Numerous solutions have been known for detecting quality data in the multicolor printing of single editions, especially in job printing and newspaper printing. The detection of quality data, e.g., of tristimulus, ink layer thickness, register, shifting and doubling values, surface coverages and the like, is used to monitor and control the coloration in multicolor printing.
A process for achieving a uniform print result on an autotypically operating multicolor offset printing press has been known from EP 0 196 431 B1. Ink layer thicknesses and full tone densities and half-tone dot sizes or surface coverage degrees, which are printed simultaneously for each printing ink in each color-setting zone of the printing press, are measured here in measuring fields. The color-controlling adjusting members of the printing press are set automatically based on the densitometric measured values. Since a plurality of measuring fields are printed simultaneously in each color-setting zone of the press, this process is suitable for job offset printing, it is not suitable for newspaper offset printing, in which the measuring fields are printed simultaneously within the printing area, contrary to the job offset printing, and they cannot be cut off after the printing. Newspaper publishers are therefore reluctant to accept these measuring fields.
The high cost of the apparatus and manpower that is needed for measuring the measuring fields can be considered to be another obstacle to the use of this prior-art process in newspaper offset printing. If the measurement is to be performed in web offset printing on-line, i.e., automatically on the running web, an optical measuring head with automatic positioning is needed for each side of the web. If the measurement were performed with commercially available manual densitometers or manual spectrophotometers, instead, personnel would have to be provided specifically for the purpose of detecting quality data in light of the large number of measuring fields and the time required for the manual positioning of the measuring device. Furthermore, the features measured according to this prior-art process in the form of the full-tone and half-tone densities of the individual colors contain little information on the color appearance of the finished multicolor printed product, even though they are directly related to the printing process.
Data on the color sensation can be obtained by printing simultaneously and colorimetrically measuring combination measuring fields, as it has been known especially from DE 44 02 784 A1 and DE 44 02 828 A1. The space requirement for the measuring field or measuring field group printed simultaneously on the printed product to be checked is markedly reduced due to the use of the measuring field group described there. However, this measuring field or the measuring field group known from this does not yet make it possible to record measured values for color uptake in multicolor printover, on the register mark or even for determining disturbances in the printing process, such as shifting and doubling.
Processes for detecting register mark errors and for measuring suitable register marks have been known from DE 44 37 603 A1 and DE 40 14 706 A1. Such register marks would have to be printed in addition to the color marks on the printed product to be checked and be measured with a corresponding measuring device. At least two measuring devices must be controlled and used here.
Another problem arises in connection with the progressive adoption of Color Management in the printing industry. As is known, the idea behind Color Management is to set color originals in the digital preliminary printing stage independently from output devices and materials. The colors of a color original are described in a colorimetric system of coordinates standardized by the Commission International de l'Eclairage (CIE), such as CIEXZY, CIELAB or CIELUV. If multicolor images thus defined are printed out on paper via a system calibrated in the sense of Color Management, it is guaranteed that the color appearance of the printed product will be comparable to the original, independently from the output process used.
Computer color printers, digital color copiers and digital proof devices are now used, among other things, as output systems that can be calibrated. It is desirable to also extend the concept of Color Management to conventional printing processes, such as newspaper offset printing. The functional chain consisting of the preparation of the printing form and the printing process is treated here as any other output device that can be calibrated.
An important prerequisite for this is met with the availability of systems for preparing color profiles of the printing process. One problem still lies in the question of how the new Color Management tools can function in a meaningful manner in conjunction with the checking and control mechanisms (densitometry and colorimetry) specific of the printing process.
When preparing color profiles, it is necessary to print and measure special test patterns under exactly defined conditions. This is expensive, because machine hours and material are consumed in the process. It would be desirable to perform the calibration of the color profiles of the printing process only when it has really become absolutely necessary, rather than preventively. However, there is no tool at present that can decide whether this is the case based on the printing of single editions.