Method for determining optical properties of a calibration device

Abstract

The invention relates to a method for determining optical properties of a calibration device (10) with respect to cameras (14) to be calibrated. The calibration device (10) comprises a laser (18) directed in the direction of the camera (14) to be calibrated, and a diffractive optical element (30) positioned between the camera (14) and the laser (18). An autocollimator (38) is provided which, via a beam splitter (34) positioned in the beam path between the laser (18) and the diffractive optical element (30), receives a reflection of the autocollimator light (42) and a laser beam (22) from the diffractive optical element (30).The method comprises the steps of determining (A) an autocollimator light angle of incidence (θ) on the diffractive optical element (30) by means of the autocollimator (38), and determining (B) a difference angle between the autocollimator light (42) and a reflection of the laser beam (22) from the diffractive optical element (30) by means of the autocollimator (38). Furthermore, the method comprises the steps of calculating (C) a laser angle of incidence (β) on the diffractive optical element (30) from the difference angle and the autocollimator light angle of incidence (θ) on the diffractive optical element (30), and calculating (D) diffraction angles of the laser beam (22) at the diffractive optical element (30) using the laser angle of incidence (β).

Description

[0001] The present invention relates to a method for determining optical properties of a calibration device with respect to cameras to be calibrated. Furthermore, the invention relates to a calibration device for carrying out such a method. State of the art

[0002] The cameras used in vehicles for driver assistance or automated driving systems perform several functions, including detecting objects in the scene using various object detection algorithms. For many of these algorithms, the world-to-camera or camera-to-world transformation must be known, which means the camera's optical parameters must be known. These parameters include the focal length, principal point, and optical distortion parameters, such as radial and non-radial distortion parameters. Intrinsic calibration is the process by which the camera's optical distortion is determined. A space- and cost-saving method for camera calibration is based on the use of a diffractive optical element.

[0003] US 3912395 A discloses a method for calibrating distortion in optical systems. A collimated light beam is diffracted in different directions by passing it through a diffraction grating. The diffracted light emerging from the grating is then refocused by the calibrated optical system, creating a series of images in the focal plane of the optical system. The relationship between the actual positions of the images in the array and the calculated positions for a distortion-free system is a measure of the distortion present in the system.

[0004] DE 10 2022 104 717 A1 describes a method for determining and correcting a center axis shift caused by the installation of an optical layer over a camera array. A pixel location shift caused by a window in front of a camera array is determined on a calibration target. This shift results from a center axis shift caused by the window. Accordingly, a transformation is generated that corrects the pixel location shift caused by the window in front of the camera array, so that the pixel locations with the window correspond to the pixel locations without the window.

[0005] The object underlying the invention is to provide a method for determining optical properties of a calibration device with respect to cameras to be calibrated, with which the accuracy of the camera calibration is increased.

[0006] The object is achieved by a method having the subject matter of patent claim 1. Preferred embodiments can be found in the dependent claims. Disclosure of the invention

[0007] The invention provides a method for determining the optical properties of a calibration device with respect to cameras to be calibrated. The calibration device comprises a laser directed toward the camera to be calibrated and a diffractive optical element positioned between the camera and the laser. An autocollimator is provided, which receives a reflection of the autocollimator light and a laser beam from the diffractive optical element via a beam splitter positioned in the beam path between the laser and the diffractive optical element.

[0008] Optical properties refer to the values ​​of the calibration device that affect the accuracy of a camera being calibrated. A diffractive optical element is an element used to shape a light beam. Autocollimator light refers to the light beam emitted by a light source of the autocollimator.

[0009] The method according to the invention comprises the steps of determining an angle of incidence of the autocollimator light on the diffractive optical element using the autocollimator, and determining a difference angle between the autocollimator light and a reflection of the laser beam from the diffractive optical element using the autocollimator. The method additionally comprises the steps of calculating a laser angle of incidence on the diffractive optical element from the difference angle and the angle of incidence of the autocollimator light on the diffractive optical element, and calculating diffraction angles of the laser beam at the diffractive optical element using the laser angle of incidence.

[0010] Accordingly, it is known which image should reach the camera to be calibrated. Based on the image captured by the camera and the computationally determined image, deviations can be determined with high accuracy. This makes it possible to calibrate the camera accordingly. This increases the accuracy of the camera calibration.

