Automated CtP Calibration for Offset Printing : Dot gain compensation, register variation and trapping evaluation

Abstract

Although offset printing has been and still is the most common printing technology for color print productions, its print productions are subject to variations due to environmental and process parameters. Therefore, it is very important to frequently control the print production quality criteria in order to make the process predictable, reproducible and stable. One of the most important parts in a modern industrial offset printing is Computer to Plate (CtP), which exposes the printing plate. One of the most important quality criteria for printing is to control the dot gain level. Dot gain refers to an important phenomenon that causes the printed elements to appear larger than their reference size sent to the CtP. It is crucial to have the dot gain level within an acceptable range, defined by ISO 12647-2 for offset printing. This is done by dot gain compensation methods in the Raster Image Processor (RIP). Dot gain compensation is however a complicated task in offset printing because of the huge number of parameters affecting dot gain. Another important quality criterion affecting the print quality in offset is the register variation caused by the misplacement of printing sheet in the printing unit. Register variation causes tone value variations, gray balance variation and blurred image details. Trapping is another important print quality criterion that should be measured in an offset printing process. Trapping occurs when the inks in different printing units are printed wet-on-wet in a multi-color offset printing machine. Trapping affects the gray balance and makes the resulting colors of overlapped inks pale. In this dissertation three different dot gain compensation methods are discussed. The most accurate and efficient dot gain compensation method, which is noniterative, has been tested, evaluated and applied using many offset printing workflows. To further increase the accuracy of this method, an approach to effectively select the correction points of a RIP with limited number of correction points, has also been proposed. Correction points are the tone values needed to be set in the RIP to define a dot gain compensation curve. To fulfill the requirement of having the register variation within the allowed range, it has to be measured and quantified. There have been two novel models proposed in this dissertation that determine the register variation value. One of the models is based on spectrophotometry and the other one on densitometry. The proposed methods have been evaluated by comparison to the industrial image processing based register variation model, which is expensive and not available in most printing companies. The results of all models were ii comparable, verifying that the proposed models are good alternatives to the image processing based model. The existing models determining the trapping values are based on densitometric measurements and quantify the trapping effect by a percentage value. In this dissertation, a novel trapping model is proposed that quantifies the trapping effect by a c...

Indirect Ink Transfer for Offset Printing, Taking Into Account the Roughness of the Offset Printing Plate Surface

“…One of the most important criteria for print quality is the control of the point gain level. A CtP calibration method used for the manufacture of a printing plate is proposed, which calibrates the point gain in accordance with the recommendations of ISO 12647-2:2013 completely automatically in parallel with printing [16].…”

Improving image quality in multi-channel printing - multilevel halftoning, color separation and graininess characterization

“…Numerous parameters influence the print result, such as the type of printing technology, paper grade, halftoning algorithm and print resolution (Hauck, 2015). To qualitatively analyze the print result, it is of importance to understand the concepts behind print quality and quality evaluation in order to choose the appropriate assessment strategies.…”