Offset spectral measurement and color adjustment

Offset printing using spectral measurement and color adjustment is no longer an incredible thing. This method is not only suitable for four-color printing, but also has advantages for spot color printing. In principle, the traditional densitometer is a "colorblind" measuring instrument, which can measure density only on printed sheets. It is possible to measure cyan, magenta, and yellow inks by using red, green, and blue optical filters. In theory, one limitation of density measurement is that the optical filters must be precisely adjusted to the colors printed, which is not a problem with standard four-color printing and is completely impossible with spot color printing.

Density measurement limitations:
1 The human eye sees colorimetry;
2 density can not explain the color;
3 polarizing filters will distort the measurement results;
4 The scope of application is limited to the printing process;
5 There is no direct relationship between density and ink layer thickness.

In 1990, Heidelberg proposed spectrum measurement, but this method once caused doubts to some printing industry. The problem is that in the papers on spectroscopy and colorimetric adjustment, there is often a lack of reference values ​​for regulating the press. It is not easy for the printer to see how the spectral colorimetric measurement is derived from these data and used to adjust the printing machine ink fountain. Key recommendation criteria.

Heidelberg Co., Ltd. and a well-known manufacturer of colorimetric measuring equipment stationed on the line, gave up the filter measurement, went to the spectral measurement, and displayed the calculated density value on the display. The advantage of this method is that the calculated density value is exactly the same as in the case of the automatic meter used by the press. The printer can also obtain the density value display calculated from the color spectrum in addition to the CIE Lab value obtained from the colorimetric measurement.

Spectral measurement and color adjustment work

In principle, all printing colors, whether it is the standard four-color or spot color, can use spectral measurement, the measured value can be automatically compared with the given rating, and the printing staff can see on the display screen. result. If the density value is used as a given value, the measured value and the rated value can be compared as before; if Lab is used as a given value, the printer can clearly see the deviation of the DE (Delta-E) value. In addition, the printer can also obtain the percentage deviation of the rated ink amount on the graph. It can be seen at a glance which of the ink fountain key inks are normal, and which ink fountain key inks are insufficient or excessive. If it is necessary to track the color of the ink, the meter can then transmit the recommended value for adjusting the position of the ink fountain key, which can be adjusted by the on-line button of the printing press. Many printers, after a short period of adaptation, felt that the DE bias value was more convenient than the density display.
PrinectImage-Control measurement method: Irradiate the printed sheet with white light, use the spectrophotometer to decompose the radiated light into the spectrum color, and the computer software analyzes the chromaticity measurement result. The values ​​derived from this are displayed on the display screen and continue to be transmitted to the press as a standard for tracking the ink color. As shown in Figure 1.

Figure 1 PrinectImage-Control works

Density cannot express shades

It is not possible to specify a nominal density value when printing spot colors, and other measures are required when spot colors are encountered. Since the density measurement ignores the pigment of the ink, the density cannot express the hue. The human eye can compare color vision, but it cannot compare density. If the type of ink is changed during printing, there is no guarantee that the same print density can produce the same color sensation. However, some printers accustomed to density measurement do not accept it. This is a topic that often quotes “standard four-color printing”.

Since the spectrometer can perform the tasks of the densitometer, Heidelberg decided long ago to use the spectral measurement and colorimetric adjustment while preserving the density display. Because some printers have been accustomed to density values, they cannot ignore the density value, but the density value loses meaning over time. Heidelberg's Prinect meter takes this into account.

However, one drawback of colorimetric adjustment is the lack of polarizing filters, and the polarization filter in the densitometer's optical path balances the gloss effect produced by wet stamps. People are accustomed to comparing dry printed sheets with wet printed sheets, hoping to get the same results. Because the customers of the printing factory only pay attention to the effect after the printed sheets are dry, after the printed sheets are dried, people often ignore the changes in the tone of the screen theme. Although the density measured by the polarizer may not change much, it has an effect on the color vision, and the old stamp can be used to determine the reference value. However, the printer always evaluates the ink with wet sheets and adjusts the amount of ink. The balanced gloss effect of the polarizer is only a numerical value, and the actual color difference between dry and wet remains, although it may not be reliable.

Other advantages of spectral color measurement

In addition to adjusting ink fountain keys during printing, the spectrophotometric performance of a measuring instrument can also be used in other ways.

When color data emerges in more and more media-neutral forms (CIELab.sRGB), it is important that printers react accordingly. Producers and media designers want to see the color proofing effect as much as possible before offset printing. The solution is the ICC color file. If you want to generate such a file, you cannot avoid the spectral measurement method, but the file only recognizes the Lab value and does not recognize the density value.

The Image-Control meter bridges the bridge from printing to prepress. The measuring instrument can be equipped with a color interface module, for example measuring 70 cm x 100 cm printed sheets with IT8/7.3 test version with 928 measured color patches. It takes only 25 seconds to complete, and comes with a corresponding test version. In the presence of the measured value file, a program is then used to continue processing the ICC color file, together with the forming software Print-Open to provide a complete set of solutions.

Personalized matching

The supplier completes the customer support in the actual conversion with the service of Print Color Management. The printing and prepress engineers at Heidelberg are responsible for the correct adjustment of the color workflow, including the creation and execution of ICC color files for digital proofing. Before using a monitor to evaluate color-constrained images, the monitor must be calibrated. The same applies to the printing plant application workflow. So Heidelberg offers customers personalized matching with Print Color Management software.

In short, spectral measurement and colorimetric adjustment are neither magical nor scientific secrets. To be precise, it is an important method. Only the spectral measurement and colorimetric evaluation can make the color consistent with the international standard, and enable the printer to adapt to the market demand in the future. For the content of spectral measurement and colorimetric adjustment, please refer to the "Printing Process Standards Manual" and "ISO12647-2 International Standard" of the German Printing and Media Association.



Source: Ke Yin Media - "Printing Technology"