Image analysis – Applications – Document or print quality inspection
Reexamination Certificate
1998-05-29
2001-01-23
Couso, Yon J. (Department: 2723)
Image analysis
Applications
Document or print quality inspection
C250S559010
Reexamination Certificate
active
06178254
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to monitoring color on a printing press. More particularly, the invention relates to a method for accurately monitoring color, based on measuring the reflectance and/or the optical density of color patches printed on a web, and correcting for the effects of imperfections in the printed color bar patches.
BACKGROUND OF THE INVENTION
In the printing industry, quality control of color printing processes typically has been achieved by measuring the optical density of a test image using a densitometer or scanning densitometer off-line of the web printing process. Optical density measurements are performed by illuminating a test image with a light source and measuring the intensity of the light reflected from the image. Optical density (D) is defined as:
D
=−log
10
(
R
)
where R is the reflectance, or ratio of reflected light intensity to incident light intensity.
The test image which is measured is often in the form of color test strips or color bars. These color bars are comprised of individual color patches of varying ink color and tone having dimensions approximately 0.2 inches by 0.2 inches, with the color patches laid out in a row adjacent one another. The color bars often extend across the width of the web, are printed in the trim area of the web, and may be utilized for registration as well as color monitoring purposes. As printers attempt to minimize paper costs through the use of “short-cutoff” presses (for example, the Harris-Heidelberg M-1000BE or M-3000 press), the trim area on the printed sheet becomes smaller, and accordingly, so do the color patches. The color patches presently may be as small as 0.10 inches by 0.0625 inches.
During the printing process, the color patches will occasionally be printed with imperfections. These imperfections include blemishes caused by a particle of foreign material caught in the ink train, spots caused by ink spray, dropouts of color caused by the blanket cylinder picking paper fiber and ink from the paper, and the overlap of two adjacent patches due to variance in registration. Another imperfection in all patches is caused by scatter within the paper. This causes the edges of a patch with white surrounding it to have a lower measured density than the rest of the patch, irrespective of the ink film thickness. This border effect may exist for approximately 100 microns at the edges of the patch.
In traditional automated color measurement devices, these imperfections will cause measurement errors. Unfortunately, heretofore there has been no reliable method to determine whether the patch has an imperfection. One approach has been to retain only the pixels within the patch having a color value which fits within a predetermined range, while excluding all other pixels having color values outside the range, labelling them invalid. The problems with this method derive from the setting of the predetermined range. If the range is too tight, there is a risk of rejecting legitimate color values. If the range is too wide, there is a risk of accepting color values associated with minor imperfections in the color patch.
One method for the optical density measurement of color patches in a color bar is disclosed in U.S. Pat. No. 5,724,259, issued Mar. 3, 1998. This patent also discloses a method and system for locating the individual color patches in an acquired image and correcting for the effects of scattered light.
SUMMARY OF THE INVENTION
It is an object of the present invention to reliably obtain higher accuracy in color measurements of color patches printed on the web. It is a further object of the invention to provide a method to avoid the rejection of valid pixels within a color patch while at the same time not accepting invalid pixels with color values indicative of minor imperfections. This is accomplished, in general, by sensing the reflectance of the patch at a multiplicity of locations or pixels throughout the patch, and employing techniques to screen these reflectance values for outlying values.
A color video camera is ideal for measuring reflectance and/or optical density on-line because many points can be measured at the same time and precise alignment of the camera with the test area is not necessary. However, optical density measurements of color patches on-line may be inaccurate in part because of the imperfect nature of the printing process and the nature of the web itself. For example, imperfections in the color bar patches lead to inaccurate optical density measurements which are not wholly reflective of the true ink colors. Better accuracy can be obtained by identifying the imperfections and eliminating the effects of the imperfections on the optical density measurements. Higher accuracy in optical density measurements of color patches printed on a web is obtained by identifying any imperfections in the color patches and eliminating the effects of the imperfections on the optical density measurements.
The present invention provides a method for the accurate reflectance and/or optical density measurement of color patches printed on a web. The method includes the steps of using an imaging device to receive light reflected from the printed image on the substrate to produce a signal. The signal is transmitted from the imaging device to a computer and is processed in the computer. The step of processing the signal in the computer includes the steps of digitizing the signal to produce a digitized array of pixels, wherein each pixel has an associated color value, and determining a portion of the pixels within the digitized array corresponding to the color patch. The method further includes the steps of determining an acceptable range of color values for the pixels in the portion of the pixels, determining for each pixel in the portion of pixels whether the color value is within the acceptable range, and computing an aggregate color value for the color patch by averaging the color values of each pixel in the portion of pixels by including only those pixels within the acceptable range.
In operation, an imaging device is positioned to have its field of view on the web. An image within the field of view is illuminated by a light source and the imaging device records an image signal which is a Likeness of the printed image. The light source is synchronized with the web such that the light source is activated when the printed image within the field of view includes a portion of the color bars. The recorded image signal is next digitized by the image capture circuitry and is stored in the computer's memory as a captured image signal array. Various signal processing techniques are utilized to correct the captured image signal array for scattered light, uneven white response across the elements of the imager, black bias, and camera nonlinearities. Additionally, the captured image array is processed to locate the individual color patches. The imperfections in the color patches are identified by tallying all the pixels and calculating the mean and standard deviation. The pixels not within a predetermined range are identified and discarded and are not included in the calculation of optical density. The optical density value for each color patch is then determined. In this manner, the effects of any color patch imperfections on the optical density measurement are eliminated.
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Rappette Jeffrey P.
Seymour John C.
Couso Yon J.
Michael & Best & Friedrich LLP
Quad Graphics, Inc.
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