Optical lens system and imaging system

Abstract

Optical lens system for a camera, comprising exactly four lenses (L1, L2, L3, L4), wherein, in an order from an object-side to an image-side end, a first lens (L1) with a positive refractive power, a second lens (L2) with a positive refractive power, a third lens (L3) with a positive refractive power, and a fourth lens (L4) with a negative refractive power are provided, wherein the second and third lenses (L2, L3) are each configured as meniscus lenses, wherein the object-side surface (F21) of the second lens (L2) is convex and the object-side surface (F31) of the third lens (L3) is concave, and wherein a radius of curvature L2R1 of the object-side surface (F21) of the second lens (L2) and a radius of curvature L3R2 of the image-side surface (F32) of the third lens (L3) satisfy the condition -1.4 < L2R1 / L3R2 < -0.7 and a radius of curvature L2R2 of the image-side surface (F22) of the second lens (L2) and a radius of curvature L3R1 of the object-side surface (F31) of the third lens (L3) satisfy the condition -1.8 < L2R2 / L3R1 < -1.0.

Description

[0001] The present invention relates to an optical lens system for a camera.

[0002] Technological developments have led to mobile devices such as mobile phones, especially smartphones, or portable computers, especially tablet computers, being equipped with one or even multiple cameras. Increasing miniaturization, which also leads to ever smaller image sensors, also requires lens systems or lenses with a compact design.

[0003] A novel feature of the aforementioned mobile devices is their equipment with a 3D imaging system, allowing the creation of three-dimensional images, i.e. images that include depth information in addition to the lateral image information. One approach to generating three-dimensional images involves using a 3D image sensor with a front-mounted lens or lens system, which generates a depth image of the scene. The depth information is obtained using the time-of-flight (TOF) method. Such 3D image sensors are also referred to as TOF sensors. An example of a TOF sensor is a photonic mixing device, also known as a PMD sensor (photonic mixing device).To obtain depth information, the scene to be recorded is illuminated with a light pulse whose wavelength is usually in the infrared range, and the depth-dependent light travel time of this light pulse is determined spatially resolved by the 3D image sensor. To generate a polychrome 3D image, a polychrome 2D image of a scene, generated by a particularly high-resolution 2D image sensor, and the corresponding 3D image generated by the 3D image sensor can be combined.

[0004] For a high-quality three-dimensional imaging system, the lens or lens system arranged in front of the 3D image sensor must have a high light intensity and, at the same time, a large depth of field, while the design should also be as compact as possible.

[0005] However, a high aperture and a high depth of field represent conflicting requirements for such a lens. Therefore, conventional lens systems or lenses usually have a relatively low aperture to meet the demand for a large depth of field. Lens systems with a higher aperture comprise more than four lenses and therefore do not meet the requirements for a compact design.

[0006] US 7,274,518 B1 and US 2010 / 0309367 A1 disclose lens systems with four lenses, wherein a first lens with a positive refractive power, a second lens with a negative refractive power, a third lens with a positive refractive power, and a fourth lens with a negative refractive power are provided in a sequence from an object-side end to an image-side end. Documents US 2014 / 0184880 A1 and US 2015 / 0168680 A1 disclose lens systems with four lenses, in which the first three lenses have a positive refractive power and the fourth lens has a negative refractive power. In the lens system according to US 2015 / 0168680 A1, the second and third lenses are each designed as meniscus lenses, wherein the object-side surface of the second lens is convex and the object-side surface of the third lens is concave.

[0007] It is the object of the present invention to provide a lens system which simultaneously has a high light intensity, a high depth of field and a compact design.

[0008] The solution is provided by an optical lens system having the features of claim 1. The optical lens system for a camera according to the present invention comprises exactly four lenses, wherein a first lens with a positive refractive power, a second lens with a positive refractive power, a third lens with a positive refractive power, and a fourth lens with a negative refractive power are provided in a sequence from an object-side to an image-side end, wherein the second and the third lenses are each designed as meniscus lenses, wherein the object-side surface of the second lens is convex and the object-side surface of the third lens is concave, and wherein a radius of curvature L2R1 of the object-side surface of the second lens and a radius of curvature L3R2 of the image-side surface of the third lens satisfy the condition -1.4 < L2R1 / L3R2 < -0.7, in particular -1.2 < L2R1 / L3R2 < -0.8,and a radius of curvature L2R2 of the image-side surface of the second lens and a radius of curvature L3R1 of the object-side surface of the third lens satisfy the condition -1.8 < L2R2 / L3R1 < -1.0, in particular -1.6 < L2R2 / L3R1 < -1.2. The lenses are advantageously arranged along an optical axis, wherein the stated order refers to the propagation direction of the light along the beam path of the lens system from an object side to an image side of the lens system. A beam-limiting aperture stop can be arranged in the region of the first lens, wherein the aperture stop can be located within or in front of the focal plane of the first lens. Furthermore, a plane-parallel plate can be arranged behind the fourth lens, wherein the plane-parallel plate can be designed as a blocking filter. The characteristic of this blocking filter can be configured as a low-pass, high-pass, or band-pass filter.In particular, the blocking filter can be transparent to infrared light and blocking to visible light. A meniscus lens is a lens with one surface convex and the other concave.

