Facsimile and static presentation processing – Natural color facsimile – Measuring – testing – and calibrating
Reexamination Certificate
1997-01-21
2001-04-24
Grant, II, Jerome (Department: 2724)
Facsimile and static presentation processing
Natural color facsimile
Measuring, testing, and calibrating
C358S501000
Reexamination Certificate
active
06222648
ABSTRACT:
This invention describes a method and apparatus for calibrating, and periodically maintaining the calibration of, the colors printed by a document output terminal.
The following patents are specifically incorporated by reference: U.S. Pat. No. 5,416,613 to Rolleston et al. for its teaching of calibrating the response of a color printer by using a plurality of color patches some of which are repeated at a plurality of spatially disparate locations on the test image; U.S. Pat. No. 4,500,919 to Schreiber for its teachings of a color conversion system converting information from RGB to CMYK; U.S. Pat. No. 4,275,413 to Sakamoto for its teachings of tetrahedral interpolation between first and second color spaces; and U.S. Pat. No. 2,790,844 to Neugebauer disclosing the desirability of defining an image in a first standard color space prior to conversion of the image coordinates to a second printer based coordinate system.
The generation of color documents can be thought of as a two step process: first, the generation of the image by means of scanning an original document with a color image input terminal or scanner or, alternatively, creating a color image on a work station operated in accordance with a color image creation program; and secondly, printing of that image with a color printer in accordance with the colors defined by the scanner or computer generated image. Color corrected scanners commonly operate with colors defined in a color space of tristimulus values, e.g., RGB (red-green-blue). Commonly, these values are a linear transformation of the standard XYZ coordinates of CIE (International Commission on Lighting) color space, or a correct transform of those values. In the case of computer generated images, colors defined by the user at the user interface of his workstation are immediately converted into color space values and directed out of the system as defined in the document colors.
Printers have an output which can be defined as existing in a color space called CMYK (cyan, magenta, yellow, key or black) which is uniquely defined for the printer by its capabilities and colorants. Printers operate by the addition of multiple layers of ink or colorant in layers or halftone dots to a page. The response of the printer tends to be non-linear. Thus, while a printer receives information in a first color space which has values defined independently of any device, it must convert that information to print in a second device dependent color space.
The desirability of operating in a tristimulus color space with subsequent conversion to a printer colorant color space is well known, as shown by U.S. Pat. No. 4,500,919 to Schreiber, U.S. Pat. No. 2,790,844 to Neugebauer, and U.S. Pat. No. 4,275,413 to Sakamoto. There are many methods of conversion between color spaces, all of which begin with the measurement of printer response to certain input values. Commonly, a printer is driven with a set of color input values, the values are printed in normal operation of the printer, and measurements are made of those colors to determine what the actual color printed was in response to the color specification. As previously noted, most printers have non-linear response characteristics.
The calibration of a printer involves the process of finding what set of signals must be sent to a printer to obtain a desired color. The desired color is described in some device independent terminology (i.e., some well defined standard), and the signals to the printer constitute a device dependent terminology. A complete calibration will transform the device independent color description into a device dependent description such that the resultant combination of materials (i.e., ink, toner, dye, etc.) on the paper produces the desired color (i.e., the color which was initially described in a device independent fashion).
In U.S. Pat. No. 4,500,919 to Schreiber, and U.S. Pat. No. 4,275,413 to Sakamoto, the information derived from patch measuring was placed into look-up tables, stored in a memory, perhaps ROM memory or RAM memory where the look-up table relates input color space to output color space. The look-up table is commonly a three dimensional table since color space is three dimensional. With a scanner or computer, the RGB space can be defined as three dimensional with black at the origin of a three dimensional coordinate system (0,0,0), and white at the maximum of a three dimensional coordinate system which, in an 8-bit system, would be located at (
255
,
255
,
255
). Each of the three axes radiating from the origin point therefore respectively define red, green, and blue. A similar construct can be made for the printer, with axes representing cyan, magenta, and yellow. Black toner is usually added separately. In the 8-bit system suggested there will be, however, over 16 million possible colors (256
3
) There are clearly too many values for a 1:1 mapping of RGB to CMYK. Accordingly, as proposed in U.S. Pat. No. 4,275,413 to Sakamoto, only a relatively small number of samples are made at the printer, perhaps on the order of 1,000, or even less. Therefore, the look-up tables consist of a set of values which could be said to be the intersections for corners of a set of rectangular parallelepipeds mounted on top of one another. Colors falling within each rectangular volume can be interpolated from the measured values, through many methods including tri-linear interpolation, tetrahedral interpolation, polynomial interpolation, linear interpolation, and any other interpolation method depending on the accuracy of the desired result.
A common approach to accomplishing calibration of a print system is by generating a set of color patches distributed in color space; printing the patches (in output color space) in a desired output medium like a paper sheet and measuring the color of the patches, typically with a high accuracy densitometer, spectrophotometer or the like to determine what was printed in terms of an input color space. This data can then be loaded into a look-up table. However, it will undoubtedly be understood that due to the vast number of measurements required to perform such a calibration, any continual, automatic updating is considered impractical and generally not performed.
Other references which disclose calibration methods for color printers utilizing look up tables and interpolation schemes include U.S. Pat. No. 5,508,826 to Lloyd et al; U.S. Pat. No. 5,471,324 to Rolleston; U.S. Pat. No. 5,491,568 to Wan; U.S. Pat. No. 5,539,522 to Yoshida; U.S. Pat. No. 5,528,386 to Rolleston et al.; and U.S. Pat. No. 5,483,360 to Rolleston et al.
All of the references cited herein are incorporated by reference for their teachings.
In accordance with one aspect of the invention, there is provided a calibration arrangement for periodically updating the calibration of a color printer, comprising: a multi-color (or monochrome) printer responsive to electronic signals to print combinations of a plurality of colorants approximating cyan, magenta, yellow and black on a substrate, a memory suitable to store electronic signals suitable to drive the color printer to reproduce a calibration image, said calibration image including patches printed on said substrate with combinations of primary (e.g., cyan, magenta, yellow, and black or a system supplemented by orange and green colorants) subtractive colorants and representing known possible colors within a gamut of said printer, a printer controller, controlling the printer to print the calibration image during normal operations, a color sensing device suitable for measuring the colorimetric response of the printer in printing the calibration image on the substrate in terms of device independent colorimetrics, a device memory storing a look-up table relating device independent colors to printer colorants derived from the colorimetric measurements of the printer response, a first compensator to convert device independent color information to calibrated device independent color information, as a function of the measured colorimetric response and a second compensator
Castelli Vittorio R.
Solcz Edward J.
Wolf Barry M.
Grant II Jerome
Xerox Corporation
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