An optical imaging method for large field of view TOF cameras

Abstract

The application relates to an optical imaging method for a large-view-field TOF camera. Incident light rays are converged by three pieces of spherical lens groups, the spherical lens groups in the light ray incident direction successively include a first piece of negative meniscus lens, a second piece of negative meniscus lens and a first piece of positive double-convex lens; the converged light rays are reduced in vignetting by an aperture diaphragm; an aspheric double-concave lens with an aspheric front surface and a spherical rear surface is used for aberration correction; then two pieces of spherical lens groups are used for twice refraction focusing light paths, correcting aberration and clearly imaging on an image plane; the two pieces of spherical lens groups successively include a positive double-concave lens and a second piece of positive double-convex lens. The imaging method provided by the application uses only one aspheric mirror, the rest of the lens surfaces are spherical, and all the lenses use the same glass material; the system uses an image far field and a reverse distance structure, and meets the requirements of the TOF camera on a large view field, miniaturization and good image quality.

Description

TECHNICAL FIELD

[0001] The present application relates to an optical imaging method suitable for a large field of view TOF camera, belonging to the field of optical technology. BACKGROUND

[0002] TOF (Time-of-Flight) is a three-dimensional measurement technology that uses infrared light of a certain wavelength as an intermediary to measure the depth information of an object by measuring the round-trip time of the infrared light. The lens in the TOF camera is used to collect the image of the infrared light irradiation area and search for the target within the field of view of the lens. The main design idea of the camera structure is to realize the miniaturization, lightweight, high imaging quality and other requirements of the spatial TOF camera. However, in order to capture as much scene range as possible, it is also necessary to meet the requirements of a large field of view and small distortion. This is the difficulty of the current TOF camera optical system design. In order to solve this difficulty, the TOF camera design usually uses a wide-angle lens and a complex optical system to expand the field of view of the camera and reduce the degree of distortion. At the same time, different optical parameters such as focal length, aperture and the number of objectives need to be balanced in the design to optimize the imaging quality and performance of the camera to the greatest extent.

[0003] Before the present application was made, Chinese invention patent CN113885178A provided an imaging method for a wide-spectrum image-side telecentric athermalization optical system, which used a large number of lenses and had a complex structure. Chinese invention patent CN106054360A disclosed an imaging method for a spatial image-side telecentric lens, but its field of view angle was only 16°, the field of view was small, the imaging range was small, and it could not meet the requirements of multiple scene ranges. SUMMARY

[0004] The present application provides an optical imaging method for a large field of view TOF camera with a large field of view and excellent image quality to overcome the deficiencies of the prior art.

[0005] In order to achieve the above-mentioned application purpose, the technical solution adopted by the present application is to provide an optical imaging method for a large field of view TOF camera, comprising the following steps:

[0006] (1) converging light rays: the incident light rays are converged by a three-piece spherical lens group, which includes a first negative meniscus lens, a second negative meniscus lens and a first positive double convex lens in order according to the direction of the incident light rays;

[0007] (2) reducing vignetting: the converged light rays are subjected to vignetting reduction through an aperture stop;

[0008] (3) correcting aberration: the light rays processed in step (2) are subjected to aberration correction by a piece of aspherical double concave lens with an aspherical front surface and a spherical rear surface;

[0009] (4) further focusing the light path, correcting aberration, and clearly imaging: a two-piece spherical lens group is further used to focus the light path by refraction twice, correct aberration, and clearly image on the image plane; the two-piece spherical lens group comprises a positive biconcave lens and a second positive biconvex lens in sequence;

[0010] The optical elements adopt a common optical axis structure, wherein the first negative meniscus lens and the second positive biconvex lens, the second negative lens and the positive biconcave lens, and the first positive biconvex lens and the negative aspheric biconcave lens are approximately symmetric about the diaphragm, respectively;