[0009] Advantageously, the refractive index of each lens is greater than 1.6, at least in the wavelength range between 800 and 1000 nm. The lenses can be made of either optical glass or plastic, e.g., polymethyl methacrylate (PMMA), polycarbonate, polystyrene, cycloolefin (co)polymers, or other suitable plastics. The use of high-index materials for the lenses results in lower angles of incidence on the lens surfaces compared to lens materials with a lower refractive index, thus reducing aberrations.

[0010] In contrast to prior art lens systems, which typically feature lenses with alternating positive and negative refractive power arranged one behind the other, the lens system according to the invention offers high light intensity while maintaining an extremely compact design. Only the fourth lens element in the lens system according to the invention has a negative refractive power, in particular to correct field curvature and distortion.

[0011] The use of meniscus lenses as carriers of the positive refractive power, at least in the second and third lenses, reduces the sensitivity to manufacturing tolerances compared to biconvex lenses, particularly to surface tolerances or adjustment tolerances such as tilt or centering tolerances. This makes it possible to manufacture the lens system according to the invention with a high and reproducible quality at a reasonable cost, even with a small aperture and compact dimensions.

[0012] Due to the above-mentioned condition for the radius of curvature L2R1 of the object-side surface of the second lens and the radius of curvature L3R2 of the image-side surface of the third lens, the second and third lenses together form an approximately symmetrical “air lens”, which enables the correction of asymmetric image errors such as coma and distortion.

[0013] Due to the above-mentioned condition for the radius of curvature L2R2 of the image-side surface of the second lens and the radius of curvature L3R1 of the object-side surface of the third lens, the surfaces of the second and third lenses facing each other also still exhibit a certain symmetry, which, however, has been somewhat weakened here due to the asymmetrical beam path.

[0014] The lens system according to the invention is particularly suitable for use with narrowband, particularly monochrome, light, especially infrared light. This eliminates the need for correcting chromatic aberrations, which generally requires the use of different lens materials and lenses with negative refractive power. Therefore, the lens system or objective according to the invention is particularly suitable for use with a 3D image sensor, in particular a 3D TOF image sensor.

[0015] According to an advantageous embodiment of the invention, each lens has at least one aspherically curved surface. Preferably, both surfaces of each lens are aspherically curved. The aspherical curvature of at least one lens surface supports the objective of the present invention to maximize light intensity, depth of field, and compactness.

[0016] According to a particularly advantageous embodiment of the invention, the lens system has spherical aberrations dimensioned such that they at least partially compensate for blurring caused by defocus in an image of an object generated by the lens system. In contrast to conventional lens systems, spherical aberrations are therefore not completely corrected, but rather are introduced in a targeted manner such that these spherical aberrations can compensate for blurring caused by a scene extending over a large depth range, thus causing parts of a captured scene to be out of focus.

[0017] Such an imaging error, referred to as defocus, on the image side of the lens system is therefore a consequence of different object distances on the object side of the lens system and is almost impossible to avoid, particularly in the aforementioned application in 3D imaging systems for objects with a large depth range. As a rule, the aim is to image a given object distance range, which can extend, for example, from infinity to 1 m, preferably to 0.5 m, particularly preferably to 0.25 m, with acceptable image sharpness. The aforementioned partial compensation is understood in particular to mean that the radius of an image-side circle of confusion for a particularly axial object point, i.e. an object point lying on the optical axis, which is located within the aforementioned object distance range, is not greater than 12 µm.In the context of this text, the radius of an image-side circle of confusion is understood to mean the radius determined using the root mean square method, i.e., the RMS light spot radius or RMS spot radius (from English "root mean sqare"). The RMS light spot radius is generally smaller than a geometrically determined radius of the circle of confusion, which can include all rays emanating from the object point, i.e., even rays located very far out. Thus, objects or scenes whose object points are within the specified object distance range can still be imaged with acceptable image sharpness into the image plane or onto an image sensor arranged there with the help of the lens system.

[0018] Advantageously, the first lens is also designed as a meniscus lens.

[0019] According to a further advantageous embodiment of the invention, the refractive power of the second lens is greater than the refractive power of the first and third lenses.

[0020] Thus, the second lens has the highest refractive power in the lens system. This further reduces sensitivity to manufacturing tolerances, especially those mentioned above.

[0021] Advantageously, the refractive power of the first lens is lower than the refractive power of the second and third lenses. This also reduces the sensitivity to manufacturing tolerances, particularly those mentioned above. A higher refractive power leads to a greater deflection of the incoming light rays and thus to a higher sensitivity to manufacturing tolerances. Lenses arranged closer to an aperture stop are more sensitive in this regard due to the associated greater illumination.

[0022] According to a further advantageous embodiment, the focal length f1 of the first lens and the total focal length f of the lens system satisfy the condition 4.0 < f1 / f < 6.5.

[0023] Furthermore, it has proven advantageous if the focal length f2 of the second lens and the total focal length f of the lens system satisfy the condition 1.0 < f2 / f < 2.0.

[0024] According to yet another advantageous embodiment, the focal length f3 of the third lens and the total focal length f of the lens system satisfy the condition 2.0 < f3 / f < 3.5.

