Calibration systems usable for distortion characterization in cameras

• US 10,623,727 B1
• Filed: 04/16/2019
• Issued: 04/14/2020
• Est. Priority Date: 04/16/2019
• Status: Active Grant

• Current Assignee: Waymo LLC (Alphabet Inc.)
• Original Assignee: Waymo LLC (Alphabet Inc.)
• Inventors: Frevert, Benjamin
• Primary Examiner(s): Gordon, Steven Whitesell
• Application Number:
  US16/385,761
• Time in Patent Office: 364 Days
• CPC Class Codes: G02B 26/124 Details of the optical system between the light source and the polygonal mirror G02B26/123, G02B26/127 take precedence
• CPC Class Codes: G02B 27/62 Optical apparatus specially adapted for adjusting optical elements during the assembly of optical systems
• CPC Class Codes: G02B 5/0808 having a single reflecting layer G02B5/0883, G02B5/0891 take precedence
• CPC Class Codes: H04N 17/002 for television cameras
• CPC Class Codes: H04N 23/55 Optical parts specially adapted for electronic image sensors; Mounting thereof

Abstract

Example embodiments relate to calibration systems usable for distortion characterization in cameras. An example embodiment includes a calibration system. The calibration system includes a first calibration target that includes a first mirror, a plurality of fiducials positioned on or adjacent to the first mirror, and an indexing fiducial positioned on or adjacent to the first mirror. The calibration system also includes a second calibration target that includes one or more second mirrors and has an aperture defined therein. The first mirror and the one or more second mirrors are separated by a distance. The first mirror faces the one or more second mirrors. The indexing fiducial is visible through the aperture in the second calibration target. Reflections of the plurality of fiducials are visible through the aperture defined in the second calibration target. The reflections of the plurality of fiducials are iterated reflections.

Claims

1. A calibration system comprising:

a first calibration target comprising;

a first mirror;

a plurality of fiducials positioned on or adjacent to the first mirror; and

an indexing fiducial positioned on or adjacent to the first mirror; and

a second calibration target comprising one or more second mirrors and having an aperture defined therein,

wherein the first calibration target and the second calibration target are positioned such that;

the first mirror and the one or more second mirrors are separated by a distance;

the first mirror is facing the one or more second mirrors;

the indexing fiducial is visible through the aperture defined in the second calibration target; and

reflections of the plurality of fiducials are visible through the aperture defined in the second calibration target, wherein the reflections of the plurality of fiducials are iterated reflections resulting from at least one reflection by the one or more second mirrors and at least one reflection by the first mirror.

2. The calibration system of claim 1, wherein the plurality of fiducials comprises at least one linear fiducial and at least one point fiducial.

3. The calibration system of claim 1,

wherein the one or more second mirrors comprise only a single mirror, and

wherein the aperture is a hole defined within the single mirror.

4. The calibration system of claim 1,

wherein the one or more second mirrors comprise two mirrors separated from one another,

wherein the aperture is between the two mirrors, and

wherein the two mirrors separated from one another are positioned such that the two mirrors are within 5.0 cm of coplanar and are within 1.0°

of parallel with one another.

5. The calibration system of claim 1,

wherein the fiducials in the plurality of fiducials emit light in a wavelength range, and

wherein the wavelength range includes at least one of a range of visible wavelengths or a range of infrared wavelengths.

6. The calibration system of claim 5, wherein the wavelength range is adjustable.

7. The calibration system of claim 1, wherein the indexing fiducial has an hourglass shape.

8. The calibration system of claim 1, further comprising a mount configured to position a camera relative to the aperture such that the camera is able to capture a calibration image of the iterated reflections of the plurality of fiducials through the aperture.

9. The calibration system of claim 8, wherein the mount comprises:

a first stage configured to rotate the camera in a yaw direction; and

a second stage configured to rotate the camera in a pitch direction.

10. The calibration system of claim 8, wherein the mount comprises a pair of camera-orientation fiducials such that, using the calibration image captured by the camera, an orientation of the camera relative to the first calibration target or the second calibration target can be determined.

11. The calibration system of claim 10, further comprising a third mirror affixed to the mount,

wherein the pair of camera-orientation fiducials comprise a first camera-orientation fiducial and a second camera-orientation fiducial,

wherein the third mirror is positioned on the mount adjacent to the camera,

wherein the first camera-orientation fiducial is positioned on the mount adjacent to the third mirror and opposite the camera, and

wherein the second camera-orientation fiducial is positioned on the mount adjacent to the camera and opposite the third mirror.

