Practical challenges when printing in extended color space (EXG).
Printing in the extended color space is currently a hotly debated and strongly marketed subject. Various manufacturers of printing inks, ink systems, software and printing presses offer solutions. But what about practical implementation? What opportunities, limitations and challenges does a print shop face when printing in the extended color space? What conditions must prevail for such a system to provide the expected benefits in practice? Can it even be used as a comprehensive substitute for spot color printing?
These are the questions this article will deal with.
DAPF TECH is involved in the practical implementation of multi-color processes in printing plants as part of its services, in addition to process optimization and increasing the performance of printing presses. In the past, DAPF TECH has successfully supported several users in the working practical implementation of Expanded Gamut Offset Printing.
Most of the readers of this article are already everyday users of “Expanded Gamut Systems” without noticing it. Namely, as is to be found in almost every print shop during proofing. Inkjet printers that cover a large color space, such as photo printers for home use, professional proof printers with advanced raster image processors or digital printing machines for commercial use. All of these systems normally work with highly saturated additional colors whose purpose is to supplement the saturation and purity of the mixed colors of cyan, magenta and yellow and thus enable a larger color space. In addition, light variants of some process colors are used. However, their purpose is not to extend the color space, but to extend the dynamic range of tonal gradations, which is particularly important in photo printing.
Many typical special colors from color palettes such as Pantone, HKS or RAL as well as some brand colors are outside the classic CMY+K spectrum. The color space-expanding colors allow your proof printer to simulate a large number of spot colors, as these fall within the extended color space.
In digital printing, there are no special requirements regarding application and process control, regardless of the number of process inks. The systems are coordinated in terms of hardware, drivers and available inks.
In offset or flexo printing, on the other hand, things are quite different. There is no fixed set of coordinated additional colors, no uniform data preparation procedure and no uniform process control strategy. In addition, there are some typical process challenges that need to be mastered for practical use.
The expected benefit of Expanded Gamut Printing:
From Expanded Gamut printing we expect the following:
- Replacement of spot colors in favor of simulating process color printing
- Reduced color changes including the necessary washing processes before and after spot color printing
- Time and cost savings
Expanded Gamut Printing in offset printing. A great solution or an unsolvable problem?
Implementation in practice:
1. An expanded gamut process in industrial mass printing requires a suitable printing press with at least 7 printing units.
2. The prepress department must be able to prepare the print data reliably.
3. the printing machine must print in a reproducible and consistent manner
Requirements for the printing press:
A printing machine with 7 printing units is recommended in order to be able to print in the Expanded Gamut process. CMYK+GOV or CMYK+RGB are typical scenarios. An expanded gamut process can also be realized with less than 7 printing units, but in this case, either the expansion into the red, green or blue range must be waived.
The press must also be well prepared in terms of process technology and have an automated color control system. Depending on the separation of the print data, instabilities in the printing process may have a drastic effect on the combined print.
The greatest challenge is the register-accuracy. Small negative fonts can become illegible at the back corners, especially with larger sheet formats or light grammages. Particularly in pharmaceutical printing, where an EXG process is intended to reduce the frequent makeready for different spot colors and thus save time and money, an impairment of the readability of contents by register offset is not acceptable. In such cases, a sheet inspection system or automatic ejection of defective printouts in finishing is recommended.
In order to take full advantage of the Expanded Gamut color systems, an individual characterization and ICC profiling of the print on the press is required. There are no official standards yet for both separation and color space-expanding inks. This print profile to be created is used for simulation in proofing and for separating the data for the individual printing plates. In order to ensure the compatibility of the profiles and in particular the classic 4-C component contained in the image, CMYK is recommended for the particular substrate on the basis of existing printing standards such as GRACoL or ProcessStandard Offset (PSO).