[0025] Furthermore, it has proven advantageous if the focal length f4 of the fourth lens and the total focal length f of the lens system satisfy the condition -12.0 < f4 / f < -6.0.

[0026] Finally, it has also been found to be advantageous if the combined focal length f23 of the second and third lenses and the total focal length f of the lens system satisfy the condition 0.9 < f23 / f < 1.3.

[0027] The conditions mentioned for the ratio of the focal lengths of a respective lens to the total focal length of the lens system achieve an advantageous balancing of the respective refractive power of the lens with the sensitivity to manufacturing tolerances, in particular with respect to the manufacturing tolerances mentioned above.

[0028] Advantageously, the second and third lenses contribute the largest amount to the total refractive power of the lens system. Thus, the combined focal length f23 of the second and third lenses approximately corresponds to the total focal length f of the lens system.

[0029] According to yet another advantageous embodiment, the object-side surface of the first lens is convex. Preferably, the image-side surface of the first lens is concave.

[0030] Advantageously, the overall length L and the total focal length f of the lens system satisfy the condition L / f < 1.1. The overall length L is understood to be the distance between the vertices of the object-side surface of the first lens and the image-side surface of the fourth lens. By specifying such a ratio between the overall length and the total focal length, the compactness of the lens system according to the invention can be characterized independently of its total focal length, since the focal length of the lens system is ultimately scalable.

[0031] Advantageously, all lenses are arranged at fixed distances from one another and cannot be moved relative to one another. The lens system therefore has a fixed focal length and is not designed as a zoom lens.

[0032] According to a further advantageous embodiment of the invention, it is provided that the lens system has a wavefront error W(p, A) at a point of the exit pupil, which is determined by a sum of orthogonal Zernike fringe polynomials P i (p, A) and corresponding coefficients Z i (also written as “Zi”) in the form W(p,A)=∑i=137Zi⋅Pi(p,A) where p is the normalized pupil height and A is the azimuthal angle, with the defocus being determined by the coefficient Z4 and the polynomial 2p 2 - 1, the 3rd order spherical aberration by the coefficient Z9 and the polynomial 6p 4 - 6p 2 + 1, the 5th order spherical aberration by the coefficient Z16 and the polynomial 20p 6 - 30p 4 + 12p 2 - 1, the 7th order spherical aberration by the coefficient Z25 and the polynomial 70p 8 - 140p 6 + 90p 4 - 20p 2+ 1, the 9th order spherical aberration by the coefficient Z36 and the polynomial 252p 10 - 630p 8 + 560p 6 - 210p 4 + 30p 2 - 1 and the 11th order spherical aberration by the coefficient Z37 and the polynomial 924p 12 - 2772p 10 + 3150p 8 - 1680p 6 + 420p 4 - 42p 2 + 1, and wherein, in particular for an axial image point and / or an object at infinity, at least one of the conditions 1.0 <Z92+Z162+Z252+Z362+Z372 / Z4<1.9 and 0.35 <Z162+Z252+Z362+Z372 / Z4<0.70 is met, in particular both of the conditions are met. The normalized pupil height p is dimensionless, while Wund Z iare specified in units of wavelength. This design of the lens system balances the low- and high-order spherical aberrations against the defocus, thus achieving the largest possible depth of field.

[0033] The present invention further relates to an imaging system with an image sensor having a plurality of light-sensitive elements arranged in rows and columns, and with an optical lens system according to one of the embodiments explained above. Advantageously, said image sensor is a 3D image sensor, in particular a 3D-TOF image sensor. Preferably, the lens system and the image sensor are designed for infrared light, in particular for narrowband, in particular monochrome, infrared light, with a wavelength between 800 and 1000 nm. The imaging system can further comprise a transmitted light source designed to emit light pulses, in particular in the infrared range. The transmitted light source can be controlled via a control unit of the 3D image sensor, so that the propagation time of a light pulse can be determined pixel by pixel, i.e., spatially resolved, separately for each light-sensitive element.

[0034] The size of the image sensor and the characteristic parameters of the lens system can be matched so that the full angle of view is 66° or more, corresponding to a half angle of view of + / - 33° or more in the corners of the image sensor. The aperture is preferably less than 1.5.

[0035] The principal ray angle, i.e., the angle of incidence of a light beam onto the surface of the image sensor relative to the normal, is a maximum of 32° across the entire image field. This ensures optimal sensitivity even when using an image sensor with microlenses arranged in front of its light-sensitive elements.

[0036] Advantageously, the distance between the image sensor and the lens system remains constant. This makes it a fixed-focus imaging system.

[0037] According to an advantageous embodiment of the invention, the lens system has spherical aberrations which are dimensioned taking into account the area of ​​a light-sensitive element in such a way that the spherical aberrations compensate for the focus-related blurring of an image of an object generated by the lens system in such a way that the area of ​​a light spot generated by imaging a beam of rays emanating from an object point on the image sensor is substantially no larger than the area of ​​the light-sensitive element within a predetermined object width range.In particular, the surfaces of the light spot and the light-sensitive element can be related to one another in such a way that, when the light spot strikes a light-sensitive element centrally, more than one-third, preferably more than half, particularly preferably more than two-thirds of the light energy of the light spot strikes the light-sensitive element. The aforementioned condition is met in particular for an object width range between infinity and 1 m, preferably 0.5 m, particularly preferably 0.25 m.

