Gretag Aktiengesellschaft

Rolf Boegli

Hans Ott

Helmut Kipphan

Gerhard Loffler

Guido Keller

Ulrich Birch

Andreas Spiess

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.

Process and apparatus for evaluating printing quality and for regulating

On a measuring table (1), a bridge (2) extends over a printed sheet (5) to be measured. A measuring carriage (3) carrying a three color-simultaneous measuring head is arranged on the bridge (2) for linear movement in a back-and-forth manner along a desired path on the printed sheet. For control of the movement of the measuring head during measuring runs and for the evaluation of the measured data, a computer (7) with an integrated image display terminal and a processor system (6) controlled by the computer, is provided with the necessary interfaces. The analysis of a color measuring strip (MS) located along the desired path on the printed sheet is effected in two phases. In a first, so-called recognition run the measuring strip is scanned stepwise with high resolution in all three color channels, for the purpose of an analysis of its configuration. By means of flat locations in the density variations, effective positions (e.g., the centers) in the measuring fields are determined as possible measuring positions for subsequent measuring runs. During the measuring runs the measuring strip is measured only at the measuring positions determined during the recognition and the measured data obtained in this manner are transmitted for processing and/or display to the computer (7). By the division of the measurements into a recognition and a measuring run, coupled with the determination of measuring positions by means of flat locations in the density variations, a practically completely automatic evaluation of the color measuring strips independently of their configuration and without the need of prior knowledge of the strips by the apparatus, is rendered possible.

Process and apparatus for the colorimetric analysis of printed material

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Regensdorf, Switzerland

Rolf Boegli