Color Management for Heidelberg Presses |
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Kurt K. Wolf, Deutscher Drucker 42/2005 - By transferring digital data for a print job, Heidelberg has created a whole range of color calibration tools. These can help the printer to match the proof as quickly and reliably as possible on the press.
Any commercial printer wishing to remain profitable in the face of the current pressure on prices needs to utilize all the time-saving and quality assurance options at his disposal. This applies to both new jobs being printed for the first time and repeat jobs, which it is now possible to set to the print parameters used for the initial run at the touch of a button. In many print shops, however, presses are still preset in the same old way based on the printer's experience and calibrated to match the proof at the start of each printing process.
The Prinect Color Solutions offered by Heidelberger Druckmaschinen AG represent the key to a workflow with a whole range of tools providing reliable color control and, at the same time, allowing very rapid operation. Unfortunately, these tools and processes are not all that easy to understand. Firstly, they all start with the name Prinect. Secondly, in most cases the functions are not obvious from the name. And thirdly, the names are all geared towards British and North American users.
In November 2005, trade journalists were invited to Heidelberg to give them a better understanding of the workflow and demonstrate just how quick it can now be to switch jobs on a Speedmaster. All the various press parameters for a job are saved and can be called up again for a reprint. In the case of new jobs too, the use of CIP4 PPF data enables the printer to get very close to the target digital proof at the very start of the printing process, so it takes just a short time and a small number of adjustments to obtain a good match.
The ideal scenario would be for prepress page production to take place in the printshop, thereby ensuring that the prepress digital proof is matched to the print parameters of the press. Unfortunately, the fact that prepress companies and offset printshops are often separate entities means that the printer has to go through the lengthy process of trying to calibrate his press to the digital proof if the prepress company and printshop are not working to the same narrow process standard.
The Heidelberg Prinect family represents a digital process chain for this very situation, forming an integrated and bi-directional closed-loop system. Here is a brief explanation of how the individual Prinect tools work.
1. Prinect Calibration Toolbox
The Prinect Calibration Toolbox creates, manages and monitors platesetter linearizations and printing characteristics in computer-to-plate systems for various output conditions encountered in production. This software can also be used as a central calibration database to supply several CtP systems simultaneously with the current printing characteristic. Used in conjunction with Prinect Image Control, the calibrations generated can be monitored and a flexible response made to changed print conditions. The role of the Quality Monitor is to monitor and analyze the quality of printing plates and characteristics.
2. Prinect Profile Toolbox
The Prinect Profile Toolbox creates ICC profiles, thus enabling proof and print to be coordinated. When used with the integrated Quality Monitor, deviations between the proof and print can be measured objectively instead of being determined and amended by feel as was previously the case. Rather than having to generate new profiles completely from scratch if print parameters such as paper type or ink change, existing profiles can be adjusted in no time at all. The Prinect Profile Toolbox supports a variety of test forms and measuring instruments. Measurement of the Mini Spots developed by Heidelberg is also supported.
3. Quality Monitor
The Quality Monitor is part of both the Prinect Profile Toolbox and the Prinect Calibration Toolbox. The name "Quality Monitor" is confusing because it refers not to a color monitor but to the process of monitoring. The tool checks and monitors color quality for the print and proof and is used to monitor and analyze the quality of printing plates and characteristics.
4. Prinect Printready System
PDF documents supplied by the customer are verified for integrity and printability using the Prinect Printready System prepress workflow. This includes matching all colors to the color space of the printing process using ICC profiles. Prinect MetaDimension then outputs the print-ready document for checking on a proofer. The print process profile and proofer profile provide a definitive simulation of the print.
5. Prinect MetaDimension
The Raster Image Processor (RIP) software from Heidelberg converts the data for all the various print sheet elements into the imaging files for the four printing colors. Images, texts and graphics supplied in the RGB or L*a*b* color space are broken down into the four colors with ICC profiles using gamut mapping, at the same time incorporating the printing characteristics required for plate imaging. This results in four imaging files for imaging the four printing plates. Pre-separated CMY or CMYK images, on the other hand, can only be adapted to the printing characteristics in the tonal value curve.
The Pantone and HKS color charts are fully integrated. The software also generates data for the parameters to preset the press and provides reference values which allow the print sheets to be monitored using Prinect Image Control.
5.1 ICC profiles
The International Color Consortium (ICC) is a manufacturer-independent body whose goal is the global standardization and dissemination of color management products. The ICC color profile is both a fingerprint of a color space, e.g. of a press, proofer or color monitor, and a set of instructions for the gamut mapping that is to be carried out, for example from the color space of a press to that of a monitor. The profiles produced on this basis are standardized and therefore reproducible at any time. ICC profiles ensure that the different color representation of input and output devices such as cameras, scanners, monitors and presses is standardized and any irregularities are eliminated.
5.2 CIP4
The CIP4 organization is an independent, global association headquartered in Switzerland. Its objective is to further the integration of computer-aided processes in the printing industry. It develops manufacturer-neutral data formats such as Print Production Format (PPF) and Job Definition Format (JDF). Based on the data entered for a particular order, Print Production Format describes all the parameters for the remaining production steps. For example, the ink fountain roller sweep and the openings of the ink zones are calculated from the four RIPped imaging files. Folder and cutter settings are calculated from the sheet dimensions and made available to the appropriate units.
Job Definition Format is a language allowing industry software systems to communicate with the prepress workflows, their proofers and imagesetters, but also with the press and postpress stages. Detailed information on the CIP4 organization can be found at www.cip4.org.