[0038] Alternatively or additionally, the condition that the stated area of ​​the light spot is essentially no larger than the area of ​​the light-sensitive element within a given object width range can also be met, for example, by the radius of the circle of confusion of the light spot, in particular the RMS light spot radius explained above, being no larger than 12 µm, and the edge length of the light-sensitive element being 14 µm. Thus, the stated conditions optimally match the blur of the lens system and the pixel size of the image sensor, i.e., the resolution of the image sensor.

[0039] Further advantageous embodiments of the invention emerge from the subclaims, the description and the drawings.

[0040] The invention is described below using exemplary embodiments with reference to the drawings. In the drawings: Fig. 1 shows a lens system according to a first embodiment; Fig. 2 diagrams of the transverse aberrations of the lens system of Fig. 1; Fig. 3 diagrams of further aberrations of the lens system of Fig. 1; Fig. 4 a longitudinal section through a section of the beam path of Fig. 1; Fig. 5 a lens system according to a second embodiment; Fig. 6 diagrams of the transverse aberrations of the lens system of Fig. 5; and Fig. 7 diagrams of further aberrations of the lens system of Fig. 5.

[0041] In the figures, identical or similar components are numbered with the same reference numerals.

[0042] Fig. Figure 1 shows an optical lens system with four refractive lens elements according to a first embodiment. An image plane B of the lens system is represented by a vertical line, while an object plane O is located outside the drawing area. The following describes the arrangement of the various elements of the lens system in a light propagation direction of the beam path, starting from the object side to the image side. Relative positional specifications such as "in front of" or "behind" refer to this order.

[0043] The lens system comprises a first lens L1 with positive refractive power, a second lens L2 with positive refractive power, a third lens L3 with positive refractive power, and a fourth lens L4 with negative refractive power. The first lens L1 is surrounded by an aperture stop A. Behind the fourth lens L4, a plane-parallel plate P serves as a cover glass. The plane-parallel plate P can be designed as a blocking filter for visible light, allowing only infrared light to pass through.

[0044] All lenses L1 to L4 are designed as meniscus lenses with one convex and one concave surface. The first, second, and fourth lenses L1, L2, and L4 are arranged so that the object-side surfaces F11, F21, and F41 are convex and the image-side surfaces F12, F22, and F42 are concave. The third lens is arranged in the opposite direction, so that the object-side surface F31 is concave and the image-side surface F32 is convex.

[0045] The lenses L1 to L4 are made of plastic and have a refractive index of 1.661 and an Abbe number of 20.4. The lens system according to the first embodiment has a total focal length f of 2.62 mm, an aperture of 1.25, a length L, measured between the vertex of the object-side surface of the first lens L1 and the image-side vertex of the fourth lens L4, of 2.66 mm, an image height of 1.97 mm, and a half angle of view of 35.5°.

[0046] The plane-parallel plate is made of glass and has a refractive index of 1.523 and an Abbe number of 54.5. All surfaces of the lenses L1 to L4 are aspherical. For an apex z of a respective lens surface parallel to the optical axis OA at a point with a height h relative to the optical axis OA and perpendicular to it, the following applies: z(h)=h2 / r01+1−(1+k)(h / r0)2+a4⋅h4+a6⋅h6+…+a16⋅h16 where r0 is the vertex curvature radius, k is the conic constant and a4, a6, ..., a16 are the asphere coefficients.

[0047] The corresponding structural data of the lens system according to the first embodiment are given in Table 1, wherein the surfaces present in the lens system are numbered according to their sequence from 0 to 12. The assignment to the various elements of the lens system results from the table, wherein for each lens L1 to L4 the object-side surface is given first and then the image-side surface and additionally in brackets the corresponding reference numerals of Fig. 1. The thickness d indicates the distance of each surface from the next. The radius r0, the thickness d, and the focal length are all given in mm. The conic constant k is zero for all lenses L1 to L4. Table 1: Fläche Element Radius r0 Dicke d Material Index Abbe # Brennweite 0 Objekt Plan ∞ 1 Blende Plan -0.366 2 (F11) Linse L1 1.421 0.595 Plastik 1.661 20.4 11.64 3 (F12) 1.472 0.134 4 (F21) Linse L2 1.121 0.302 Plastik 1.661 20.4 3.69 5 (F22) 1.922 0.559 6 (F31) Linse L3 -1.303 0.296 Plastik 1.661 20.4 8.21 7 (F32) -1.135 0.059 8 (F41) Linse L4 2.447 0.712 Plastik 1.661 20.4 -29.47 9 (F42) 1.920 0.128 10 Deckglas Plan 0.210 Glas 1.523 54.5 11 Plan 0.590 12 Bild Plan Sensor

