Test and Calibration of Printing System Regulated by GRACoL7 (I)

It is one of the dreams of many printing companies to establish a standardized printing production environment to improve the overall quality of printed materials. The development of various printing standards and the development of digital inspection and control technologies are all aimed at achieving this goal. Standardized and standardized production environments (systems) and corresponding detection methods are a hot topic for many research and production organizations. In this regard, GRACoL of the United States has made useful attempts and achieved very good results.

GRACoL (General Recommendations for Applications in Offset Lithography) is a United States application and promotion organization for offset printing technology and is part of the International Digital Enterprise Alliance (IDE). GRACoL developed a standard set of test and implementation methods for printing production data according to the ISO 12647-2 offset printing standard Beijing Screen Printing, and in June 2006 released the latest version of this implementation method - GRACoL7.

Based on extensive experiments, GRACoL7 has extracted a series of new concepts and corresponding parameters for print inspection. By measuring and calculating these parameters, it finally obtains some data. Based on these results, it can adjust the RIP output calibration curve of CTP platesetter or imagesetter. Therefore, on the basis of ensuring the reproducible work of the system, the tone and color reproduction quality of the printed matter can be further improved.

GracoL7 test principle and implementation steps

1. Related concepts and parameters

GRACoL puts forward many new concepts and parameters for print testing and judgment, including: Neutral Print Density Curve (NPDC), Highlight Range HR (Highlight Range), Shadow Contrast SC (Shadow Contrast) ) and bright contrast HC (Highlight Contrast) and other concepts. Among them, the neutral printing density curve NPDC and the brightening range HR are the two most important concepts.

The NPDC contains two curves, CMY and K, which reflect the relationship between the measured neutral density and the percentage of the original dot on the neutral gray scale. Since the neutral density is an absolute measure, The dot enlargement value is a relative quantity related to the density of the field, so this concept can guarantee the consistency of the measurement and the apparent sense of the tone scale and density.

Dot enlargement is a very important parameter in the field of printing control, but there are some defects in the operation comparison and so on. For this reason, GRACoL has also proposed the parameter of bright tuning range. The bright tuning range has replaced the dot expansion in its proposed method to reflect the entire printed matter. Tone and gray balance parameters. HR needs to measure 4 times (2 times for CMY and K) and calculate 2 times; while the traditional dot expansion needs to measure at least 12 times for each color plate 3 times, and because HR is an absolute measurement value and dot gain is A relative value, so the contrast and visual brightness of the printed image obtained by HR is more reliable than dot gain. among them:

HR_cmy=ND(50c, 40m, 40y)-ND(paper)HR_k=ND(50k)-ND(paper)

Dark contrast SC and Brightness contrast HC are parameters that quickly detect the correctness of the neutral density curve in dark and bright tone, where:

SC_cmy=ND(75c, 66m, 66y)-ND(paper) SC_k=ND(75k)-ND(paper) Darkness contrast depends on the dynamic range of the printed product.