[0011] In a preferred embodiment of the invention, a pitch, yaw, and roll angle of a camera mount formed on a camera mount is determined. The method comprises the steps of determining an angle of incidence of the autocollimator light on a mirror arranged in the camera mount of the camera mount using the autocollimator, determining an angular deviation between the autocollimator light and a reflection of the laser beam from the mirror using the autocollimator, and calculating a laser angle of incidence on the mirror from the angular deviation and the angle of incidence of the autocollimator light.Furthermore, the method comprises the steps of calculating a pitch and yaw angle of the camera image using the laser angle of incidence on the mirror, determining an element rotation angle of the diffractive optical element relative to an element mount of the diffractive optical element, and tactilely determining a mount rotation angle between the element mount and the camera image. In a final step of the method, a rotation angle of the diffractive optical element relative to the camera image is determined from the element rotation angle and the mount rotation angle.

[0012] A pitch angle is the angle of camera movement around a transverse axis. A yaw angle describes the angle of movement around a vertical axis. Similarly, a rotation angle describes the angle of movement around a longitudinal axis. Instead of the camera, a mirror is arranged in the camera mount. The mirror is arranged in the same way as the camera. The mirror can therefore be used to simulate the position of the camera in the shot. Unlike the camera, however, the mirror makes it possible to determine the position in the camera mount using the autocollimator.

[0013] The tactile determination is performed by a separate tactile measuring device. This measuring device moves to specific measuring points on the element mount and the camera mount. The corresponding values ​​can then be used to determine the rotation angle between the element mount and the camera mount. By additionally determining the pitch, yaw, and rotation angles of the camera mount, the position of the camera in space and relative to the diffractive optical element can be precisely determined. This procedure allows the absolute alignment of the camera to be determined.

[0014] In a further preferred embodiment of the invention, the element rotation angle is determined by determining an angle between an actual reference line of the diffractive optical element or a reference line formed by reference points and an actual reference line of the element holder or a reference line formed by reference points. A reference line is understood to be an object shaped as a line which serves to determine an angle of rotation to another reference line with high accuracy. An actual reference line is also formed as a line on or in the body. In contrast, a reference line formed by reference points is not actually present, but can be formed imaginarily by connecting these reference points. Such reference lines have the advantage that they can be formed with high accuracy and an angle between these reference lines can be determined easily.

[0015] Preferably, the autocollimator is aligned so that the autocollimator light falls perpendicularly onto the diffractive optical element. Aligning the autocollimator in this way can simplify the calculation of the optical properties.

[0016] In an advantageous further development, the laser angle of incidence on the diffractive optical element is continuously determined. The laser angle of incidence is thus measured not only after the calibration device has been set up, but also at predetermined intervals during camera calibration. A change in the laser angle of incidence can thus be easily corrected, thus increasing the accuracy of the camera calibration. Continuous measurement of the laser angle of incidence also eliminates the need for a rigid and expensive mechanical calibration device, making such a calibration with high accuracy economically feasible.

[0017] The object underlying the invention is achieved by a calibration device for calibrating cameras. The calibration device has optical properties with respect to the camera, which are determined according to the method according to the invention. The calibration device comprises a camera mount for holding a camera to be calibrated, a laser directed in the direction of the camera to be calibrated, and a diffractive optical element positioned between the camera and the laser. Furthermore, the calibration device comprises an element mount for holding the diffractive optical element, a beam splitter arranged between the diffractive optical element and the laser, and an autocollimator which receives a reflection from the direction of the diffractive optical element via the beam splitter.

[0018] Such a calibration device essentially has the advantages and properties described above.

[0019] Advantageously, the diffractive optical element has at least one reference line and / or reference points. Such reference lines or reference points increase the accuracy of determining the element's rotation angle. Such reference points can be created during the manufacture of the diffractive optical element. Since the manufacture of the diffractive optical element requires a high degree of precision, the reference points can be formed with equally high precision during production.

[0020] In a further advantageous embodiment, at least one reference line and / or reference points are formed on the element holder. Such a reference line or reference points increase the accuracy of determining the element's rotation angle. This also simplifies the determination of the element's rotation angle.

[0021] According to a practical embodiment, the reference line and / or the reference points are designed such that they can be measured tactilely. Advantageously, the reference points are designed as drilled holes, which allows both tactile and optical determination. Alternatively, it is also possible to design the reference points as 3D objects. This design of the reference line and the reference points simplifies tactile measurement of these elements.