6. Prinect Prepress Interface
The Prepress Interface is the most important interface between the prepress and press stages. Prinect Prepress Interface uses the prepress data to calculate the color presetting data for the press (the zonal ink coverage data) and transfers this together with the ink and paper information to the central control unit of the press, the Prinect CP2000 Center. This prepress data is used to automatically select the characteristics for the press. The software also produces thumbnails of the print product for the CP2000 Center and Image Control.
7. Prinect CP2000 Center
As the central press control unit, the Prinect CP2000 Center combines press operation with remote ink and register control in a single unit. Consequently, it is also where all color presets for print production are made.
7.1 Spectral color measurement
The spectral color measurement process involves light diffraction using a grid instead of the color filters usually found in densitometers. It records the visible spectrum, enabling a very precise conversion into color values. Measuring instruments such as Prinect Axis Control or Prinect Image Control use this method to measure the spectral distribution of light and are able to detect even slight color deviations and automatically rectify these in the press. In order to do so, the data from the spectra has to be converted to the CMYK color space so that the amount of ink applied can be adjusted in the individual inking units.
7.2 Prinect Axis Control
Prinect Axis Control measures the color bar in the individual colors and the composite print spectrophotometrically, thus controlling the amount of ink applied in all inking units at the same time. Setting the print conditions provides the system with data on the press, paper type and inks. This also allows the L*a*b* values and dot gain to be defined. Calibrating the dot gain in the plate manufacturing process also determines the tonal value curves to be achieved. When printing starts, the ink zone settings are already adjusted to the ink flow required to achieve the end densities and also to the CIP4 PPF job data. With these settings and the Prinect Axis Control measuring results, the press reaches the appropriate standard soon after the start of printing. Axis Control can record data for up to eight colors in a single measuring run which means that it can also be used for presses equipped with a perfecting device. On achieving the target values in the color bar, the printer compares the print sheet with the proofs and any improvements required to the ink zone setting and ink flow can be made using the control station's zone controllers.
7.3 Prinect Image Control
Prinect Image Control monitors the print result for the whole of the print image. A scanner moves over the entire print sheet, recording and storing up to 160,000 image points spectrophotometrically. The process normally begins with the target values in the color bar, and any visual adjustments required are made subsequently. Once the printer has obtained the first acceptable print sheet using this method, it is scanned and saved as a reference sheet.
Each additional print sheet the printer takes from the press is scanned using Image Control and compared to the reference sheet. Based on the measurements deviating from the reference values, the operator is automatically told what adjustments are required. These adjustments are sent to the press online via the Prinect CP2000 Center. The press then adjusts the ink zones in all the printing units simultaneously.
It is also possible to use Mini Spots to report deviations resulting from changes in production conditions to the Prinect Profile Toolbox and Prinect Calibration Toolbox. Color profiles and plate calibration can then be adjusted accordingly. Prinect Image Control is the only color measurement system in the world to support spectrophotometric measurement of the entire print image in process.
7.4 Mini Spots
Mini Spots are small measuring elements for monitoring the color process. These are placed randomly on the print sheet by the user and Prinect Image Control automatically carries out spectrophotometric measurements. When defining colors for the Mini Spots, the printer can specify in particular those colors that are very important for the print image. The data measured is recorded and tested using the Quality Monitor. If there are any deviations, plate calibration or the color profiles can be adjusted automatically. This allows the printer to adjust the calibrations during production without having to print special test forms. The measuring data can be used to record print quality.
7.5 Characteristics database
This is part of the Prinect CP2000 Center and can store as many presettings as needed for various paper grades, inks and coloring standards. Instead of having to amend ink presettings manually when changing paper grades, this can be done at the touch of a button. If the Prinect Printready System or Prinect MetaDimension and Prinect Prepress Interface are used, the production parameters for selecting the presetting are automatically transferred to the Prinect CP2000 Center, substantially reducing setup times and material waste.
8. Densitometry or spectrophotometry?
The standard offset printing process currently uses the colors cyan, magenta, yellow and black (CMYK for short), and an ISO standard stipulates the relevant color loci for ink manufacturers. Consequently, print parameters are checked densitometrically by measuring color bars. This involves measuring and evaluating not only the end density, but also the tonal value curve. The dot gain has to be determined based on the screen dots and the characteristics of the three chromatic colors have to be arranged in such a way relative to one another that gray image elements also appear in gray in the print, i.e. the gray balance is correct. In four-color printing, these changes can only be made in the four inking units of the press. Consequently, densitometers have always been used since offset printing was first standardized in the 1970s.
The argument Heidelberg uses to justify spectrophotometric measurement rather than densitometric to control the four inks is that this method can also be used to measure special colors too and to control them using a patented colorimetric process without recourse to density measurements. Whereas densitometric measurement and control concentrate solely on the thickness of ink on the paper, the objective of colorimetry is to minimize the visible variation in coloring. And given that all offset printers use special colors to varying degrees, the spectrophotometric method Heidelberg has opted for is accepted as cutting-edge technology. Moreover, this is the only technology that can be used for Color Management measurements and accurately achieves the full tone values stipulated in the latest version of ISO 12647-2 for offset printing.
Image Control went into series production in 1999. Recognition quickly followed, for example in the form of the GATF Award in the USA in 2000 and, more recently, the Deutscher Drucker magazine's innovation award for the Color Interface option with Mini Spots. Since drupa 2004, Prinect Image Control has been a key component of Prinect Color Solutions. The 1000th system was installed in the Rasch print shop in Bramsche near Osnabrück in mid-November 2005, proving that printers have been thoroughly won over by Image Control.
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