Optical imaging method for an airborne camera

Abstract

The present application relates to an optical imaging method of an airborne camera. Incident light rays are incident to a convex-concave lens through a protection lens, the light beam is converged to be incident to a first plane mirror, the reflected light rays are deflected by 90 degrees to enter a double cemented lens to correct chromatic aberration and spherical aberration, the refracted light rays are incident to a concave-convex thick lens through a second plane mirror after the direction of the reflected light rays is deflected by 180 degrees with the direction of the incident light rays, the refracted light beam is focused on an image plane in the same plane with the protection lens, and high-resolution imaging is obtained. The optical imaging method of the present application adopts a U-shaped structure to fold the optical path, so that the optical system of the airborne camera has a long focal length while reducing the total length of the system and the rotational inertia of the system, ensuring the stability and maneuverability of the aircraft attitude during rotation, and being suitable as an optical load of the aircraft.

Description

Technical Field

[0001] This invention relates to an optical imaging method for an airborne camera, and more particularly to an optical imaging method employing a U-shaped folded optical path, belonging to the field of optical imaging technology. Background Technology

[0002] Drone aerial photography uses airborne remote sensing equipment, such as high-resolution cameras, lightweight optical cameras, and infrared scanners, to acquire information. The image information is then processed by computers and processed into images according to specific precision requirements. It is a new application technology integrating drones, photography, and computer information processing. Drone aerial images have the advantages of high definition, large scale, small area coverage, and high timeliness.

[0003] Unmanned aerial vehicles (UAVs) provide a convenient and easily relocatable remote sensing platform for aerial photography. Takeoff and landing are less restricted by location, and can be conducted on playgrounds, roads, or other relatively open ground. Furthermore, UAVs offer advantages such as high stability, no risk of personnel injury, strong survivability, good maneuverability, and ease of use, making them extremely promising for applications in civilian fields such as aviation, transportation, military, fire and rescue, agriculture, and healthcare. Small-area remote sensing aerial photography using UAVs has achieved significant results and accumulated valuable experience in practice, meeting the needs of national economic and cultural development and providing effective technical services for regional economic and cultural construction, thus playing a vital role in promoting my country's economic development.

[0004] Airborne cameras acquire scene information across a wider space using wide-field-of-view lenses or rotating scanning imaging. However, wide-field-of-view lenses often have relatively short focal lengths, resulting in limitations such as a large instantaneous field of view and low spatial resolution. To acquire higher-resolution images across a wider space, higher-resolution lenses with longer focal lengths are typically required. However, long-focal-length high-resolution lenses have a large barrel length and a large moment of inertia, which can severely affect the stability and maneuverability of the aircraft when rotating. For example, Chinese invention patent CN105950683A discloses an imaging method using a telephoto lens with a total optical length of 60.5mm and an effective focal length of 72.5mm. The ratio of the total length to the focal length of the optical system is 0.83, resulting in a large moment of inertia that cannot meet the requirements for further applications in the field of UAV aerial photography. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an optical imaging method for airborne cameras that employs a U-shaped folded optical path. The optical system features low rotational inertia, a simple and compact structure, and a long focal length.

[0006] To achieve the above-mentioned objectives, the present invention provides an optical imaging method for an airborne camera, comprising the following steps:

[0007] (1) After passing through the protective lens, the incident light enters the convex and concave lens, and the beam is converged and incident on the first plane mirror;

[0008] (2) The first plane mirror is used to deflect the light path, deflecting the reflected light by 90° before it enters the doublet lens, and the refracted light, after being corrected for chromatic aberration and spherical aberration, enters the second plane mirror.

[0009] The cemented doublet lens is formed by cementing the rear surface of a biconvex lens with the front surface of a biconcave lens. The refractive index of the material of the biconvex lens is 1.57 to 1.62, and the refractive index of the material of the biconcave lens is 1.73 to 1.78. The difference between their refractive indices is greater than 0.15.

[0010] (3) The second plane mirror is set perpendicular to the first plane mirror to bend the light path again, so that the propagation direction of the reflected light is deflected by 180° from the direction of the incident light and then incident on the concave-convex thick lens.

