On our previous post “Do you know how to design for digital“, several comments were made about matching spot colors. Over the next month, we will have a series of posts going up on the following topics related to Spot Color and Substitute Color Processing on Fiery Controllers:
- – Factors that influence spot color accuracy
- – Using Fiery Spot-On to manage and fine tune spot colors
- – Creating and using a new Spot Color group, and capturing spot colors with an ES-1000 spectrophotometer
- – Creating and using substitute colors with Spot-On
Written by Malcolm Crawford, Senior Worldwide Technical Marketing Manager at EFI
But first, what are Spot Colors?
A spot color is a single ink (or colorant) that is traditionally used in offset printing. It is normally comprised of various pigments that when mixed, create a single color that can be used on an offset press. Xerox digital printers use cyan, magenta, yellow, and black (CMYK) inks to reproduce all colors, including spot colors. CMYK printers have a defined gamut (range of printable colors) that when combined with special processing and a profiled and calibrated Fiery controller, can simulate spot colors to the best capabilities of the printer.
Commercial Color Matching Systems, such as those provided by PANTONE®, include spot color palettes in their products that can be used by designers to create color schemes for projects. These palettes include both spot color names and alternate color values that will be used if the printer does not support the specific spot color ink requested. These alternate values are traditionally declared as CMYK that has been tuned to simulating the spot color using an offset press standard on coated papers however, some applications now use red, green blue (RGB) which is tuned to an average viewing monitor. Since Xerox digital presses use only CMYK inks these alternate values would normally be used. This would not be optimal since a digital CMYK press, which uses dry inks and digitally optimized papers, is not the same as an offset press and offset coated stock. This would result in spot colors that are not optimized for the digital press.
In order to ensure the Xerox digital press produces spot colors that are optimal for the printer, the Fiery controller provides look up tables (LUT’s) of most major commercial color matching systems, such as PANTONE. These LUT’s contain the spot color name as a key, and associated true color value (L*a*b*) as intended by the color matching system. L*a*b* color is a way of defining any color visible to the human eye without any limiting factors such as inks, paper and light. When the Fiery controller receives a spot color reference it searches these LUT’s for a corresponding match. If no match is found it will use the alternate CMYK or RGB values that were provided by the application. Alternate CMYK will go through the CMYK path and RGB will go through the RGB path. If a match is found the Fiery generates optimal CMYK values for the Xerox press by rendering the true (L*a*b*) color with the active output profile. The output profile is specific to the actual printing conditions. This method ensures that the true color is rendered with the actual printing conditions of the press delivering the most accurate and optimal color possible.
To illustrate the importance of the output profile consider the example shown to the left. You have two papers from different sources. One paper is a subdued white, slightly cool. The other paper has a faint green cast. Both papers have been profiled on your printer so each profile knows exactly what “paper white” is for each paper and exactly how the dry ink responds in your printing environment. Now we want to print PANTONE 377, which is a green. If I was to print this color using the same output profile on each paper, the greenish paper would be showing more green (dry inks are translucent therefore the paper white is added to the color). This means that we would not see the same green for PANTONE 377 on each different paper. If we used the correct profiles for each paper, as described above, the profile for the greenish paper would determine that less C and Y is required to achieve the correct color. This example has been exaggerated to demonstrate the importance of profiles, however even the slightest differences in paper white, texture, humidity and coating can cause significant shifts in spot colors, as well as other colors and objects. Profiling each paper will improve your overall color accuracy (Fiery Color Profiler Suite is recommended for best results).
Factors that influence spot color accuracy:
By default the Fiery is set to process spot colors using its extensive set of color matching system tables. The user does not need to set anything special, simply print the job. This allows for minimal intervention with an excellent interpretation and rendering of spot colors.
Here is a multi-level approach to improving spot color matching on the Fiery. Each level will cumulatively help improve the accuracy of spot color processing.
Level 1: Calibrate your Fiery regularly. Every Fiery includes one or more calibration methods, which should be used to ensure that the Fiery is in line with the engine. Methods that use an instrument, such as the ES-1000, will provide a more precise calibration. This may not apply to Fiery for iGen3/4, and calibration procedures may differ for engines equipped with an In-line Spectrophotometer (ILS)
Level 2: Ensure that the most appropriate output profile is selected when printing a job. The output profile directly influences the specific Xerox press CMYK values generated by the spot color tables (LUT’s). All Fiery’s include factory standard output profiles for a variety of Xerox papers, weights and coatings. Select the profile that best matches your paper.
From the print driver properties or job properties in Command WorkStation5 select [Color > Basic Settings…]. Select the most appropriate output profile from the drop down menu.
Level 3: Build and use your own output profiles. This allows you to characterize your exact printing conditions with your specific paper. Build profiles for each paper you use. You will need profiling software that includes an instrument in order to build output profiles. All Xerox production Fiery’s come standard with Fiery Color Profiler Suite, which includes an ES-1000 spectrophotometer. Fiery Color Profiler Suite can also be purchased as an option from your Xerox reseller.
Level 4: Manually tweak the spot color using Spot-On. Details are in my next post entitled Using Fiery Spot-On to manage and fine tune spot colors (Part 2)