[0011] The light passing aperture of the first negative meniscus lens is 3.5mm-4.5mm, and the thickness is 2mm-2.6mm; the light passing aperture of the second negative meniscus lens is 2mm-3mm, and the thickness is 0.5mm-1mm; the light passing aperture of the first positive biconcave lens is 1mm-2mm, and the thickness is 1mm-1.5mm; the light passing aperture of the negative aspheric lens is 1mm-2mm, and the thickness is 0.5mm-1.5mm; the light passing aperture of the positive biconcave lens is 2mm-3mm, and the thickness is 1mm-2mm; and the light passing aperture of the second positive biconvex lens is 2mm-3mm, and the thickness is 0.5mm-1.5mm;

[0012] The aspheric front surface of the aspheric biconcave lens is used to construct a Cartesian space rectangular coordinate system with the intersection point of the surface and the optical axis as the origin O, the light incidence direction as the positive direction of the Z axis, the positive direction of the Y axis upward, and the positive direction of the X axis perpendicular to the paper and inward, and the sagitta equation z of the even aspheric surface is:

[0013]

[0014] Wherein, r is the light passing aperture radius of the aspheric lens, and satisfies the condition: 1 -10 a3<3×10 -10 a4<-7×10 -14 a5<7×10 -14 a6<-4×10 -17 a7<4×10 -17 a8<-5×10 -10 a5<5×10 -20 .

[0015] The optical imaging method for the large field of view TOF camera adopts the same glass material with a refractive index >1.5 and an Abbe number <30.

[0016] Compared with the prior art, the present application has the following advantages:

[0017] 1. The present application provides an optical imaging method suitable for a large field of view TOF camera, the optical system adopts six lenses, only one aspheric lens is used, and the mold cost is saved and the tolerance is advantageous; the lenses used in the optical system are selected from the same ZF7 flint glass material, which solves the contradiction that ordinary materials cannot be completely transmitted in the short wave infrared band, and is conducive to reducing processing errors.

[0018] 2. The optical imaging method provided by the present application adopts an optical imaging system which is telecentric on the image side, and the maximum distortion percentage is 1.38%, which can be used for high-precision target positioning and measurement; the maximum outline of the optical imaging system is The center of curvature of the last lens is 2.5mm away from the image plane, and the structure is small and compact. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a work flow chart of an optical imaging method for a large field of view TOF camera provided by an embodiment of the present application;

[0020] Figure 2 is an optical system light path diagram of an optical imaging method for a large field of view TOF camera provided by an embodiment of the present application;

[0021] Figure 3 is a transfer function MTF curve diagram of an optical imaging method for a large field of view TOF camera provided by an embodiment of the present application;

[0022] Figure 4 is a point column diagram of an optical imaging method for a large field of view TOF camera provided by an embodiment of the present application, which is an optical system of each field of view in the visible light band;

[0023] Figure 5 is a distortion curve diagram of an optical imaging method for a large field of view TOF camera provided by an embodiment of the present application.

[0024] In the figure, 1. The first negative meniscus lens; 2. The second negative meniscus lens; 3. The first positive double convex lens; 4. The aperture stop; 5. The aspheric double concave lens; 6. The positive double concave lens; 7. The second positive double convex lens; 8. The imaging surface. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be further described below in combination with the drawings and embodiments.

[0026] Embodiment 1

[0027] The embodiment provides an optical imaging method and an optical imaging system for a large field of view TOF camera.

[0028] Referring to the accompanying drawings Figure 1 , 2 , respectively, are a work flow chart of the optical imaging method and an optical path chart of the optical system of the optical imaging system for a large field of view TOF camera provided by the embodiment.

[0029] Combined with Figure 1 and Figure 2 It can be seen that the imaging method comprises the following steps:

[0030] (1) converging light rays: converging incident light rays by using a three-piece spherical lens group, and the spherical lens group in the light ray incident direction comprises a first negative meniscus lens 1, a second negative meniscus lens 2 and a first positive double convex lens 3 in sequence;

[0031] (2) reducing vignetting: reducing vignetting of the converged light rays by using an aperture stop 4;

[0032] (3) correcting aberration: correcting aberration of the light rays processed in the step (2) by using a non-spherical double concave lens 5 with a non-spherical front surface and a spherical rear surface;

[0033] (4) further focusing the light path, correcting aberration and clearly imaging: further focusing the light path, correcting aberration and clearly imaging on an image plane 8 by using a two-piece spherical lens group for secondary refraction; the two-piece spherical lens group comprises a positive double concave lens 6 and a second positive double convex lens 7 in sequence.