[0022] According to a further expedient embodiment, a camera receptacle formed on the camera mount forms a reference surface. The reference surface is aligned such that a rotation of the camera receptacle can be determined via it. Advantageously, the reference surface extends such that a normal vector of the reference surface is orthogonal to the axis of rotation of the camera mount. A reference surface has the advantage that it can be easily detected by touch. Particularly advantageously, the reference surface forms part of the camera receptacle. The camera thus lies directly against the reference surface and has the same rotation as the reference surface. The reference surface thus forms the camera receptacle on the one hand, and this surface also serves as a reference surface, so that no separate surface needs to be formed.

[0023] The invention additionally provides a method for calibrating a camera in a calibration device. The method comprises the steps of inserting the camera into the camera mount, recording an image generated in the camera by the laser beams diffracted by the diffractive optical element, and determining pixel coordinates of image points generated by the diffracted laser beams. In a final step, the optical properties of the camera are determined from the determined pixel coordinates and the diffraction angle of the diffracted laser beams calculated based on the optical properties of the calibration device.

[0024] Diffracted laser beams are defined as the laser beams that have been split into individual laser beams by the diffractive optical element. In other words, the previously single laser beam is split into a multitude of individual laser beams. Pixel coordinates are defined as the spatial location of the image points generated by the diffracted laser beams on the camera's image sensor.

[0025] Embodiments of the invention are illustrated in the drawing and explained in more detail in the following description. It shows: Fig. 1 Schematic representation of a calibration device for calibrating cameras according to an embodiment of the invention, Fig. 2 Illustration of an element holder and the diffractive optical element, Fig. 3 Illustration of the camera mount with a camera mount and a mirror, Fig. 4 Embodiment of a method for determining optical properties of the calibration device, Fig. 5 Representation of a camera with pitch, yaw and rotation angles, and Fig. 6 Embodiment of a method for calibrating the camera in the calibration device.

[0026] In Fig. 1 shows a schematic representation of a calibration device 10 for calibrating cameras 14 according to an embodiment of the invention. The calibration device 10 comprises a laser 18, which is directed towards the camera 14 to be calibrated. A laser beam 22 emitted by the laser 18 can be optically adjusted via a lens 26 arranged downstream of the laser 18. A diffractive optical element 30 is arranged between the camera 14 and the laser 18. The calibration device 10 additionally has a beam splitter 34, which is positioned in the beam path between the laser 18 and the diffractive optical element 30. A reflection of the laser beam 22 from the diffractive optical element 30 can be transmitted to an autocollimator 38 via this beam splitter 34.

[0027] In the illustrated embodiment, the autocollimator 38 is arranged orthogonally to the beam path between the laser 18 and the diffractive optical element 30. An autocollimator light 42 is redirected via the beam splitter 34 so that it impinges on the diffractive optical element 30. A camera mount 46, by which the camera 14 is held, is arranged downstream of the diffractive optical element 30.

[0028] Fig. 2 shows an illustration of an element holder 50 and the diffractive optical element 30. The diffractive optical element 30 is held by the element holder 50. The element holder 50 forms a plurality of reference points 54 arranged perpendicular to one another and a plurality of reference points 54 arranged horizontally to one another. In this exemplary embodiment, the reference points 54 are designed as drill holes. The reference points 54 form an imaginary reference line 58. A rotation of the diffractive optical element 30 relative to the element holder 50 can be determined via the reference lines 58. The diffractive optical element 30 is formed from a substrate 62 and an active surface 66 positioned on the substrate 62. Additionally, three crosses 70 arranged horizontally to one another are formed on the substrate 62 and serve as reference points of the diffractive optical element 30.An outer edge 74 of the active surface 66 of the diffractive optical element 30 forms a reference line 74.

[0029] In Fig. 3 shows a representation of the camera mount 46 with a camera receptacle 78 and a mirror 82. In the exemplary embodiment shown, the camera receptacle 78 is designed as a rectangular cutout in the camera mount 46, so that a camera 14 can be accommodated in the camera receptacle 78. The camera receptacle 78 forms four webs 86 which protrude into the camera receptacle 78. The webs 86 form a defined contact surface for the camera 14. The camera receptacle 78 forms reference surfaces 90, via which a holder rotation angle between the element holder 50 and the camera receptacle 78 can be tactilely determined. In the exemplary embodiment shown here, the mirror 82 is arranged in the camera receptacle 78 instead of a camera 14. Calibration of the camera mount 46 and the camera receptacle 78 is possible via this mirror 82.

