -1.66E-002 5.33E-002 6.75E-004 -8.50E-003 6.67E-003 -2.06E-003 3.05E-004 -3.14E-005 -3.33E-006 S5 -1.80E-002 -8.16E-003 1.52E-002 -7.82E-003 2.93E-003 -6.57E-004 6.44E-005 5.22E-006 -2.13E-006 S6 5.88E-002 -5.46E-002 2.50E-002 -7.07E-003 8.70E-004 5.31E-005 -2.06E-005 1.10E-006 -6.43E-009 S7 -3.06E-002 2.80E-002 -1.38E-002 2.50E-003 1.29E-004 -8.61E-005 -7.19E-006 5.02E-006 -4.50E-007 S8 4.41E-003 -7.52E-003 5.74E-004 1.27E-004 -2.04E-005 -1.08E-006 4.96E-007 -4.58E-008 1.49E-009 S9 -4.61E-003 -7.44E-003 1.75E-003 -1.62E-004 3.16E-007 8.99E-007 -1.38E-008 -5.37E-009 2.65E-010

[0092] Among them, -4.92E-002 means that the coefficient A of the surface number S2 is -4.92*10 -2 , and so on.

[0093] Table 6 shows the focal lengths f1 to f4 of each lens in the fixed-focus lens provided in Example 2, the focal length f of the fixed-focus lens, the total optical length TTL, the reciprocal of the relative aperture F#, the field of view FOV and the image height ImgH.

[0094] Table 6 Related parameters of fixed-focus lens

[0095] f1=4.66mm f2=-3.75mm f3=6.14mm f4=10.73mm f=4.47mm TTL=7.58mm F#＝2.0 FOV = 75° ImgH=6.90mm

[0096] Furthermore, Figure 5 This is a distortion curve diagram of the fixed-focus lens provided in the second embodiment of the present invention, such as Figure 5 As shown, the horizontal coordinate represents the size of the distortion, the unit is %; the vertical coordinate represents the normalized image height, no unit; Figure 5 It can be seen that the optical distortion of the fixed-focus lens provided in this embodiment is less than 2% in the full field of view, and the distortion is well corrected, meeting the requirement of low distortion.

[0097] Figure 6 The vertical axis chromatic aberration diagram of the fixed focus lens provided in the second embodiment of the present invention is as follows: Figure 6 As shown in the figure, the vertical direction represents the normalized field of view angle, 0 represents the optical axis, and the vertical vertex represents the maximum field of view radius; the horizontal direction represents the offset of the meridian range based on 0.587μm, in μm. The numbers on the curve in the figure represent the wavelength represented by the curve, in μm, which is Figure 6 It can be seen that the vertical axis chromatic aberration can be controlled within the range of (-5μm, 3μm).

[0098] Embodiment 3

[0099] Figure 7 This is a schematic diagram of the structure of a fixed-focus lens provided in Embodiment 3 of the present invention, as shown in FIG. Figure 7 As shown, the fixed-focus lens provided in the second embodiment of the present invention includes an aperture 100, a first lens 110, a second lens 120, a third lens 130, a fourth lens 140 and a protective glass plate 150 which are sequentially arranged along the optical axis from the object plane to the image plane.

[0100] As a feasible implementation, Table 7 shows the surface shape, radius of curvature, thickness, K value, refractive index and Abbe number of each lens in the fixed-focus lens provided in Example 2, wherein the units of the radius of curvature and thickness are both millimeters (mm).

[0101] Table 7 Design values ​​of fixed-focus lenses

[0102]

[0103] Among them, the surface numbers are numbered according to the surface order of each lens, where "S2" represents the object side surface of the first lens 110, "S3" represents the image side surface of the first lens 110, and so on; "S10" and "S11" are the object side surface and image side surface of the protective glass plate 150 of the lens; the radius of curvature represents the curvature of the lens surface, a positive value represents that the surface is bent toward the image side, and a negative value represents that the surface is bent toward the object side, where "PL" represents that the surface is a plane and the radius of curvature is infinite; the thickness represents the central axial distance from the current surface to the next surface, the refractive index represents the ability of the material between the current surface and the next surface to deflect light, and a blank space represents that the current position is air and the refractive index is 1; the Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light, and a blank space represents that the current position is air; the K value represents the numerical value of the best fitting cone coefficient of the aspheric surface, and a blank space represents that the K value of the current position is 0.

[0104] The aspheric cone coefficient can be defined by the following aspheric formula, but is not limited to the following representation method:

[0105]

[0106] Wherein, r is the height of the aspheric surface; z is the distance vector height from the vertex of the aspheric surface when the aspheric surface is at a height of r along the optical axis; c is the curvature of the fitted sphere, which is the inverse of the radius of curvature; k is the fitted cone coefficient; AI is the coefficient of the 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, and 20th order terms of the aspheric polynomial, and the units of z and r are both mm.

[0107] Exemplarily, Table 5 describes in detail the aspheric coefficients of each lens in this embodiment in a feasible implementation manner.