[0034] By Figure 2It can be seen that the structure of the image-side telecentric lens suitable for the TOF camera imaging method adopted in the embodiment is a co-optical axis structure, and along the light incident direction, the optical elements include a first negative meniscus lens 1, a second negative meniscus lens 2, a first positive lenticular lens 3, a negative aspheric biconcave lens 5, a positive biconcave lens 6, a second positive lenticular lens 7, and an imaging surface 8 in sequence. An aperture stop 4 is arranged between the positive lenticular lens 3 and the negative aspheric biconcave lens 6. The first negative meniscus lens and the second positive lenticular lens, the second negative lens and the positive biconcave lens, and the first positive lenticular lens and the negative aspheric biconcave lens are approximately symmetric about the stop, respectively. The light passing aperture of the first negative meniscus lens is 3.5 mm to 4.5 mm, and the thickness is 2 mm to 2.6 mm. The light passing aperture of the second negative meniscus lens is 2 mm to 3 mm, and the thickness is 0.5 mm to 1 mm. The light passing aperture of the first positive biconcave lens is 1 mm to 2 mm, and the thickness is 1 mm to 1.5 mm. The front surface of the negative aspheric biconcave lens is aspheric, and the rear surface is spherical. The light passing aperture is 1 mm to 2 mm, and the thickness is 0.5 mm to 1.5 mm. The light passing aperture of the positive biconcave lens is 2 mm to 3 mm, and the thickness is 1 mm to 2 mm. The light passing aperture of the second positive lenticular lens is 2 mm to 3 mm, and the thickness is 0.5 mm to 1.5 mm.

[0035] In the embodiment, the parameters of each optical element (surface) are shown in Table 1.

[0036] Table 1:

[0037]

[0038] In the embodiment, one negative aspheric biconcave lens is adopted to correct aberration to the maximum extent by adjusting the conic coefficient and the aspheric coefficient. A Cartesian space rectangular coordinate system is constructed with the intersection point of the lens surface and the optical axis as the origin O. The positive direction of the Z axis is the light incident direction, the positive direction of the Y axis is upward, and the positive direction of the X axis is inward perpendicular to the paper. The sag equation z of the even aspheric surface is:

[0039] In the embodiment, the coefficients of the even aspheric lens are shown in Table 2.

[0040] Table 2:

[0041] Element No. Conical coefficient [a1] [a2] [a3] [a4] [a5] [a6] 8 -5.632 0 -0.021 0.013 -0.011 4.545 x 10 -3 ]] -7.368 x 10 -4 ]]

[0042] Referring to FIG. 8, Figure 3 which is a large field of view TOF camera optical system working wave band transfer function MTF curve provided in the embodiment; in the figure, the (a) graph, the (b) graph, and the (c) graph are the transfer function MTF curves of the large field of view TOF camera optical system provided in the embodiment on the image surface corresponding to the wavelengths of 840 nm, 850 nm, and 860 nm, respectively. It can be seen from the figure that the MTF curves of the large field of view TOF camera optical system provided in the embodiment are all greater than 0.5 at the spatial frequency of 50 lp / mm, and the MTF curves of the large field of view TOF camera optical system provided in the embodiment are all greater than 0.5 at the spatial frequency of 50 lp / mm.Figure 3 It can be seen that the optical transfer functions of each field of view in each working waveband at 28p / mm are all greater than 0.8, most of the curves are smooth and compact, which indicates that the system has clear and uniform imaging, and the system has good imaging quality and resolution in the full field of view in each working waveband.