[0048] The aspheric coefficients a4, a6, ..., a16 are given below in Table 2 for surfaces 2 to 9 of lenses L1 to L4. Table 2: Fläche a4 a6 a8 a10 2 (F11) -0.072375119 0.11612465 0.2111098 0.18270985 3 (F12) -0.23567085 -0.098423245 0.23908259 -0.50931788 4 (F21) -0.2485888 0.35034196 -1.2113076 -0.008843515 5 (F22) 0.217257 -0.46390236 0.25516898 -0.38707056 6 (F31) 0.43683291 -1.3557549 2.6451936 -1.6661149 7 (F32) 0.023106241 0.45518488 -1.6958001 3.9359768 8 (F41) -0.3894922 0.43761569 -0.29996824 0.12677858 9 (F42) -0.28038751 0.15754306 -0.070084236 0.017339992 Fläche a12 a14 a16 2 (F11) -0.08073422 0 0 3 (F12) 0.47199749 -0.15230936 0 4 (F21) 1.4837768 -0.76996347 0 5 (F22) 0.38433529 0 0 6 (F31) 0.14141413 0 0 7 (F32) -3.061051 0.38969131 0.28545062 8 (F41) -0.030379063 0.0030922295 0 9 (F42) -0.001955181 0 0

[0049] The lens system according to Fig. 1 has a wavefront error W(p, A) for a point of the exit pupil, which is determined by a sum of orthogonal Zernike fringe polynomials P i (p, A) and corresponding coefficients Z i (or Zi) in the form W(p,A)=∑i=137Zi⋅Pi(p,A) where p is the normalized pupil height and A is the azimuthal angle. Accordingly, p can take values ​​between 0 and 1 and A between 0 and 2π. The wavefront error W and the corresponding coefficients Z i (or Zi) are given in units of wavelength. Table 3 shows the values ​​of the Zernike fringe coefficients Z1 to Z37 and the corresponding polynomials P for an axial image point and an object at infinity.i specified. Table 3: Z 1 -0.57920179 1 Z 2 0.00000000 (p) * COS (A) Z 3 0.00000000 (p) * SIN (A) Z 4 -0.16329097 (2p^2 - 1) Z 5 0.00000000 (p^2) * COS (2A) Z 6 0.00000000 (p^2) * SIN (2A) Z 7 0.00000000 (3p^2 - 2) p * COS (A) Z 8 0.00000000 (3p^2 - 2) p * SIN (A) Z 9 0.54962059 (6p^4 - 6p^2 + 1) Z 10 0.00000000 (p^3) * COS (3A) Z 11 0.00000000 (p^3) * SIN (3A) Z 12 0.00000000 (4p^2-3) p^2 * COS (2A) Z 13 0.00000000 (4p^2-3) p^2 * SIN (2A) Z 14 0.00000000 (10p^4 - 12p^2 + 3) p * COS (A) Z 15 0.00000000 (10p^4 - 12p^2 + 3) p * SIN (A) Z 16 -0.04107681 (20p^6 - 30p^4 + 12p^2 - 1) Z 17 -0.00009667 (p^4) * COS (4A) Z 18 0.00000000 (p^4) * SIN (4A) Z 19 0.00000000 (5p^2 - 4) p^3 * COS (3A) Z 20 0.00000000 (5p^2 - 4) p^3 * SIN (3A) Z 21 0.00000000 (15p^4 - 20p^2 + 6) p^2 * COS (2A) Z 22 0.00000000 (15p^4 - 20p^2 + 6) p^2 * SIN (2A) Z 23 0.00000000 (35p^6 - 60p^4 + 30p^2 - 4) p * COS (A) Z 24 0.00000000 (35p^6 - 60p^4 + 30p^2 - 4) p * SIN (A) Z 25 0.06376645 (70p^8 - 140p^6 + 90p^4 - 20p^2 + 1) Z 26 0.00000000 (p^5) * COS (5A) Z 27 0.00000000 (p^5) * SIN (5A) Z 28 -0.00013088 (6p^2 - 5) p^4 * COS (4A) Z 29 0.00000000 (6p^2 - 5) p^4 * SIN (4A) Z 30 0.00000000 (21p^4 - 30p^2 + 10) p^3 * COS (3A) Z 31 0.00000000 (21p^4 - 30p^2 + 10) p^3 * SIN (3A) Z 32 0.00000000 (56p^6 - 105p^4 + 60p^2 - 10) p^2 * COS (2A) Z 33 0.00000000 (56p^6 - 105p^4 + 60p^2 - 10) p^2 * SIN (2A) Z 34 0.00000000 (126 p^8 - 280p^6 + 210p^4 - 60p^2 + 5) p * COS (A) Z 35 0.00000000 (126 p^8 - 280p^6 + 210p^4 - 60p^2 + 5) p * SIN (A) Z 36 0.07678453 (252p^10 - 630p^8 + 560p^6 - 210p^4 + 30p^2 - 1) Z 37 -0.13813242 (924p^12 - 2772p^10 + 3150p^8 - 1680p^6 + 420p^4 - 42p^2 + 1)

[0050] It is noted that the Zernike fringe coefficients Z i were calculated using numerical methods. Although, due to the rotationally symmetric design of the lens system, a non-zero value is in principle only expected for the coefficients Z1, Z4, Z9, Z16, Z25, Z36, and Z37 for an axial image point, Table 3 and the corresponding Table 6 contain several additional non-zero coefficients, namely the coefficients Z17 and Z28. However, their value is several orders of magnitude lower than the values ​​of the rotationally symmetric coefficients mentioned above. These values ​​are due to computational inaccuracies, particularly rounding inaccuracies.