When creating profiles and especially when separating data, it is important to note that a high level of process reliability on the press can only be achieved if the composite colors of the printed image are rendered from a maximum of 3 process colors + black. Otherwise, there will be uncertainties in color control and process disturbances due to insufficient area coverage values on the individual printing plates. (See Process Control)
For the creation of print data, one needs a suitable RIP or appropriate image processing software. If necessary, plug-ins for industry-standard programs are available. Ideally, color servers with intelligent color space conversion options or device-link technologies are used. The color channels of the output color separations must be clearly named and kept consistent for a reproducible workflow. (See Printing Plate Production)
Printing plate production and process calibration:
Just like the classic CMYK components, the tonal value compensation must also be adjustable for the expanded gamut colors OGV or RGB. The color space expanding colors are treated as special colors, whereby it must be ensured that their tonal value transfer curves can be set individually for a successful process calibration. Modern evaluation methods such as the SCTV method ISO 20654 (Graphic technology – Measurement and calculation of spot colour tonevalue) exist for the evaluation of tone value increases. In the case of Expanded Gamut process color printing, however, we recommend using the classic dot-gain formula. It is not important which formula is used. The only important thing for printing is that the set increments are kept constant.
An automatic color measurement and control system is essential for process control in Expanded Gamut printing. Standard density control is sufficient to check the solid ink densities suitable for the process ink set. Depending on the measuring system used, special requirements may be placed on the print control strip. In order to observe the dot gain in print, the measuring system should have the possibility of displaying the dot gain values.
Color deviations due to process variations and material influences apply to expanded gamut printing as well as to classic 4-C printing. Depending on the separation of the print data, it can be a great challenge for the machine operator to determine which color separation has to be adjusted in which way.
Color adjustments. A decision-making process.
While in classic 4-C printing, an overall color deviation is easily corrected. If the overall image is reddish, this can either to be corrected by reducing yellow and magenta or by adding cyan. In Expanded Gamut printing, decision processes are much more difficult. In order to decide on which printing units a targeted color correction can be performed, the operator or the control system must be clearly aware of the respective separation portion in the image.
A saturated red area in the image can be generated by several valid separations. If a CMYK+OGV set is used, the color deviation can be caused by excessive inking of parts of magenta+yellow, orange+yellow, or exclusively orange. If, due to the print data generation, the system is allowed to use the color space-expanding print color components not only in the highly saturated image areas, but also close to the gray axis, then the color control becomes a challenge for the operator.
According to our current knowledge, only one manufacturer of color control systems currently offers a fully automated and successfully tested color control system for offset expanded gamut printing. (System Brunner InstrumentFlight)
Currently unsolved problems:
- Metamerism. Special colors and in particular prints already produced with real special colors show a certain behavior due to the pigments used when these are viewed under changing light conditions. This particular behavior cannot be taken into account in the Expanded Gamut process in offset and flexo printing. Color conformity with the special color requested by the customer is only guaranteed under standard lighting conditions such as D50. But not with the customer or at the point-of-sale.
- Since the Expanded Gamut process is not standardized in its practical implementation and printing companies’ own methods are very likely, it is practically impossible to pass the print job on to another print service provider on the basis of the print data created in the event of a printing press downtime.
- Expanded Gamut printing only considers the color match between the artwork and the print. The other existing parameters such as surface appearance, register accuracy, the optically effective influence of visible halftone dots and metamerism are not taken into account under purely color considerations. These properties play a significant role in high-quality packaging printing.
These above-mentioned conclusions are the result of a successful implementation of the Expanded Gamut process in offset printing at a Swiss packaging printer in 2015. Until today, in addition to a few simplifications in the necessary color management, there have been only a few developments with regard to standardization or uniform methodology in process control. For prepress in offset and flexo, expanded gamut means a relatively simple path that can be followed with ready-made software solutions.
Only the Swiss company System Brunner currently bridges the gap to automated process control on the press with its InstrumentFlight product.
Expanded Gamut Printing is practically feasible. However, it requires a comprehensive understanding of all parameters involved in the printing process. Rapid implementation is only possible if the necessary operational know-how is built up and the interfaces between print data generation, printing plate production and process control in printing are precisely defined. In order to prevent possible customer complaints in packaging printing, the color management process must take the lighting conditions of the planned presentation condition into account.
Are you interested in implementing Expanded Gamut printing in your company? DAPF TECH and System Brunner are at your disposal as competent and experienced EXG professionals.