JP3095411B2 - Calibration method of CCD camera - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CCD camera calibration method for calibrating distortion of a solid-state sensor or a lens system of a high-resolution CCD camera.

[Prior art] In recent years, CCD cameras have been used as image sensors that capture images of objects in various processes such as processing, assembly, and inspection in the production process of automobiles, etc. due to the increase in the number of pixels of solid-state sensors and the increase in resolution. Has been expanded. Image information captured by the CCD camera is processed by a computer via an image processing processor, and measurement and inspection of the appearance of the object are performed.

It is known that one pixel of a CCD camera has a size of about several microns, and a CCD camera having a number of pixels of about 1000 × 1000 can measure the appearance of an object in a micron under a certain condition. . In such a high-resolution CCD camera, there are subtle distortions on the order of microns in the arrangement of pixels in the solid-state sensor and the lens system that projects light from an object onto the solid-state sensor, in which a large number of pixels are arranged in a grid. When,
Alternatively, if there is a slight deviation between the assembly of the solid-state sensor and the lens system, the distortion or an image at a portion corresponding to the deviation may cause a non-negligible error in the measured value.

For example, as shown in FIG. 4, when the CCD camera (1) is normally accurate, light from a point P on the line of sight b at an angle α with respect to the optical axis a of the lens system (4) is solid. It is assumed that one pixel 3m of the sensor (2) is incident. On the other hand, suppose that the lens system (4) is distorted and light from the point P is incident on one pixel 3n adjacent to the pixel 3m of the solid-state sensor (2).
Then, if the lens system (4) is distorted, the CCD camera (1) erroneously determines that there is a point P in the direction of the line of sight c indicated by the dashed line in the figure connecting the pixel 3n and the principal point Q of the lens system (4). Acknowledge.
This is because the angles α and α ′ of the line of sight b and the line of sight c with respect to the optical axis a are different, and as the distance to the point P increases,
The accuracy of the measured values of the three-dimensional coordinates of the point P is deteriorated.

Therefore, in a CCD camera that requires high accuracy,
It is necessary to perform a calibration for correcting the measurement value due to the distortion or the like in advance. For example, as shown in FIG.
A calibration method for rotating the CCD camera (1) up, down, left, and right has been developed.

FIG. 5 shows a case where the CCD camera (1) is fixed on a swing table (10) that rotates vertically and horizontally, and the CCD camera (1) is moved by the swing table (10) to the principal point Q of the lens system (4). It is to rotate up and down and left and right around the center. The swing table (10) is rotated by a rotary encoder (not shown) whose top, bottom, left and right rotation angles are measured. Light source (11) at fixed position in front of CCD camera (1)
The CCD camera (1) is first rotated so that light from the light source (11) is incident on the pixel 3s at the center of the solid-state sensor (2) of the CCD camera (1). At this time, the CCD
The light source (11) is located on the optical axis a of the camera (1), and the direction of the optical axis a is set. Next, for example, a CCD camera (1)
Is rotated upward by an angle α about the principal point Q as shown by the chain line in FIG. Then, if the CCD camera (1) is equivalent to that shown in FIG. 4 and the accuracy is normal, light from the light source (11) is incident on one pixel 3m of the solid-state sensor (2). If there is distortion in 4), the light source (11)
Is incident on another pixel 3n. Therefore, pixel 3m,
Regardless of the light incident on any of 3n, if the light is corrected to be light incident at a predetermined angle α with respect to the optical axis a, the measured value accompanying the distortion of the lens system (4) can be obtained. Is corrected. The above calibration work is repeatedly performed by rotating the CCD camera (1) up, down, left, and right with respect to the optical axis a while changing the angle in a stepwise manner. After the calibration is completed, the CCD camera
It is used for measuring the appearance of an object, and performs the measurement of the appearance of the object with high accuracy based on calibration data.

[Problem to be Solved by the Invention] As described above, the method of performing calibration by rotating the CCD camera is easy to accurately obtain the three-dimensional rotation angle of the CCD camera with the rotary encoder, but the CCD is easy. It is very difficult to rotate the camera about the principal point of the lens system. That is, the lens system is a combination of various lenses, and the principal point is a virtual optical point, and cannot be stopped accurately. In addition, CC
Even if the D-camera can be rotated accurately, the correspondence between the incident angle of light with respect to the optical axis and the pixels of the solid-state sensor is not accurate when viewed in micron units, and high-precision calibration has been difficult. Furthermore, the calibration accuracy of the CCD camera can be increased as the rotation of the CCD camera is performed at a small unit angle of seconds or less, but it is technically difficult to accurately perform such a small unit rotation. This, too, was a factor that prevented highly accurate calibration.

The present invention has been made in view of the problems of the related art, and an object of the present invention is to perform technical calibration for correcting distortion or the like of a solid-state sensor of a CCD camera that requires high resolution. CCD that can be easily and accurately performed
An object of the present invention is to provide a camera calibration method.

Means for Solving the Problems In order to achieve the above object, the present invention provides a point light source supported by a positioning device in front of a fixed CCD camera, and a light source for one pixel of a solid-state sensor of the CCD camera. The step of moving between two points on one line of sight is performed on a plurality of lines of sight including the optical axis of the CCD camera so that the light of the center of Since the angle to the optical axis of the CCD camera is obtained from the position information of the positioning device, the correspondence between the incident angle of light to the CCD camera and the pixel of the solid-state sensor is calibrated based on the obtained angle data. is there.

