CAMERA CALIBRATING APPARATUS AND METHOD

• US 20080303906A1
• Filed: 08/21/2008
• Published: 12/11/2008
• Est. Priority Date: 05/24/2004
• Status: Active Grant

• Current Assignee: Centre de recherche industrielle du Québec
• Original Assignee: Centre de recherche industrielle du Québec
• Inventors: Lessard, Jean-Luc, Lessard, Denis, Poulin, Pierre
• Granted Patent: US 8,049,779 B2
• Time in Patent Office: Days
• US Class Current: 348/187
• CPC Class Codes: G01B 11/2504 Calibration devices
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light

Abstract

A camera calibrating apparatus uses a calibration target in combination with a laser source generating a laser beam defining a calibration plane and intersecting the camera field of view to define an object imaging area. The calibration target includes a frame defining a coplanar arrangement of more than five reference points having known position coordinates within an object reference system. The frame is disposed in a camera calibration position wherein the coplanar arrangement of reference points is substantially parallel to the calibration plane. The target includes an arrangement of light reflecting members protruding from the arrangement of reference points in a direction substantially perpendicular thereto so as to extend within the object imaging area, to allow the camera to capture an image formed by illuminated portions of the light reflecting members. The apparatus further includes image processor means programmed on the basis a calibrated camera model to associate any position coordinates in the object reference system with corresponding position coordinates in an image reference system.

Claims

1. A camera calibrating target for use with a laser source capable of generating a beam of coherent light defining a calibration plane, said camera being characterized by a field of view in the direction of an optical axis forming a predetermined angle with said calibration plane, said beam intersecting the field of view of said camera to define an object imaging area, said target comprising:

a frame defining a coplanar arrangement of more than five reference points having known position coordinates within an object reference system associated with said frame and defining at least two dimensions, said frame being capable of being disposed in a camera calibration position wherein said coplanar arrangement of reference points is substantially parallel to said calibration plane; and

an arrangement of light reflecting members protruding from said arrangement of reference points in a direction substantially perpendicular thereto so as to extend within said object imaging area when said frame is disposed in said camera calibration position to allow said camera to capture an image formed by illuminated portions of said light reflecting members.

2. The camera calibrating target according to claim 1, wherein said reference points are located with one another within said arrangement and with respect to said coherent light beam so as to prevent shading of said reflecting members which may adversely affect said illuminated portions in forming said image.

3. The camera calibrating target according to claim 1, wherein said coplanar arrangement includes a plurality of reference points series extending substantially transversely to said coherent light beam and being disposed in parallel spaced apart relationship in the direction of said light beam from a foremost position on said frame proximal to said laser source to a rearmost position on said frame distal to said laser source.

4. The camera calibration target according to claim 3, wherein the respective points of adjacent ones of said series are transversely shifted with one another within said arrangement and with respect to the coherent light beam so as to prevent shading of said reflecting members which may adversely affect said illuminated portions in forming said image.

5. A camera calibrating apparatus for use with a laser source capable of generating a beam of coherent light defining a calibration plane, said camera being characterized by intrinsic imaging parameters and a field of view in the direction of an optical axis forming a predetermined angle with said calibration plane, said beam intersecting the field of view of said camera to define an object imaging area, said apparatus comprising:

a calibration target including;

a frame defining a coplanar arrangement of more than five reference points having known position coordinates within an object reference system associated with said frame and defining at least two dimensions, said frame being capable of being disposed in a camera calibration position wherein said coplanar arrangement of reference points is substantially parallel to said calibration plane; and

an arrangement of light reflecting members protruding from said arrangement of reference points in a direction substantially perpendicular thereto so as to extend within said object imaging area when said frame is disposed in said camera calibration position to allow said camera to capture an image formed by illuminated portions of said light reflecting members; and

image processor means for estimating position coordinates of said illuminated portions within an image reference system associated with said camera, for associating the estimated position coordinates of each said illuminated portion within said image reference system with the known position coordinates of a corresponding one of said reference points in relation with a corresponding one of said calibration planes within said object reference system, for calibrating a camera model based on said intrinsic imaging parameters with said associated known position coordinates and estimated position coordinates, and for using said calibrated camera model to associate any position coordinates in said object reference system with corresponding position coordinates in said image reference system.

6. The camera calibrating apparatus according to claim 5, wherein said reference points are located with one another within said arrangement and with respect to the coherent light beam so as to prevent shading of said reflecting members which may adversely affect said illuminated portions in forming said image.

7. The camera calibrating apparatus according to claim 5, wherein said coplanar arrangement includes a plurality of reference points series extending substantially transversely to said coherent light beam and being disposed in parallel spaced apart relationship in the direction of said light beam from a foremost position on said frame proximal to said laser source to a rearmost position on said frame distal to said laser source.

8. The camera calibration apparatus according to claim 7, wherein the respective points of adjacent ones of said series are transversely shifted with one another within said arrangement and with respect to the coherent light beam so as to prevent shading of said reflecting members which may adversely affect said illuminated portions in forming said image.

9. A method of calibrating a camera characterized by intrinsic imaging parameters and a field of view in the direction of an optical axis, said method comprising the steps of:

i) generating a beam of coherent light defining a calibration plane;

ii) disposing said camera so that its field of view in the direction of its optical axis forms a predetermined angle with said calibration plane and so that said beam intersects the field of view of said camera to define an object imaging area;

iii) providing a calibration target including;

a) a frame defining a coplanar arrangement of more than five reference points having known position coordinates within an object reference system associated with said frame and defining at least two dimensions; and

b) an arrangement of light reflecting members protruding from said arrangement of reference points in a direction substantially perpendicular thereto;

iv) disposing said frame in a camera calibration position wherein said coplanar arrangement of reference points is substantially parallel to said calibration plane so that said light reflecting members extend within said object imaging area;

v) generating with said camera an image formed by illuminated portions of said light reflecting members;

vi) estimating position coordinates of said illuminated portions within an image reference system associated with said camera;

vii) associating the estimated position coordinates of each said illuminated portion within said image reference system with the known position coordinates of a corresponding one of said reference points within said object reference system;

viii) calibrating a camera model based on said intrinsic imaging parameters with said associated known position coordinates and estimated position coordinates; and

ix) using said calibrated camera model to associate any position coordinates in said object reference system with corresponding position coordinates in said image reference system.

10. The camera calibrating method according to claim 9, wherein said reference points are located with one another within said arrangement and with respect to the coherent light beam so as to prevent shading of said reflecting members which may adversely affect said illuminated portions in forming said image.

11. The camera calibrating method according to claim 9, wherein said coplanar arrangement includes a plurality of reference points series extending substantially transversely to said coherent light beam and being disposed in parallel spaced apart relationship in the direction of said light beam from a foremost position on said frame proximal to said laser source to a rearmost position on said frame distal to said laser source.

12. The camera calibration method according to claim 11, wherein the respective points of adjacent ones of said series are transversely shifted with one another within said arrangement and with respect to the coherent light beam so as to prevent shading of said reflecting members which may adversely affect said illuminated portions in forming said image.