Scanner and printer profiling system

Details Scanner and printer profiling system Scanner and printer profiling system

2000-01-05

2003-12-30

Coles, Edward (Department: 2624)

Facsimile and static presentation processing

Static presentation processing

Specific to image source

C358S003230, C358S406000, C358S504000, C358S518000, C382S162000

Reexamination Certificate

active

06671067

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates most generally to color correction of computer peripheral devices, and more particularly to a color profiling system for a printer and scanner.
BACKGROUND OF THE INVENTION
Color is defined as the perceptual result of light in the visible region of the spectrum. The human retina has three types of color photoreceptor cells for illumination, therefore, it is possible to define color using only three numerical components.
The Commission Internationale de L'Eclairage (CIE) created a standardized system for representing color illuminations using three numerical components to represent the mathematical coordinates of color space. The colors produced by reflective systems are a function not only of the colorants but also of the ambient illumination that requires further spectral matching. The most familiar color systems include CIE XYZ, CIE xyY, CIE L*u*v* and CIE L*a*b*.
The CIE system is based on the description of color as luminance component Y and spectral weighting curves components X and Z. The spectral weighting curves for X and Z were standardized by the CIE based on statistics from experiments involving human observers. The magnitudes of the XYZ components are proportional to physical energy, but their spectral composition corresponds to the color matching characteristics of human vision.
Most devices employ a device-dependent color-coordinate system to specify the colors, and there are several different systems in the market. The coordinate system is defined in a color space that maps the color coordinates to the color mechanism used by the device. Color space refers to an N-dimensional space in which each point in the space corresponds to a color. The cyan (C), magenta (M), yellow (Y) and black (K) (CMYK) color space is commonly used for color printers, where each point in the CMYK color space corresponds to a color produced using a formula for the CMYK colorants. The color space could be represented solely by CMY, but black is added as a colorant for print matter for several reasons. Printing black by overlaying cyan, yellow and magenta ink is expensive and time-consuming, and the edges of the lettering tend to blur. The printing of three ink layers to produce black also causes the printed paper to become wet requiring more time to dry.
The red, green and blue (RGB) system is a color space system that is complementary to the CMYK color space. The RGB system is a three-dimensional color space wherein each point in the color space is formed by some combination of RGB colorants. The RGB system is typically used for computer monitors, TV screens and scanners—illuminating devices.
The term color gamut is used to refer to a range of colors that can be produced within a color space by a particular device from a set of colorants. The color gamut of a device corresponds to the visible colors that can possibly be produced by the device.
A digitized color image is represented as an array of pixels, wherein each pixel contains numerical components that define a color. The three components are required to represent an image, and printing necessitates a fourth component, namely black. Color printers and color copy machines typically use three or four colorants, such as CMYK to produce the color image. The combination mix and density of the colorants produce a wide array of shades and colors.
While the three numerical values for digitized images could be provided by a color specification system, the color coding systems require faster processing. Several other systems have developed for image coding, including linear RGB, nonlinear R′G′B′, nonlinear CMY, nonlinear CMYK, and derivatives of nonlinear R′G′B′ such as Y′CBCR. RGB values can be transformed to and from the CIE XYZ values by a three-by-three matrix transform.
A scanner is used for converting print mediums such as pictures, artwork, documents, transparencies, and photographs into an electronic format. The scanner captures an image by measuring colors reflected from or transmitted through an image at many points and assigns numerical values to the colors at those points. Normally, the scanned image is represented as digital data, called pixels, in a Red-Green-Blue (RGB) representation. The pixels are arranged into a table of rows and columns, and contain information about the image such as the color information for a particular pixel defined as some formula of the primary colors R-G-B. Some scanners convert the RGB values to CMYK values.
The reproduction of color information from multiple devices and varying environments is a common occurrence in the industry. Colored works are transferred among variety of peripheral devices and the color information processing systems within the various systems seek to ensure the accuracy of the original work. For example, a computer with a color monitor can interact with a colored printer, a scanner, digital camera, color copy machine, color facsimile and various other devices. As the color data passes from one medium to another, digital processing attempts to maintain a visual match within the capabilities of the devices.
Advances in technology and computing means have made color reproduction systems available to the general public. Many desktop publishing systems employ hardware and software that are affordable to users that need to produce quality color work products. Unfortunately, the concept of ‘What you see is what you get’ is normally lacking, and it is not uncommon to see the desired image on the monitor but produce a print product that lacks the quality characteristics desired.
Colors produced by two different devices based on the same input will differ, in part because of distortion of the signals which occur due to nonlinear response characteristics of the electronics of the devices and the method of selecting a color within a device color gamut. An input signal representing a particular color provided to two different devices typically results in the devices producing two different colors. This is true even when the input signal represents a color within the color gamuts of both of devices.
In order to accomplish accurate color transfer, the individual devices employ color calibration techniques. Calibration is necessary to set the color response of the color reproduction devices. The process of deriving a transform by comparing the device output to some reference output and generating a lookup table is called system color calibration. A transform derived for a particular scanner-printer combination is referred to as a closed system and the process is called closed system color calibration.
The purpose of the calibration is to account for the color differences. The color differences actually refers to numerical differences between the color specifications and more specifically refers to the perception of color differences in XYZ or RGB. Perceptual uniformity concerns numerical differences that correspond to color differences at the threshold of perceptibility. A perceptually uniform system is one where a small change to a component value is equally perceptible across the entire range. XYZ and RGB systems are not perceptually uniform and are actually highly nonuniform. In order to transform XYZ into a uniform standard, two systems developed, L*u*v* and L*a*b*, also written CIELUV and CIELAB. L*u*v* and L*a*b* improve perceptual nonuniformity but require highly complex computations to accommodate real-time display.
In most cases, an initial factory calibration creates calibration tables that are used by the digital processing schemes to make the color reproduction devices conform to standards and to compensate for drift and other changes.
Various instruments and methods are used to calibrate devices for color reproduction, including densitometers and colorimeters. A densitometer measures the density of ink on a print patch in each of CMYK colorants. The densities are then compared to a scale of desired densities to produce calibration curves. The data from the calibration cu

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