Method and system for automatically calibrating a color camera in a machine vision system

• US 5,821,993 A
• Filed: 01/25/1996
• Issued: 10/13/1998
• Est. Priority Date: 01/25/1996
• Status: Expired due to Term

• Current Assignee: Microscan Systems Inc (Omron Corporation)
• Original Assignee: Medar Incorporated (Nadex (Thailand) Co. Ltd.)
• Inventors: Robinson, David A.
• Primary Examiner(s): HSIA, SHERRIE Y
• Application Number:
  US08/591,130
• Time in Patent Office: 992 Days
• Field of Search: 348/175, 348/176, 348/187, 348/188, 348/652, 348/653, 348/654, 348/655, 348/656, 348/657, 348/658, 348/672, 364/571.01, 364/571.02, 364/571.04, 382/167, 382/274, 382/275
• US Class Current: 348/187
• CPC Class Codes: H04N 17/002 for television cameras
• CPC Class Codes: H04N 23/88 for colour balance, e.g. white-balance circuits or colour temperature control

Abstract

A pair of calibration algorithms are provided for calibrating a color camera in a machine vision system. The first and most general form of the calibration algorithm requires that a calibration target be shown to a well adjusted machine vision system. The calibration target contains samples of red, green and blue. A gray level histogram is taken of each of the resulting red, green and blue images and is used to generate corresponding standard cumulative probability functions. The standard cumulative probability functions, together with a copy of the calibration target, is provided to a machine vision system having its own color camera which requires calibration. The same above-noted steps are repeated for this system and the resulting cumulative probability functions are compared to the corresponding standard cumulative probability functions to obtain gains for the red, green and blue channels of the camera to be calibrated . In the second simplified version of the calibration algorithm, a target patch of pure white is provided and camera outputs for red, green and blue are determined while observing the patch. The average gain for each of the output channels of the color camera is determined. This simplifies to taking the average white level measured for each of the channels and dividing the average white value into the full scale value of the machine vision system to be calibrated. Applying the three gain factors determined under either version of the algorithm to a subsequent image allows one to end up with a color-corrected image.

Claims

1. A computer-controlled method for automatically calibrating a first color camera having output channels in a machine vision system, the method comprising the steps of:

providing a calibration target to an adjusted machine vision system including a second color camera having output channels;

generating a calibration image from each output channel of the second color camera from the calibration target;

generating an initial set of calibration data based on the calibration images;

utilizing the first color camera to generate an initial target image containing the calibration target for each of the output channels;

measuring an output of each of the output channels during the step of utilizing to obtain output measurements;

computing a final set of calibration data from the output measurements and the initial set of calibration data; and

applying the final set of calibration data to each subsequent image acquired by the color camera to obtain a color-corrected image.

2. The method as claimed in claim 1 wherein the initial set of calibration data includes a first set of cumulative probability functions based on the calibration images.

3. The method as claimed in claim 2 wherein the step of computing includes the steps of generating a second set of cumulative probability functions corresponding to the target images and comparing the first and second sets of cumulative probability functions to obtain the final set of calibration data.

4. The method as claimed in claim 1 wherein the first color camera is a RGB color camera.

9. The method as claimed in claim 1 wherein the calibration target has red, green and blue components.

10. The method as claimed in claim 1, wherein the calibration target has a pure white region.

5. A computer-controlled system for automatically calibrating a first color camera having output channels in a machine vision system, the system comprising:

an adjusted machine vision system including a second color camera having output channels, the adjusted machine vision system including means for generating a calibration image from each output channel of the second color camera from a calibration target and means for generating an initial set of calibration data based on the calibration image;

means for storing the initial set of calibration data;

the first color camera generating an initial target image containing the calibration target for each of the output channels;

means for measuring an output of each of the output channels during generation of the initial target images to obtain output measurements, computing a final set of calibration data from the output measurements and the initial set of calibration data and applying the final set of calibration data to each subsequent image acquired by the first color camera to obtain a color corrected image.

6. The system as claimed in claim 5 wherein the initial set of calibration data includes a first set of cumulative probability functions based on the calibration images.

7. The system as claimed in claim 6 wherein the means for computing includes means for generating a second set of cumulative probability functions corresponding to the target images and means for comparing the first and second sets of cumulative probability functions to obtain the final set of calibration data.

8. The system as claimed in claim 5 wherein the first color camera is a RGB color camera.

11. The system as claimed in claim 5 wherein the calibration target has red, green and blue components.

12. The system as claimed in claim 5 wherein the calibration target has a pure white region.