[0011] The center thickness d1 and center thickness d2 of the convex and concave lenses satisfy the condition 0.90≤d1 / d2≤1.1; the focal length f1 and focal length f2 of the convex and concave lenses satisfy the condition 0.98≤f1 / f2≤1.02; the air gap d3 between the rear surface of the convex and concave lenses and the center position of the first plane mirror surface, and the air gap d4 between the center position of the second plane mirror surface and the front surface of the convex and concave lenses satisfy the condition 0.98≤d3 / d4≤1.02.

[0012] (4) The light beam refracted by the concave-convex thick lens is focused on the image plane in the same plane as the protective lens, resulting in high-resolution imaging.

[0013] The optical imaging method for an airborne camera described in this invention employs an optical system with a U-shaped folded optical path. Its optical components include a protective lens, a convex-concave lens, a first plane mirror, a cemented doublet lens, a second plane mirror, a thick concave-convex lens, and an image plane. The center of gravity D of the optical system satisfies 0.1mm≤l1-l3≤0.5mm, 0.6mm≤l2-l1≤0.8mm, and 0.6mm≤l2-l3≤0.8mm, where l1, l2, and l3 are the distances from the center of gravity of the convex-concave lens, the cemented doublet lens, and the concave-convex lens, respectively, to D. The rotation center of the system coincides with the center of gravity.

[0014] Compared with existing technologies, the beneficial effects of this invention are as follows: The optical imaging method for airborne cameras provided by this invention shortens the distance from the lens's center of gravity to the rotation center by employing a U-shaped structure to fold the optical path, thereby reducing the lens's moment of inertia. The moment of inertia of the optical system used in this invention mainly depends on three lenses: a convex-concave lens, a cemented doublet lens, and a concave-convex lens. By maintaining approximately equal distances from the three lenses to the rotation center, equal distances between any two lenses, and the center of gravity of the three lenses coinciding with the system's rotation center, the system's moment of inertia is evenly distributed among the three lenses, reducing the overall moment of inertia of the system and ensuring the stability of the airborne camera during rotation and the maneuverability of the aircraft. Attached Figure Description

[0015] Figure 1 This is an optical imaging flowchart of the airborne camera optical system provided in an embodiment of the present invention;

[0016] Figure 2 This is a schematic diagram of the optical path of the airborne camera optical system provided in an embodiment of the present invention;

[0017] Figure 3 This is a ray tracing point array diagram of the airborne camera optical system provided in the embodiments of the present invention;

[0018] Figure 4 This is the energy concentration curve of the airborne camera optical system provided in this embodiment of the invention at the center wavelength;

[0019] Figure 5 This is a graph of the MTF (Mean Transfer Function) curve of the airborne camera optical system provided in an embodiment of the present invention.

[0020] Figure 6 This is an axial chromatic aberration curve of the airborne camera optical system provided in an embodiment of the present invention;

[0021] In the diagram: 1. Protective lens; 2. Convex and concave lens; 3. First plane mirror; 4. Cemented doublet lens; 4.1 Biconvex lens; 4.2 Biconcave lens; 5. Second plane mirror; 6. Concave and convex lens; 7. Image plane. Detailed Implementation

[0022] The specific implementation schemes of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Example 1:

[0024] To reduce the rotational inertia of the airborne camera, this embodiment provides an optical imaging method employing a U-shaped folded optical path, see appendix. Figure 1 The imaging method of an airborne camera optical system includes the following steps:

[0025] Step 1: By using a convex-concave lens to refract and converge the incident light beam behind the protective lens;

[0026] Step 2: The converging beam formed by refraction is deflected by a plane mirror (the first one) at 90° before entering the cemented doublet lens;

[0027] Step 3: The cemented doublet is made of two lenses with different refractive indices and dispersion rates cemented together. It is used to correct chromatic aberration and spherical aberration. After the light is refracted by the biconvex and biconcave lenses of the cemented doublet, it is reflected again by the surface of another plane mirror (the second mirror), causing the light to be deflected by 90° again. Steps 2 and 3 reduce the overall length of the optical system and reduce the rotational inertia of the system by using two plane mirrors.

[0028] Step 4: After two reflections by the plane mirror in Step 2 and Step 3, the light is deflected by 180° and the direction of light propagation is opposite to the direction of incident. Then, a concave-convex lens is used to refract and focus the light to form an image on the image plane. The image plane and the protective lens are in the same plane, thus achieving high-resolution imaging.

