Heidelberger Druckmaschinen Ag

Gretag Aktiengesellschaft

Heidelberger Druckmaschinen Aktiegesellschaft

Gretag Atkiengesellschaft

Gretak Aktiengesellschaft

Guido Keller

Helmut Kipphan

Hans Ott

Gerhard Loffler

Rolf Boegli

Andreas Spiess

To improve the control of the inking process in an offset printing machine, color measuring fields provided on printed sheets are evaluated not as heretofore densitometrically but colorimetrically by means of spectral measurements. Spectral reflections are used to match colors, or color coordinates are calculated from them and compared with corresponding set reflections or set color coordinates. The color deviations obtained in this manner are used to control the inking process. For the stabilization of printing runs the spectral reflections are converted into filter color densities and the inking process is controlled on the basis of these color densities in a conventional manner. The control of the inking process using color deviations and control using color density may be superposed upon each other. The process makes it possible to adapt color impressions in delicate locations of importance for the image in the print to the corresponding locations of the proof. Color deviations due to different material properties and other error sources may also be equalized to some extent.

Process and apparatus for controlling the inking process in a printing

To improve the control of the inking process in an offset printing machine, color measuring fields provided on printed sheets are evaluated not as heretofore densitometrically but colorimetrically by means of spectral measurements. Spectral reflections are used to match colors, or color coordinates are calculated from them and compared with corresponding set reflections or set color coordinates. The color deviations obtained in this manner are used to control the inking process. For the stabilization of printing runs the spectral reflections are converted into filter color densities and the inking process is controlled on the basis of these color densities in a conventional manner. The control of the inking process using color deviations and control using color denisty may be superposed upon each other. The process makes it possible to adapt color impressions in delicate locations of importance for the image in the print to the corresponding locations of the proof. Color deviations due to different material properties and other error sources may also be equalized to some extent.

A three-color offset printing machine produces, during the printing of sheets, color measuring strips with several color measuring fields. A gray half-tone field produced by the overprinting of three colors serves as the reference field. The color location of the reference field is compared in a color deviation computer with the color location of a correlated desired reference field. From the color deviation, a layer thickness variation computer calculates a layer thickness variation control vector by means of a sensitivity matrix calculated on the basis of a linear model by a matrix computer. The matrix computer evaluates a series of secondary fields comprising three single color half-tone fields, three single color full-tone fields, three two-color full-tone fields, and a three-color full-tone field.

Process and apparatus for the ink control of a printing machine

The printed sheets are scanned, by image elements, immediately behind the last printing mechanism, with one or several measuring heads. Suitable dimensions of an image element range from approximately 1.times.1 mm.sup.2 to 10.times.10 mm.sup.2. In each image element the reflectance is measured in four spectral ranges (infrared for black, red for cyan, green for magenta and blue for yellow). The measured reflectance values are converted by means of Neugebauer equations into surface coverages in a demasking step and inputted to a computer for evaluation. With the same measuring device, both the reference values associated with OK sheets, and the actual values associated with continuous printing, are measured. The computer compares the measured data, weights them with respect to such factors as surface coverage, foreign color component and the environment of a respective image element, and produces a regulating signal to control the ink feed elements. The color coordinates (X,Y,Z) may be determined from the surface coverage values of the four colors in a parallel computer program. Image elements which are important for the visual appearance of an image are given a high weight. A quality measure may be determined from the weighted comparison of reference and actual values to change the visual appearance. With this process it is possible to control the ink feed elements of a multicolor printing press by direct on-line measurement of the printed image, without using color measuring strips.

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Zurich, Switzerland

Guido Keller