[Operation] The movement of the light source between two points on one line of sight including the optical axis in front of the CCD camera can be easily performed by confirming that light is incident on the same pixel of the solid-state sensor of the CCD camera. Can be done accurately. As the moving distance of the light source in one line of sight is longer, the angle of the line of sight with respect to the optical axis can be obtained more accurately and at a smaller angle from the information on the position of the positioning device. If the angle of the line of sight at which such a light source moves is determined and the coordinates of the pixel of the solid-state sensor on which the light of the light source is incident are determined, the pixel corresponding to the light incident on the CCD camera at a predetermined angle is set, and the CCD camera is set. Calibration is performed with high accuracy.

Embodiment FIG. 1 is a view schematically showing a calibration apparatus for carrying out the method of the present invention. The CCD camera (1) is fixed by a fixing means (12) such as a fixing table, and a positioning device having a point-like light source (5) in front of the CCD camera (1), in this example, a coordinate measuring machine (6) is arranged. The coordinate measuring machine (6) supports the light source (5) via a movable arm (7) so as to be movable in three dimensions, and the light source (5)
Is detected in micron units.

The operation of calibrating the CCD camera (1) by the calibration device will be described.

First, the light source (5) is placed at a desired point A on the optical axis a of the CCD camera (1) by the coordinate measuring machine (6). . This is CC
In the pixel 3s at the center of the solid-state sensor (2) of the D camera (1),
As shown by the solid circle in the center of FIG. 2, the light source (5)
This is performed by confirming that the light at the center of is incident. Next, the light source (5) is retracted along the optical axis a, and the point A
At a point B sufficiently away from This retreat may be performed by monitoring so that the light of the light source (5) does not deviate from the pixel 3s at the center of the solid-state sensor (2). In this case, as shown by a broken circle in FIG. The amount of incident light at the pixel 3s corresponding to the axis a decreases. At this point, if the coordinates of the points A and B of the light source (5) on the optical axis a are obtained by the three-dimensional measuring machine (6), the three-dimensional coordinates of the optical axis a of the CCD camera (1) can be obtained from the coordinates. The direction at is determined.

Next, the light source (5) is moved to a desired point C away from the optical axis a by the coordinate measuring machine (6). At this time, the light of the light source (5) is changed to a solid line shown in the lower part of FIG. It is assumed that the light enters the pixel 3r indicated by a circle. Next, the light source (5) is monitored so that the incident light does not shift from the pixel 3r, and is retracted from the point C to a point D at a desired distance. When the coordinates of the point C and the point D are obtained by the three-dimensional measuring device (6), the three-dimensional angle of the line of sight e connecting the point C and the point D is obtained from the coordinates, and the angle β of the line of sight e with respect to the optical axis a is obtained.
Is determined.

In addition, the light source (5) is advanced from the point B to the point A,
Further, the angle β between the optical axis a and the line of sight e may be obtained by moving forward from the point D to the point C.

By moving the optical axis (5) in various directions as described above,
The coordinates of the pixels of the solid-state sensor (2) corresponding to the lines of sight at various angles with respect to the optical axis a are obtained, and the calibration is completed. That is,
For example, when the lens system (4) of the CCD camera (1) is distorted and the light from the line of sight e is incident on the pixel 3r, if the light is not corrected, the CCD camera (1) will be replaced with the one shown in FIG. It is erroneously recognized that light has entered from the direction of the line of sight f indicated by the chain line. Thus, the light incident on the pixel 3r may be calibrated by data processing to be from the line of sight e at an angle β with respect to the light source a.

In this example, a CCD camera has been described, but it goes without saying that the present invention can be applied to other one-dimensional and two-dimensional optical sensors.

[Effects of the Invention] According to the present invention, the angle of the line of sight to the optical axis due to the movement of the light source measured and driven by the coordinate measuring machine in front of the CCD camera is such that the longer the movement distance of the light source in one line of sight, the longer the distance. It is measured accurately, reliably and with a small angle of less than a second. Therefore, the number of pixels of the solid-state sensor is 1000
Even a CCD camera with a high resolution of × 1000 or more can calibrate the distortion of the solid-state sensor with high accuracy.

In addition, since it is only necessary to fix the CCD camera and move the light source in the three-dimensional direction with a positioning device such as a coordinate measuring machine in front of it, expensive and special equipment such as rotating the CCD camera with high precision is required In addition, since the existing CMM can be used, the calibration of the CCD camera which is advantageous in terms of equipment can be performed.

[Brief description of the drawings]

1 is a diagram schematically showing an example of an apparatus for implementing a calibration method according to the present invention, FIG. 2 is a front view of a solid-state sensor of the CCD camera in FIG. 1, and FIG. 3 is a relationship between the solid-state sensor and incident light thereof. FIG. FIG. 4 is a diagram showing the relationship between a solid-state sensor and incident light in a CCD camera, and FIG. 5 is a diagram schematically showing a calibration device for explaining a conventional method of calibrating a CCD camera. (1) CCD camera, (2) solid-state sensor, (3) pixel, (5) light source, (6) three-dimensional measuring device, a optical axis, e line of sight .

Claims (1)

(57) [Claims] 1. A point-like light source supported by a positioning device in front of a fixed CCD camera is positioned on one line of sight such that light at the center of the light source is incident on one pixel of a solid-state sensor of the CCD camera. The step of moving the light source between two points is performed on a plurality of lines of sight including the optical axis of the CCD camera, and the angle of each of the plurality of lines of sight with the light source moved with respect to the optical axis of the CCD camera is information on the position of the positioning device. And correcting the correspondence between the incident angle of light to the CCD camera and the pixels of the solid-state sensor based on the obtained angle data.