[0029] Using the optical imaging method of the airborne camera in this embodiment, an airborne camera optical system is provided, the optical system having a working wavelength of 450nm to 650nm, a focal length of 45mm, and an image-side F number of 3.

[0030] See appendix Figure 2 This is a schematic diagram of the optical path of the airborne camera optical system provided in this embodiment. The optical system adopts a U-shaped folded optical path, with the positive Z-axis as the incident direction of the light. The light entering the optical system along the incident direction passes through the protective lens 1, is refracted by the convex and concave lens 2, and is reflected on the surface of the first plane mirror 3, which is set at an angle of 45° to the Z-axis. The light propagates along the negative X-axis. The light enters the cemented doublet lens 4, and after being refracted by the biconvex lens 4.1 and biconcave lens 4.2 of the cemented doublet lens, the light is reflected again on the surface of the second plane mirror 5. The light propagates along the negative Z-axis. Finally, the light is refracted by the concave and convex lens 6 and imaged at the image plane 7. The protective lens and the image plane are located on the same plane.

[0031] The airborne camera optical system provided in this embodiment has each optical element (surface) that meets the conditions in Table 1.

[0032] Table 1:

[0033]

[0034] The moment of inertia of the airborne camera optical system provided in this embodiment is calculated as follows, based on the moment of inertia of a homogeneous cylindrical rigid body about an arbitrary axis:

[0035]

[0036] Where R, r, and h are the outer diameter, inner diameter, and length of the cylinder, respectively; M is the mass of the cylinder; α is the angle between the axis of rotation and the central axis of the cylinder; and d is the distance from the center of gravity of the cylinder to the axis of rotation.

[0037] To calculate the moment of inertia of a folding optical system, the center of gravity is first selected as the center of rotation. Then, the moment of inertia at all positions is integrated to obtain the moment of inertia of the folding optical system. In this embodiment, the moment of inertia mainly comes from three lenses: a convex-concave lens, a cemented doublet lens, and a concave-convex lens. In this embodiment, the inner diameter of the lens is 0, and the angle α between the axis of rotation and the central axis of the cylinder is 90°. Therefore, the formula for the moment of inertia can be expressed as:

[0038]

[0039] The total moment of inertia is I, and the moments of inertia of the three lenses are I1, I2, and I3, respectively. M i R i h i d i These represent the mass, outer diameter, thickness, and distance from the lens's center of gravity to the rotation axis of the three lenses after folding. The specific parameters are as follows:

[0040]

[0041] Using a conventional straight-tube telephoto lens system that does not incorporate a plane mirror but only employs the same three-lens system as in this embodiment as a comparative example, let the moment of inertia of the conventional optical path system be I′, and the specific parameters of its three lenses are as follows:

[0042]

[0043] Substituting all parameters into equation (2), we can calculate I′ / I = 37%. The U-shaped folded optical path system provided in this embodiment has reduced the moment of inertia by more than 60% compared to the unfolded system. While keeping the focal length unchanged, the total optical length of the system is reduced from 60.9 mm when unfolded to 27 mm.

[0044] See appendix Figure 3 It is a ray tracing point diagram of light passing through an airborne camera optical system provided in this embodiment. The radius of the Airy disk is 2.2 μm. The root mean square radius of the point diagrams corresponding to the three different wavelengths in the figure is less than 1.1 μm, which has good imaging quality and meets the system usage requirements.

[0045] See appendix Figure 4This is the energy concentration curve of the airborne camera optical system provided in this embodiment at the center wavelength. The horizontal axis is the distance from the centroid width, and the vertical axis is the proportion of diffraction energy concentrated within a single pixel size range. The top curve is the diffraction limit, and the other curves represent the energy concentration curves of different fields of view. It can be seen that 80% of the energy is concentrated within the detector pixel range.

[0046] See appendix Figure 5 This is a full-band MTF (Mean Transfer Function) curve of the airborne camera optical system provided in this embodiment; in the figure, (a), (b), and (c) are the MTF curves of all fields of view on the image planes corresponding to wavelengths of 450nm, 550nm, and 650nm, respectively. Figure 4 It can be seen that, at 120 lp / mm, the optical transfer function of the entire field of view in the 450nm to 650nm working band is greater than 0.5, close to the diffraction limit, and the curve is smooth and compact, indicating that the system has clear and uniform imaging, and the system has good imaging quality in the entire band and the entire field of view.