[0030] Fig. 4 shows an embodiment of a method for determining optical properties of the calibration device 10. In a first step A of the method, an autocollimator light angle of incidence θ on the diffractive optical element 30 is determined by means of the autocollimator 38. For this purpose, an angle between a reflection of the autocollimator light 42 from the diffractive optical element 30 and the autocollimator light 42 emitted by the autocollimator 38 is determined. In a next step B, a difference angle between the emitted autocollimator light 42 and a reflection of the laser beam 22 received by the autocollimator 38 from the diffractive optical element 30 is determined by means of the autocollimator 38.

[0031] In a subsequent step C, a laser angle of incidence β on the diffractive optical element 30 is determined. For this purpose, the angle between the difference angle and the autocollimator light angle of incidence θ is determined. This angle corresponds to the laser angle of incidence β on the diffractive optical element 30. In a next step D, the diffraction angle of the laser beam 22 at the diffractive optical element 30 is calculated from the laser angle of incidence β and the properties of the diffractive optical element 30.

[0032] In order to determine the orientation of the camera 14 in the calibration device 10, knowledge of the pitch angle γ, yaw angle δ and rotation angle ε is important. The pitch angle γ describes, as in Fig. 5, an angle around a transverse axis y of the camera 14. The transverse axis y runs orthogonally to a longitudinal axis x and a vertical axis z of the camera 14. The yaw angle δ, on the other hand, describes, as in Fig. 5, an angle around a vertical axis z of the camera 14, while the rotation angle ε describes an angle around a longitudinal axis x of the camera 14.

[0033] In order to determine the pitch γ and yaw angle δ of the camera receptacle 78 of the camera mount 46, the angle of incidence of the autocollimator light on the mirror 82 arranged in the camera receptacle 78 of the camera mount 46 is determined in a next step E. For this purpose, the angle between the autocollimator light 42 emitted by the autocollimator 38 and the autocollimator light 42 reflected by the mirror 82 is determined by means of the autocollimator 38. Subsequently, in a next step F, an angular deviation between the autocollimator light 42 and a reflection of the laser beam 22 from the mirror 82 is determined by means of the autocollimator 38. From the angular deviation and the angle of incidence of the autocollimator light on the mirror 82, a laser angle of incidence on the mirror 82 is determined in a further step G. The pitch γ and yaw δ angles of the camera image 78 can then be determined based on the laser incidence angle.

[0034] To determine the rotation angle ε, in a next step I, an element rotation angle of the diffractive optical element 30 relative to the element holder 50 of the diffractive optical element 30 is determined. In this case, an angle is determined between an imaginary reference line formed by the reference points 70 of the diffractive optical element 30 and the imaginary reference line 58 formed by the reference points 54 of the element holder 50. It is also possible to determine an angle between a reference line formed by the outer edge 74 of the active surface 66 of the diffractive optical element 30 and the reference line 58 of the element holder 50. The determined angle corresponds to the element rotation angle of the diffractive optical element 30 relative to the element holder 50.

[0035] In a next step J, a mounting rotation angle between the element mount and the camera mount 78 is determined by tactile determination. For this purpose, the reference points 54 of the element mount 50 and the reference surface 90 of the camera mount 78 are tactilely approached. In a next step K, a rotation angle ε of the diffractive optical element 30 relative to the camera mount 78 is determined from the element rotation angle and the mounting rotation angle.

[0036] In Fig. 6 shows an embodiment of a method for calibrating the camera 14 in the calibration device 10. In a first step A K of the process, the camera 14 is inserted into the camera mount 46. In a next step B KAn image generated in the camera 14 is determined using the laser beam 22 emitted by the laser 18. Before impinging on the camera 14, the laser beam 22 strikes the diffractive optical element 30 at the calculated laser angle of incidence β. According to this angle of incidence β, the laser beam 22 is diffracted by the diffractive optical element 30 before the diffracted laser beams 22 strike an image sensor of the camera 14.

