The How
Once you have the basic scanning of the color charts with the spectrophotometer, it comes time to build the actual color profiles. We are in an industry heavily conditioned by the tools available, as not all the profile-building software, proofing systems, color conversion workflows, etc. have the same features and options. So, you will need to decide the best options in using your available tools for the project. In general, my advice is to build slightly different profiles with the same data using, if needed, different black generation (UCR/GCR), total area coverage or total ink limit (TAC), black ink limit, black starting point, etc., all according again to the options in your tools.
The TAC, black ink limit and type of black generation (UCR/GCR) are among the specs you will need to confirm with the printer even before preparing the layout for fingerprint, since they may be set in the images and files included in this layout. But, there is always some room to play and test other options if you consider what will be needed. Depending on the actual space available on the press for the fingerprint, you may want to add some options to test, like same image with different TAC values, etc.
The slightly different profiles with the same data may be some options with different black generation (see Figure 3):
- Black generation set to “none” (the converted image will have only CMY and no black at all) can be used, for example, in a background where process colors and a spot color are mixed in gradations or any other halftones, so the risk of moire increases if five colors (including K) were to be mixed simultaneously in some areas
- Black generation set to “medium” can be used in standard CMYK full-color images with average tonal and color ranges, like fruits, for example
- Black generation set to “heavy” (or even maximum) can be used in images with a limited tonal and/or color range like, for example, a very light and quite neutral vanilla flower, where the tonal range in the white petals is between 15 percent and 2 percent. Here, the heavy black generation can help maintain the neutrality of the gray balance in the press and, at the same time, avoid having too small dots in CMY by leaving in the black channel all or most of the dots in the neutral areas
The way to work with different black generation profiles on the conversion from the source to target color spaces in the prepress process may vary according to the tools used. If you are planning to use “profile to profile” conversion in Adobe Photoshop, you may want to use different layers with “smart objects” included, as smart objects are not affected by this type of color conversion. By using a custom automation script in Photoshop (or manually), smart objects can be converted back and forth to standard layers and thus use different black generation profiles for different areas of the images, even if they are in the same main layered file.
Once you are sure and committed to making your own printer-specific profiles, you’ll need to schedule time on the press—which is always scarce in my experience.
As a special note, beware of embedded profiles that can have an uncertain result in the printer’s workflow when making the plates, since the recognition and use of embedded profiles depends on the workflow’s settings. It is better and more reliable to use straight actual color conversions of the files to make sure the results obtained in prepress and proofing will be the same when the plates are created and used in the press, with no difference in separations, black generation, small dots (highlights), etc.
The Extra
Another potential use for these custom color profiles is high-accuracy conversion of spot color to process whenever needed with a tight control of Delta E, and final process components of the converted CMYK color (see Figure 4). Using this type of tool, a very close conversion (whenever the target gamut allows) can be achieved by minimizing the Delta E and at the same time controlling the number of separations and dot percentage of the resulting conversion that can later be applied in prepress, either manually in a software like Adobe Illustrator, or in other conversion tools either in a more automatic way or using conversion scripts.
In this case, a custom spot color was measured using a connected spectrophotometer. Then, selecting the type of Delta E to be used (Delta E 2000, in this example), the number of resulting channels or separations (two channels) was manually chosen. The individual components of the two participating channels (cyan and magenta, in this case) can be also manually adjusted and have a live feedback on screen of the approximate visual difference and the exact Delta E to be expected after the conversion. This way, any very small dot percentage of an undesirable component (0.7 percent yellow, for example) can eventually be eliminated and still have a workable conversion within the chosen Delta E range. The result, however, needs to be verified in practice, and for this the high-end proofing system is an invaluable tool, since it has also been calibrated using the color chart scanned from the fingerprint layout, thus being able to predict with high accuracy the color results to be obtained later on the press.
This approach is particularly useful in packaging prepress versus a more generic or automatic conversion that is less controlled, thus producing less predictable results that can include an undesirable small dots percentage of some process channels and/or higher Delta E difference with the original spot color.
The Recap
- Sometimes “printer specific” color profiles from the press may be needed for a particular packaging prepress project, especially if it involves same type of complex graphics being printed in different printers and/or different printing systems. For this, it is necessary to fingerprint (characterize) the press for the particular substrate and printing conditions required by the project
- Never compromise in the quality of the fingerprinting run and beware of the potential “it’s just another test” attitude of the press crew. The later color conversion in prepress depends on the quality and accuracy of the fingerprint
- Slightly different color profiles can be created from the same spectrometric data set obtained from the color charts in the fingerprint. These can be used to make different separations as needed in different areas of the graphics by independently controlling the black generation (UCR/GCR) and even the TAC, black ink limit, etc.
- Special conversion software can be used to convert spot color to process when needed by tightly controlling the Delta E and the results in the separations, avoiding by example small dot percentage in undesirable channels that can affect the quality or evenness of the color reproduction in the press
About the Author: Antonio de Llamas is head of prepress/R&D at Stora Enso, professor of graphic technology and developer of a patented color management system for flexible packaging in flexo and gravure.