[0047] See appendix Figure 6 This is the axial chromatic aberration curve of the airborne camera optical system provided in this embodiment. The axial chromatic aberration curve represents the degree to which different wavelengths deviate from the ideal image plane position at different pupils. The horizontal axis represents the offset, and the vertical axis represents the normalized pupil coordinates. We mainly focus on the smallest offset among all wavelengths near the 0.707 pupil, derived from... Figure 6 It can be seen that the horizontal distance from the intersection of the wavelengths of 450nm and 650nm at the 0.86 pupil to the wavelength of 550nm is approximately 0.02mm.

[0048] The results show that the optical system of the airborne camera provided by this invention has a root mean square radius of the dot plot of each field of view corresponding to different wavelengths that is smaller than the Ally disk; at a sampling frequency of 120 lp / mm, the optical transfer function of the entire working band and the entire field of view is greater than 0.5, the imaging quality is good, and high-resolution images can be acquired.

[0049] The optical imaging method for an airborne camera provided by this invention has an optical system with a working wavelength of 450nm to 650nm, a focal length of 45mm, an image-side F-number of 3, and a total length to focal length ratio of 0.55. While maintaining a long focal length, it uses two plane mirrors to reduce the total length of the optical system, which greatly reduces the rotational inertia of the system and ensures the stability and maneuverability of the aircraft when the airborne camera is rotating. It can play a role in a wider range of application scenarios and has practical application value.

Claims

1. An optical imaging method for an airborne camera, characterized in that... Includes the following steps: (1) After passing through the protective lens (1), the incident light enters the convex-concave lens (2), and the light beam is converged and incident on the first plane mirror (3); (2) The first plane mirror (3) is used to deflect the light path, deflect the reflected light by 90° and enter the doublet lens (4), and the refracted light after chromatic aberration and spherical aberration correction is incident on the second plane mirror (5). The doublet lens (4) is formed by cementing the rear surface of the biconvex lens (4.1) with the front surface of the biconcave lens (4.2). The refractive index of the material of the biconvex lens is 1.57 to 1.62, and the refractive index of the material of the biconcave lens is 1.73 to 1.

78. The difference between their refractive indices is greater than 0.

15. (3) The second plane mirror (5) is set perpendicular to the first plane mirror (3) to bend the light path again, so that the propagation direction of the reflected light is deflected by 180° from the direction of the incident light and then enters the concave-convex lens (6). The center thickness of the convex and concave lens (2) d 1 and the center thickness of the concave and convex lens (6) d 2. The condition 0.90 ≤ d 1 / d 2≤1.1; Focal length of convex and concave lens (2) f 1 and the focal length of the concave and convex lenses (6) f 2. The condition 0.98 ≤ f 1 / f 2≤1.02; the air gap between the rear surface of the convex-concave lens (2) and the center position of the surface of the first plane mirror (3). d 3. The air gap between the center of the surface of the second plane mirror (5) and the front surface of the concave-convex lens (6). d 4. The condition 0.98 ≤ is satisfied. d 3 / d 4≤1.02; (4) The light beam refracted by the concave and convex lens (6) is focused on the image plane (7) in the same plane as the protective lens, thus obtaining a high-resolution image.

2. The optical imaging method for an airborne camera according to claim 1, characterized in that: The optical system adopted has a U-shaped folded optical path. Its optical components consist of a protective lens (1), a convex and concave lens (2), a first plane mirror (3), a doublet lens (4), a second plane mirror (5), a concave and convex lens (6), and an image plane (7). The position D of the center of gravity of the optical system satisfies 0.1mm ≤ l 1- l 3≤0.5mm, 0.6mm≤ l 2- l 1≤0.8mm, 0.6mm≤ l 2- l 3≤0.8mm, where, l 1. l 2. l 3 represents the distance from the center of gravity of the convex-concave lens (2), the doublet lens (4), and the concave-convex lens (6) to the center of gravity of the optical system, respectively. The rotation center of the system coincides with the center of gravity of the optical system.

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