[0043] Referring to the attached Figure 4 It is a spot diagram of the optical system of the large-field-of-view TOF camera provided in the embodiment in each field of view in each working waveband. Figure 4 It can be seen that in each working waveband in each field of view, the meridional and sagittal direction spots are uniform and close to the diffraction limit, the aberration of the system is well corrected, the energy is relatively concentrated, and the use requirement is met.

[0044] Referring to the attached Figure 5 It is a distortion curve of the optical system of the large-field-of-view TOF camera provided in the embodiment. The vertical coordinate represents the normalized field of view, and the horizontal coordinate represents the F-Tan distortion percentage of the image in each field of view. Distortion will deform the image, and the smaller the distortion percentage, the smaller the degree of image deformation. The optical system structure uses the approximate symmetrical structure and the telecentric design, so that the maximum distortion in the normalized field of view is not more than 1.38%.

[0045] The above results prove that the optical imaging method for the large-field-of-view TOF camera provided in the embodiment uses only 6 lenses, all the lens materials are ZF7, the working F number is 2.6, the focal length is 3.0mm, and the total length of the system is 18mm. In the compact structure, the optical transfer functions of each working waveband and the full field of view are all greater than 0.8 at 28p / mm, the distortion percentage of each field of view is less than 1.37%, and the imaging quality is good.

Claims

1. An optical imaging method for a large field-of-view TOF camera, characterized in that... Includes the following steps: (1) Converging rays: The incident rays are converged by a group of three spherical lenses. The spherical lens group, according to the direction of ray incident, includes a first negative meniscus lens (1), a second negative meniscus lens (2), and a first positive biconvex lens (3). (2) Reduce vignetting: The converged light rays pass through the aperture stop (4) to reduce vignetting; (3) Correcting aberrations: Aberrations are corrected for the light processed in step (2) using a negative aspherical biconcave lens (5) with an aspherical front surface and a spherical rear surface; (4) Further focus the light path and correct aberrations to achieve clear imaging: Then use two spherical lens groups to perform secondary refraction focusing of the light path and correct aberrations to achieve clear imaging on the image plane (8); the two spherical lens groups include a positive biconcave lens (6) and a second positive biconvex lens (7). The optical elements adopt a coaxial structure, wherein the first negative meniscus lens and the second positive biconvex lens, the second negative lens and the positive biconcave lens, and the first positive biconvex lens and the negative aspherical biconcave lens are approximately symmetrical about the aperture stop. The first negative meniscus lens has an aperture of 3.5mm–4.5mm and a thickness of 2mm–2.6mm; the second negative meniscus lens has an aperture of 2mm–3mm and a thickness of 0.5mm–1mm; the first positive biconcave lens has an aperture of 1mm–2mm and a thickness of 1mm–1.5mm; the negative aspherical biconcave lens has an aperture of 1mm–2mm and a thickness of 0.5mm–1.5mm; the positive biconcave lens has an aperture of 2mm–3mm and a thickness of 1mm–2mm; the second positive biconvex lens has an aperture of 2mm–3mm and a thickness of 0.5mm–1.5mm. The aspherical front surface of the negative aspherical biconcave lens (5) is used to construct a Cartesian coordinate system with the intersection of its surface and the optical axis as the origin O. The incident direction of the light is the positive Z-axis, the positive Y-axis is upward, and the positive X-axis is perpendicular to the paper and inward. The equation for the sag of its even-order aspherical surface, z, is: ； Among them, r is the clear aperture radius of the aspherical lens, satisfying the condition: 1 < r < 1.5; c is the reciprocal of the curvature radius of the aspherical lens, satisfying the condition: 0.3 < c < 0.4; k is the conic coefficient of the aspherical lens, satisfying the condition: -1 < k < 1; a1 to a8 are the coefficients of each monomial, satisfying the condition: -3×10 -10 <a3 < 3×10 -10 、-7×10 -14 <a4 < 7×10 -14 、-4×10 -17 <a5 < 4×10 -17 、-5×10 -10 <a5 < 5×10 -20 .

2. The optical imaging method for a large field-of-view TOF camera according to claim 1, characterized in that: The optical components are made of the same glass material with a refractive index >1.5 and an Abbe number <30.

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