[0051] In Fig. 2 shows the transverse aberrations ex, ey (in µm) for the normalized pupil coordinates in x- and y-direction.

[0052] In Fig. 3a is the spherical aberration (in mm) as a function of the normalized pupil coordinate, in Fig. 3b the astigmatism (in mm) as a function of the half angle of view (in degrees) and in Fig. 3c shows the distortion (in percent) as a function of the half angle of view (in degrees).

[0053] In Fig. Figure 4 shows various rays with a normalized pupil height of 0, 0.25, 0.5, 0.75, and 1, which intersect at different intersection planes. The intersection planes are represented by vertical lines labeled with the corresponding pupil coordinate. The intersection planes are located within a focal length range of 75 µm. It should be noted that for pupil height 0, no actual intersection point can be identified, since the corresponding rays are infinitesimally close to the optical axis OA.

[0054] In Fig. Figure 5 shows a lens system according to a second embodiment of the invention. Since the structural design of the lens system differs from Fig. 5 does not depend on the design of the lens system of Fig. 1, please refer to the corresponding description Fig. 1.

[0055] In deviation from the embodiment according to Fig. 1, the lenses L1 to L4 are made of glass and have a refractive index of 1.743 and an Abbe number of 49.3. The lens system according to Fig. 5 has a total focal length f of 2.54 mm, an aperture of 1.25, an overall length L of 2.61 mm, an image height of 1.97 mm and a half angle of view of 35.5°.

[0056] The corresponding structural data of the lens system of Fig. 5 are given in Table 4 and the corresponding asphere coefficients a4, a6, ..., a16 in Table 5, where the sizes and their units correspond to the information in Tables 1 and 2. The conical constant k is zero for all lenses L1 to L4. Table 4: Fläche Element Radius r0 Dicke d Material Index Abbe # Brennweite 0 Objekt Plan ∞ 1 Blende A Plan -0.293 2 (F11) Linse L1 1.516 0.566 Glas 1.743 49.3 15.34 3 (F12) 1.477 0.128 4 (F21) Linse L2 1.128 0.297 Glas 1.743 49.3 3.36 5 (F22) 1.855 0.583 6 (F31) Linse L3 -1.364 0.301 Glas 1.743 49.3 6.16 7 (F32) -1.144 0.184 8 (F41) Linse L4 2.457 0.546 Glas 1.743 49.3 -22.47 9 (F42) 1. 937 0.184 10 Deckglas Plan 0.210 Glas 1.523 54.5 11 Plan 0.590 12 Bild Plan Sensor Table 5: Fläche a4 a6 a8 a10 2 (F11) -0.075937186 0.054712247 -0.073160234 0.051208664 3 (F12) -0.24510218 -0.36049224 0.91239014 -1.254869 4 (F21) -0.29531824 0.53305516 -2.2321386 2.2734468 5 (F22) 0.22392329 -0.52272802 0.16748942 0.063541082 6 (F31) 0.27432111 -0.96865964 2.2034305 -1.5438713 7 (F32) 0.039573477 0.4435482 -1.5793287 3.2461717 8 (F41) -0.26565332 0.23303717 -0.13924911 0.054903712 9 (F42) -0.27380619 0.16252771 -0.080990904 0.024674249 Fläche a12 a14 a16 2 (F11) -0.038024183 0 0 3 (F12) 0.88357042 -0.24509103 0 4 (F21) -0.50546073 -0.16841905 0 5 (F22) 0.084928232 0 0 6 (F31) 0.21758246 0 0 7 (F32) -2.1376274 -0.043280605 0.33521613 8 (F41) -0.012826538 0.0012799542 0 9 (F42) -0.004113719 0.0002384066 0