[0108] Table 8 Design values ​​of aspheric coefficients in fixed-focus lenses

[0109]

[0110]

[0111] Among them, -6.94E-002 means that the coefficient A of the surface number S2 is -6.94*10 -2, and so on.

[0112] Table 9 shows the focal lengths f1 to f4 of each lens in the fixed-focus lens provided in Example 2, the focal length f of the fixed-focus lens, the total optical length TTL, the reciprocal of the relative aperture F#, the field of view FOV, and the image height ImgH.

[0113] Table 9 Related parameters of fixed-focus lens

[0114] f1=4.38mm f2=-3.69mm f3=3.418mm f4=-16.26mm f=4.38mm TTL=8.16mm F#＝2.0 FOV = 75° ImgH=6.80mm

[0115] Furthermore, Figure 8 This is a distortion curve diagram of the fixed-focus lens provided in Embodiment 3 of the present invention, such as Figure 8 As shown, the horizontal coordinate represents the size of the distortion, the unit is %; the vertical coordinate represents the normalized image height, no unit; Figure 8 It can be seen that the optical distortion of the fixed-focus lens provided in this embodiment is less than 2% in the full field of view, and the distortion is well corrected, meeting the requirement of low distortion.

[0116] Fig. 9 The vertical axis chromatic aberration diagram of the fixed focus lens provided in the third embodiment of the present invention is as follows: Fig. 9 As shown in the figure, the vertical direction represents the normalized field of view angle, 0 represents the optical axis, and the vertical vertex represents the maximum field of view radius; the horizontal direction represents the offset of the meridian range based on 0.587μm, in μm. The numbers on the curve in the figure represent the wavelength represented by the curve, in μm, which is Fig. 9 It can be seen that the vertical axis chromatic aberration can be controlled within the range of (-4μm, 3μm).

[0117] In summary, the fixed-focus lens provided in the embodiment of the present invention adopts a 4-piece plastic aspherical lens structure, which greatly reduces the lens cost. The fixed-focus lens meets TTL / f<1.9 and IC / TTL>0.85, which meets the requirements of miniaturization and large image plane of the access control system, and the optical distortion is less than 2% under the full field of view, and meets the imaging requirements when used in an environment of -20 to 60°C.

[0118] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A fixed-focus lens, characterized in that: It includes a stop, a first lens, a second lens, a third lens and a fourth lens which are arranged in sequence from the object plane to the image plane along the optical axis; The first lens is a positive power lens, the second lens is a negative power lens, and the third lens is a positive power lens; The fourth lens is a positive power lens, or the fourth lens is a negative power lens; The first lens, the second lens, the third lens and the fourth lens are all plastic aspherical lenses; 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, and the focal length of the fixed focus lens is f, wherein: 0.7<|f1 / f|<1.4; 0.6<|f2 / f|<1.2; 0.6<|f3 / f|<1.5; 1.5<|f4 / f|<4.0; The sag height of the image side surface of the first lens is SAG12, and the central thickness of the first lens is CT1, wherein 0.5<|SAG12 / CT1|<0.8; The effective image plane diameter of the fixed-focus lens is IC, and the distance from the optical axis center of the object side of the first lens to the image plane is TTL, wherein IC / TTL>0.

85.

2. The fixed-focus lens according to claim 1, characterized in that: The object side surface of the first lens is a convex surface, or the object side surface of the first lens is a concave surface; The image side surface of the first lens is a convex surface; The object side surface of the second lens is a concave surface, and the image side surface of the second lens is a convex surface; The object side surface of the third lens is a convex surface, or the object side surface of the third lens is a concave surface; The image side surface of the third lens is a convex surface; The object side surface of the fourth lens is convex, and the image side surface of the fourth lens is concave.

3. The fixed-focus lens according to claim 1, characterized in that: The combined focal length of the first lens and the second lens is f12, where |f12 / f|>5.

0.

4. The fixed-focus lens according to claim 1, wherein: The center thickness of the third lens is CT3, and the edge thickness of the third lens is ET3, wherein 2.4 <CT3 / ET3<3.0。 5. The fixed-focus lens according to claim 1, wherein: The object-side surface of the fourth lens includes at least one inflection point; the image-side surface of the fourth lens includes at least one inflection point.

6. The fixed-focus lens according to claim 1, wherein: The distance from the optical axis center of the image side of the fourth lens to the image plane is BFL, and the distance from the optical axis center of the object side of the first lens to the image plane is TTL, wherein 0.27 <BFL / TTL<0.4。 7. The fixed-focus lens according to claim 1, wherein: The distance from the center of the optical axis of the object side of the first lens to the image plane is TTL, and the sum of the intervals between any two lenses from the first lens to the fourth lens on the optical axis is ∑AT, where TTL / ∑AT>5.

0.

8. The fixed-focus lens according to claim 1, wherein: The distance from the center of the optical axis of the object side surface of the first lens to the image plane is TTL, where TTL / f<1.9.

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