Technique for accurate color-color registration measurements

• US 6,529,616 B1
• Filed: 11/29/1999
• Issued: 03/04/2003
• Est. Priority Date: 11/29/1999
• Status: Expired due to Fees

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Rasmussen, D. Rene, Dalal, Edul N., Zoltner, Susan June
• Primary Examiner(s): Chang, Jon
• Assistant Examiner(s): KIM, CHONG R
• Application Number:
  US09/450,181
• Time in Patent Office: 1,191 Days
• Field of Search: 382/112, 382/162-167, 382/287-289, 250/559.01, 250/559.49, 358/1.9, 358/3.26, 358/504, 358/406, 358/540, 347/19, 347/116
• US Class Current: 382/112
• CPC Class Codes: H04N 1/00002 Diagnosis, testing or measuring; Detecting, analysing or monitoring not otherwise provided for error detection, error correction or monitoring in digital computers or digital computer components G06F11/00
• CPC Class Codes: H04N 1/00015 Reproducing apparatus
• CPC Class Codes: H04N 1/00023 Colour systems
• CPC Class Codes: H04N 1/00031 Testing, i.e. determining the result of a trial H04N1/00029 takes precedence; using test signals for transmission mode changing H04N2201/33371
• CPC Class Codes: H04N 1/00045 using a reference pattern designed for the purpose, e.g. a test chart
• CPC Class Codes: H04N 1/00053 out of service, i.e. outside of normal operation
• CPC Class Codes: H04N 1/00058 using a separate apparatus
• CPC Class Codes: H04N 1/00061 using a remote apparatus H04N1/00066 takes precedence
• CPC Class Codes: H04N 1/00068 Calculating or estimating
• CPC Class Codes: H04N 1/00079 remotely
• CPC Class Codes: H04N 1/6033 using test pattern analysis H04N1/6055 takes precedence

Abstract

A special test pattern and image quality analysis system and process for evaluating the test pattern are provided to accurately measure color-color registration in an image output device that prints in CMYK color. The process is robust even when using relatively low-resolution CCD-based scanners and also is unique in that the process automatically factors out errors originating from skew between the detector and the subject of measurement. Further, registration of each of C, M, and Y relative to K are independently determined. The test pattern includes an upper part and a lower part, with both the upper and lower parts having a black K component and the upper part also including color components for C, M and Y. The system and method have potential for being built in to color printers or copiers to allow measurement of color misregistration within the paper path and automatically correct for color registration errors.

Claims

1. A method of performing image quality analysis on a color image output device having an output station that generates a hardcopy color image output from an input image, the method comprising:

providing a test pattern having first and second sections of spaced geometrical marks, the first section of marks being black while the second section includes black marks and at least one color mark of a color other than black;

generating a hardcopy image output from the image output device using the test pattern as an input image;

scanning the hardcopy image to form a digital raster image using a scanner; and

performing image quality analysis on the digital raster image, including determination of color-color misregistration with skew correction,

wherein the step of performing image quality analysis includes;

taking reflectance profiles across both the first section and the second section using a channel of the scanner complementary to the color other than black;

calculating centroids of lines having the color and black;

measuring a first offset between a centroid of a color line and a centroid of a corresponding line from the first section having black marks to determine a total misregistration;

measuring a second offset between a centroid of a black mark from the second section and a centroid of a corresponding black mark from the first section having black marks to determine the contribution from skew to the previously calculated misregistration; and

subtracting the second offset from the first offset to determine color misregistration compensated for skew of the color other than black relative to black.

2. The method of claim 1, wherein the geometric marks are parallel lines.

3. The method of claim 2, wherein the step of scanning uses an RGB scanner.

4. The method of claim 1, wherein the color is cyan.

5. The method of claim 4, wherein the step of performing image quality analysis further includes:

taking reflectance profiles across both the first section and the second section using a channel of the scanner complementary to magenta;

calculating centroids of marks having magenta and black;

measuring a first offset between a centroid of a magenta mark and a centroid of a corresponding mark from the one section having black marks to determine a total misregistration;

measuring a second offset between a centroid of a black mark from the second section and a centroid of a corresponding black mark from the first section having black marks to determine misregistration due to skew; and

subtracting the second offset from the first offset to determine magenta to black misregistration compensated for skew.

6. The method of claim 4, wherein the step of performing image quality analysis further includes:

taking reflectance profiles across both the first section and the second section using a channel of the scanner complementary to yellow;

calculating centroids of marks having yellow and black;

measuring a first offset between a centroid of a yellow mark and a centroid of a corresponding mark from the first section having black marks to determine a total misregistration;

measuring a second offset between a centroid of a black mark from the second section and a centroid of a corresponding black mark from the first section having black marks to determine misregistration due to skew; and

subtracting the second offset from the first offset to determine yellow to black misregistration compensated for skew.

7. The method of claim 1, wherein the geometric marks are parallel lines, the step of calculating centroids includes:

determining an approximate edge position for a line;

defining a line surround;

calculating a weight profile; and

calculating a centroid of the line.

8. The method of claim 1, wherein the geometric marks are parallel lines and the test pattern includes 10 black lines L1 to L10 in the first section and 10 lines U1 to U10 in the second section, with U1 and U2 being black lines, U3 and U4 being cyan lines, U5 and U6 being magenta lines, U7 and U8 being yellow lines and U9 and U10 being black lines, the method comprising the steps of:

taking reflectance profiles across both the first section and the second section using a channel of the scanner complementary to cyan;

calculating centroids of lines U1, U2, U3, U4, U9 and U10 as XU1, XU2, XU3, XU4, XU9 and XU10, respectively;

calculating centroids of lines L1, L2, L3, L4, L9 and L10 as XL1, XL2, XL3, XL4, XL9 and XL10, respectively;

obtaining a measurement of misregistration UMC uncorrected for skew using the following equation,

9. The method of claim 1, wherein only a central region CR of the first and second sections are used for image quality analysis.

10. The method of claim 1, wherein the test pattern is stored in memory in the image output device.

11. The method of claim 10, wherein the image output device is a printer.

12. The method of claim 10, wherein the image output device is a copier.

13. An image quality analysis system for performing image quality analysis on a color image output device having an output station that generates a hardcopy color image output from an input image, the system comprising:

a test pattern having first and second sections of spaced geometrical marks, the first section of marks being black while the second section includes black marks and at least one color mark;

a scanner that forms a digital raster image from a hardcopy image output from an image output device to be tested that used the test pattern as an input image;

an image quality analysis module that performs image quality analysis on the digital raster image to determine color-color misregistration corrected for skew,

wherein the geometric marks are lines, and the test pattern includes 10 black lines L1 to L10 in the first section and 10 lines U1 to U10 in the second section, with U1 and U2 being black lines, U3 and U4 being cyan lines, U5 and U6 being magenta lines, U7 and U8 being yellow lines and U9 and U10 being black lines.

14. The system of claim 13, wherein the geometric marks are parallel lines.

15. The system of claim 13, wherein the scanner is an RGB scanner.

16. The system of claim 15, wherein the image quality analysis module isolates misregistration for each color by conducting image quality analysis using signals from individual RGB channels of the RGB scanner.

17. The system of claim 13, wherein the scanner is integral with the image output device.

18. The system of claim 13, wherein the scanner is separate from the image output device.