[0057] The wavefront error W(p, A) is analogous to the first embodiment by a sum of Zernike fringe polynomials P i with the corresponding coefficients Z i as described in Table 6. Table 6: Z 1 0.48647117 1 Z 2 0.00000000 (p) * COS (A) Z 3 0.00000000 (p) * SIN (A) Z 4 0.51499558 (2p^2 - 1) Z 5 0.00000000 (p^2) * COS (2A) Z 6 0.00000000 (p^2) * SIN (2A) Z 7 0.00000000 (3p^2 - 2) p * COS (A) Z 8 0.00000000 (3p^2 - 2) p * SIN (A) Z 9 0.43007385 (6p^4 - 6p^2 + 1) Z 10 0.00000000 (p^3) * COS (3A) Z 11 0.00000000 (p^3) * SIN (3A) Z 12 0.00000000 (4p^2-3) p^2 * COS (2A) Z 13 0.00000000 (4p^2-3) p^2 * SIN (2A) Z 14 0.00000000 (10p^4 - 12p^2 + 3) p * COS (A) Z 15 0.00000000 (10p^4 - 12p^2 + 3) p * SIN (A) Z 16 0.20751425 (20p^6 - 30p^4 + 12p^2 - 1) Z 17 -0.00003308 (p^4) * COS (4A) Z 18 0.00000000 (p^4) * SIN (4A) Z 19 0.00000000 (5p^2 - 4) p^3 * COS (3A) Z 20 0.00000000 (5p^2 - 4) p^3 * SIN (3A) Z 21 0.00000000 (15p^4 - 20p^2 + 6) p^2 * COS (2A) Z 22 0.00000000 (15p^4 - 20p^2 + 6) p^2 * SIN (2A) Z 23 0.00000000 (35p^6 - 60p^4 + 30p^2 - 4) p * COS (A) Z 24 0.00000000 (35p^6 - 60p^4 + 30p^2 - 4) p * SIN (A) Z 25 0.14462009 (70p^8 - 140p^6 + 90p^4 - 20p^2 + 1) Z 26 0.00000000 (p^5) * COS (5A) Z 27 0.00000000 (p^5) * SIN (5A) Z 28 -0.00004471 (6p^2 - 5) p^4 * COS (4A) Z 29 0.00000000 (6p^2 - 5) p^4 * SIN (4A) Z 30 0.00000000 (21p^4 - 30p^2 + 10) p^3 * COS (3A) Z 31 0.00000000 (21p^4 - 30p^2 + 10) p^3 * SIN (3A) Z 32 0.00000000 (56p^6 - 105p^4 + 60p^2 - 10) p^2 * COS (2A) Z 33 0.00000000 (56p^6 - 105p^4 + 60p^2 - 10) p^2 * SIN (2A) Z 34 0.00000000 (126 p^8 - 280p^6 + 210p^4 - 60p^2 + 5) p * COS (A) Z 35 0.00000000 (126 p^8 - 280p^6 + 210p^4 - 60p^2 + 5) p * SIN (A) Z 36 0.16152218 (252p^10 - 630p^8 + 560p^6 - 210p^4 + 30p^2 - 1) Z 37 -0.15785051 (924p^12 - 2772p^10 + 3150p^8 - 1680p^6 + 420p^4 - 42p^2 + 1)

[0058] The diagrams of the Fig. 6 and 7a to 7c show the transverse aberrations, the spherical aberration, the astigmatism and the distortion analogous to the diagrams of Fig. 2 and 3a to 3c.

[0059] If a lens system according to one of the two embodiments is to be used as a component of an imaging system, a corresponding image sensor, in particular a 3D image sensor, can be arranged in the image plane B.

[0060] The two exemplary embodiments explained above show two exemplary configurations for a lens system according to the invention, in which the targeted introduction of spherical aberrations can be used to at least partially compensate for defocus-induced image blur, thereby achieving acceptable image sharpness over a very wide object distance range. Such a lens system can be used to realize a fixed-focal-length imaging system, in particular a 3D imaging system for use with 3D-TOF image sensors, which has a high light intensity, a large depth of field, and a compact design.

[0061] The example lens systems are optimized for a wavelength range of 800 to 1000 nm. The calculation was performed for a wavelength of 890 nm.

[0062] The structural data and other parameters of the two embodiments of a lens system according to the invention are merely examples. It is understood that lens systems with different structural data and other parameters may also be encompassed by the present invention.

[0063] Compared to known lens systems, which have an aperture of 1.58 or higher, the aperture can be reduced by 21% with the aid of a lens system according to the invention, in particular with the aid of a lens system according to one of the exemplary embodiments explained above. Thus, a lens system according to the invention has 1.6 times the light intensity compared to a conventional lens system. The increase in light intensity improves the signal-to-noise ratio at the image sensor.

[0064] This can be used, in particular, to reduce the pixel size of the image sensor from 17.5 µm, as is common in conventional 3D imaging systems, to 14 µm, allowing the use of a significantly more compact 3D image sensor compared to a conventional 3D imaging system. The compact length of the lens system allows for the realization of an imaging system that can be shortened by up to 31% compared to a conventional imaging system. The lens system according to the invention is particularly suitable for installation in mobile devices such as mobile phones or tablet computers. List of reference symbols A aperture diaphragm B Image plane F11 object-side surface of the first lens F12 image-side surface of the first lens F21 object-side surface of the second lens F22 image-side surface of the second lens F31 object-side surface of the third lens F32 image-side surface of the third lens F41 object-side surface of the fourth lens F42 image-side surface of the fourth lens L Length of the lens system L1 first lens L2 second lens L3 third lens L4 fourth lens O Object level OA optical axis P plane-parallel plate

Claims

[1] An optical lens system for a camera, comprising exactly four lenses (L1, L2, L3, L4), wherein a first lens (L1) having a positive refractive power, a second lens (L2) having a positive refractive power, a third lens (L3) having a positive refractive power, and a fourth lens (L4) having a negative refractive power are provided in a sequence from an object-side end to an image-side end, wherein the second and the third lens (L2, L3) are each designed as a meniscus lens, wherein the object-side surface (F21) of the second lens (L2) is convex and the object-side surface (F31) of the third lens (L3) is concave, and wherein a radius of curvature L2R1 of the object-side surface (F21) of the second lens (L2) and a radius of curvature L3R2 of the image-side surface (F32) of the third lens (L3) satisfy the condition -1.4 < L2R1 / L3R2 < -0,7 and a radius of curvature L2R2 of the image-side surface (F22) of the second lens (L2) and a radius of curvature L3R1 of the object-side surface (F31) of the third lens (L3) satisfy the condition -1.8 < L2R2 / L3R1 < -1.