12. The calibration system of claim 11, wherein the first camera-orientation fiducial and the second camera-orientation fiducial are the same shape.

13. The calibration system of claim 11, wherein the calibration image captured by the camera is usable to determine an angle between the mount and the first mirror based on reflections of the pair of camera-orientation fiducials from the third mirror.

14. The calibration system of claim 1, wherein the first calibration target and the second calibration target are positioned such that the first mirror and each of the one or more second mirrors are greater than 0.1°

within parallel of one another but less than 10.0°

within parallel of one another.

15. The calibration system of claim 1, wherein the first calibration target and the second calibration target are positioned such that reflections of the indexing fiducial are not visible through the aperture defined in the second calibration target.

16. A method comprising:

positioning a first calibration target and a second calibration target such that;

a first mirror of the first calibration target and one or more second mirrors of the second calibration target are separated by a distance;

the first mirror is facing the one or more second mirrors;

an indexing fiducial positioned on or adjacent to the first mirror of the first calibration target is visible through an aperture defined in the second calibration target; and

reflections of a plurality of fiducials positioned on or adjacent to the first mirror of the first calibration target are visible through the aperture defined in the second calibration target, wherein the reflections of the plurality of fiducials are iterated reflections resulting from at least one reflection by the one or more second mirrors and at least one reflection by the first mirror;

positioning a camera behind the second calibration target such that it observes the first calibration target through the aperture defined in the second calibration target;

capturing a calibration image using the camera; and

characterizing, based on the calibration image, an intrinsic distortion of the camera.

17. The method of claim 16, wherein characterizing the intrinsic distortion of the camera comprises using the iterated reflections to determine a vanishing point within the calibration image.

18. The method of claim 16, wherein characterizing the intrinsic distortion of the camera comprises determining an angle of the camera relative to the first mirror or relative to at least one of the one or more second mirrors.

19. The method of claim 18,

further comprising positioning the camera relative to the aperture using a mount comprising a pair of camera-orientation fiducials and a third mirror,

wherein determining the angle of the camera relative to the first mirror comprises identifying the pair of camera-orientation fiducials within the calibration image, and

wherein the pair of camera-orientation fiducials is identified based on reflections from the first mirror and the third mirror.

20. The method of claim 19, further comprising:

rotating the mount relative to the first calibration target and the second calibration target in a yaw direction using a first stage of the mount or in a pitch direction using a second stage of the mount;

capturing an additional calibration image; and

determining, based on the calibration image and the additional calibration image, an angle of the third mirror relative to the camera.

21. The method of claim 16, further comprising determining a depth of field of the camera based on the iterated reflections in the calibration image.

22. The method of claim 16, further comprising:

adjusting a wavelength range emitted by the fiducials in the plurality of fiducials to a different wavelength range than emitted when the calibration image was captured by the camera, wherein the intrinsic distortion of the camera comprises an intrinsic distortion for a first channel of the camera;

capturing an additional calibration image using the camera; and

characterizing, based on the additional calibration image, an additional intrinsic distortion of the camera, wherein the additional intrinsic distortion of the camera comprises an intrinsic distortion of a second channel of the camera.

23. The method of claim 16, wherein positioning the first calibration target and the second calibration target comprises positioning the first calibration target and the second calibration target such that reflections of the indexing fiducial are not visible through the aperture defined in the second calibration target.

24. A non-transitory, computer-readable medium having instructions stored thereon,

wherein the instructions, when executed by a processor, cause the processor to execute a method comprising:

receiving a calibration image captured using a camera positioned behind a second calibration target to observe a first calibration target through an aperture defined in the second calibration target,

wherein the first calibration target comprises;

a first mirror;

a plurality of fiducials positioned on or adjacent to the first mirror; and

an indexing fiducial positioned on or adjacent to the first mirror,

wherein the second calibration target comprises one or more second mirrors, and

wherein the first calibration target and the second calibration target are positioned such that;

the first mirror and the one or more second mirrors are separated by a distance;

the first mirror is facing the one or more second mirrors;

the indexing fiducial is visible through the aperture defined in the second calibration target; and

reflections of the plurality of fiducials are visible through the aperture defined in the second calibration target, wherein the reflections of the plurality of fiducials are iterated reflections resulting from at least one reflection by the one or more second mirrors and at least one reflection by the first mirror; and

characterizing, based on the calibration image, an intrinsic distortion of the camera.

25. The non-transitory, computer-readable medium of claim 24, wherein the first calibration target and the second calibration target are positioned such that reflections of the indexing fiducial are not visible through the aperture defined in the second calibration target.