[0037] In a further process step C K Pixel coordinates of image points generated by the laser beams 22 are determined. In a final step D KThe optical properties of camera 14 are determined. This is done using the determined pixel coordinates and the previously determined optical properties of calibration device 10. For this purpose, the required pixel coordinates are calculated based on the optical properties of calibration device 10 and the known laser angle of incidence β. The optical properties of camera 14 are thus determined based on the calculated coordinates and the actual pixel coordinates. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 3912395 A

[0003] DE 10 2022 104 717 A1

[0004]

Claims

[1] Method for determining optical properties of a calibration device (10) with respect to cameras (14) to be calibrated, the calibration device (10) comprising a laser (18) directed in the direction of the camera (14) to be calibrated, and a diffractive optical element (30) positioned between the camera (14) and the laser (18), wherein an autocollimator (38) is provided which receives a reflection of the autocollimator light (42) and a laser beam (22) from the diffractive optical element (30) via a beam splitter (34) positioned in the beam path between the laser (18) and the diffractive optical element (30), the method comprising the steps: - determining (A) an autocollimator light angle of incidence (θ) on the diffractive optical element (30) by means of the autocollimator (38), - determining (B) a difference angle between the autocollimator light (42) and a reflection of the laser beam (22) from the diffractive optical element (30) by means of the autocollimator (38), - calculating (C) a laser angle of incidence (β) on the diffractive optical element (30) from the difference angle and the autocollimator light angle of incidence (θ) on the diffractive optical element (30), and - Calculating (D) diffraction angles of the laser beam (22) at the diffractive optical element (30) by means of the laser angle of incidence (β). [2] Method according to claim 1, characterized by that a pitch (γ), yaw (δ) and rotation angle (ε) of a camera holder (78) formed on a camera mount (46) is determined, the method comprising the steps of: - determining (E) an angle of incidence of the autocollimator light onto a mirror (82) arranged in the camera receptacle (78) of the camera mount (46) by means of the autocollimator (38), - determining (F) an angular deviation between the autocollimator light (42) and a reflection of the laser beam (22) from the mirror (82) by means of the autocollimator (38), - calculating (G) a laser angle of incidence on the mirror (82) from the angular deviation and the autocollimator light angle of incidence, - Calculating (H) a pitch (γ) and yaw angle (δ) of the camera image (78) by means of the laser angle of incidence on the mirror (82), - determining (I) an element rotation angle of the diffractive optical element (30) to an element holder (50) of the diffractive optical element (30), - Tactile determination (J) of a bracket rotation angle between element holder (50) and camera holder (78), - Determining (K) a rotation angle (ε) of the diffractive optical element (30) to the camera receptacle (78) from the element rotation angle and the holder rotation angle. [3] Method according to claim 2, characterized bythat the element rotation angle is determined by determining an angle between an actual reference line (74) of the diffractive optical element (30) or a reference line formed by reference points (70) to an actual reference line (58) of the element holder (50) or a reference line formed by reference points (54). [4] Method according to one of the preceding claims, characterized by that the autocollimator (38) is aligned so that the autocollimator light (42) falls perpendicularly onto the diffractive optical element (30). [5] Method according to one of the preceding claims, characterized by that the laser angle of incidence (β) on the diffractive optical element (30) is continuously determined. [6] Calibration device (10) for calibrating cameras (14), which has optical properties with respect to the camera (14) which are determined according to a method according to one of the preceding claims, comprising, - A camera mount (46) for holding a camera (14) to be calibrated, - A laser (18) directed towards the camera (14) to be calibrated, - A diffractive optical element (30) positioned between the camera (14) and the laser (18), - An element holder (50) for holding the diffractive optical element (30), - a beam splitter (34) arranged between the diffractive optical element (30) and the laser (18), and - An autocollimator (38) which receives a reflection from the direction of the diffractive optical element (30) via the beam splitter (34). [7] Calibration device (10) according to one of the preceding claims, characterized by that the diffractive optical element (30) has at least one reference line (74) and / or reference points (70). [8] Calibration device (10) according to one of the preceding claims, characterized bythat at least one reference line (58) and / or reference points (54) are formed on the element holder (50). [9] Calibration device (10) according to claim 8, characterized by that the reference line (58) and / or the reference points (54) are designed such that they can be measured tactilely. [10] Calibration device (10) according to one of the preceding claims, characterized by that a camera receptacle (78) formed on the camera holder (46) forms a reference surface (90). [11] Method for calibrating a camera (14) in a calibration device (10) according to one of claims 6 to 10, comprising the steps: - Introduction (A K ) of the camera (14) into the camera holder (46), - Recording (B K ) an image generated in the camera (14) by the laser beams (22) diffracted at the diffractive optical element (30), - Determine (C K) of pixel coordinates, of image points generated by the diffracted laser beams (22), - Determine (D K ) the optical properties of the camera (14), from the determined pixel coordinates and the diffraction angle of the diffracted laser beams (22) calculated on the basis of the optical properties of the calibration device (10) according to one of claims 1 to 5.

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