0. [2] Optical lens system according to claim 1, characterized by that each lens (L1 - L4) has at least one aspherically curved surface (F11 - F42). [3] Optical lens system according to one of the preceding claims, characterized by that the first lens (L1) is designed as a meniscus lens. [4] Optical lens system according to one of the preceding claims, characterized by that the refractive power of the second lens (L2) is greater than the refractive power of the first and third lenses (L1, L3). [5] Optical lens system according to one of the preceding claims, characterized by that the refractive power of the first lens (L1) is smaller than the refractive power of the second and third lenses (L2, L3). [6] Optical lens system according to one of the preceding claims, characterized by that the focal length f1 of the first lens (L1) and the total focal length f of the optical lens system satisfy the condition 4.0 < f1 / f < 6.

5. [7] Optical lens system according to one of the preceding claims, characterized by that the focal length f2 of the second lens (L2) and the total focal length f of the optical lens system satisfy the condition 1.0 < f2 / f < 2.

0. [8] Optical lens system according to one of the preceding claims, characterized by that the focal length f3 of the third lens (L3) and the total focal length f of the optical lens system satisfy the condition 2.0 < f3 / f < 3.

5. [9] Optical lens system according to one of the preceding claims, characterized by that the focal length f4 of the fourth lens (L4) and the total focal length f of the optical lens system satisfy the condition -12.0 < f4 / f < -6.

0. [10] Optical lens system according to one of the preceding claims, characterized by that the common focal length f23 of the second and third lenses (L2, L3) and the total focal length f of the optical lens system satisfy the condition 0.9 < f23 / f < 1.

3. [11] Optical lens system according to one of the preceding claims, characterized by that the object-side surface (F11) of the first lens (L1) is convex. [12] Optical lens system according to one of the preceding claims, characterized by that the overall length L and the total focal length f of the optical lens system satisfy the condition L / f < 1.

1. [13] Optical lens system according to one of the preceding claims, characterized by that all lenses (L1 - L4) are arranged at fixed distances from each other. [14] Optical lens system according to one of the preceding claims, characterized bythat the optical lens system has spherical aberrations which are dimensioned in such a way that they at least partially compensate for a blurring caused by defocus of an image of an object produced by the optical lens system. [15] Optical lens system according to one of the preceding claims, characterized by that the optical lens system has a wavefront error W(p, A) at a point of the exit pupil, which is determined by a sum of orthogonal Zernike-Fringe polynomials P i (p, A) and corresponding coefficients Z i in the form W(p,A)=∑i=137Zi⋅Pi(p,A) where p is the normalized pupil height and A is the azimuthal angle, with the defocus being determined by the coefficient Z4 and the polynomial 2p 2 - 1, the 3rd order spherical aberration by the coefficient Z9 and the polynomial 6p 4 - 6p 2+ 1, the 5th order spherical aberration by the coefficient Z16 and the polynomial 20p 6 - 30p 4 + 12p 2 - 1, the 7th order spherical aberration by the coefficient Z25 and the polynomial 70p 8 - 140p 6 + 90p 4 - 20p 2 + 1, the 9th order spherical aberration by the coefficient Z36 and the polynomial 252p 10 - 630p 8 + 560p 6 - 210p 4 + 30p 2 - 1 and the spherical aberration of the 11th order by the coefficient Z37 and the polynomial 924p 12 - 2772p 10 + 3150p 8 - 1680p 6 + 420p 4 - 42p 2 + 1, and wherein at least one of the conditions 1.0 <Z92+Z162+Z252+Z362+Z372 / Z4<1.9 and 0.35 <Z162+Z252+Z362+Z372 / Z4<0.70 is fulfilled. [16] Optical lens system according to one of the preceding claims, characterized bythat the radius of curvature L2R1 of the object-side surface (F21) of the second lens (L2) and the radius of curvature L3R2 of the image-side surface (F32) of the third lens (L3) satisfy the condition -1.2 < L2R1 / L3R2 < -0.8 and the radius of curvature L2R2 of the image-side surface (F22) of the second lens (L2) and the radius of curvature L3R1 of the object-side surface (F31) of the third lens (L3) satisfy the condition -1.6 < L2R2 / L3R1 < -1.2 [17] An imaging system comprising an image sensor having a plurality of light-sensitive elements arranged in rows and columns, and an optical lens system according to any one of the preceding claims. [18] Imaging system according to claim 17, characterized by that the distance between the image sensor and the optical lens system is constant. [19] Imaging system according to claim 17 or 18, characterized bythat the optical lens system has spherical aberrations which, taking into account the area of a light-sensitive element, are dimensioned in such a way that the spherical aberrations compensate for a blurring caused by defocus of an image of an object produced by the optical lens system in such a way that the area of a light spot which is produced by imaging a beam of rays emanating from an object point on the image sensor is essentially not larger than the area of the light-sensitive element within a predetermined object width range.

Images