Spot Color Processing

ColorLogic’s spot color solution is aimed at printers whose customers impose stringent demands on the color accurate rendering of spot colors. Packaging printer that process files with many different spot colors looking to optimize their printing process by converting spot colors into CMYK or Multicolor process colors. The ColorLogic solution offers printers the security of getting the best possible conversion of spot colors to process colors, in terms of colorimetry and printability.

Classic methods of spot conversion

To appreciate the special features of the new solution, we first need to take a look back in order to understand how the previous method for converting spot colors used to work in ZePrA and many other solutions: The Convert spot colors to target color space checkbox resolves spot colors (DeviceN colors without CMYK components) and converts them to the target profile. The substitute color (referred to as the alternate color space in the PDF) assigned to each spot color in the PDF file is used in this context and is processed using the color management settings in the Images/Vectors tab in ZePrA. The PDF alternate color value for a spot color is usually indicated in CMYK or Lab, and is specified in the graphic or layout application used to create the PDF (e.g. Adobe Illustrator, InDesign or QuarkXPress).

Given appropriate settings, ZePrA performs DeviceLink conversion from the alternate color space to the target color space. Nevertheless, conversion using the alternate color value is only an inaccurate solution, particularly if the alternate color for the spot color is indicated by device-dependent CMYK values. Even if the alternate color were to be indicated by a measured Lab value, ICC color conversion would usually result in converted colors that are not sufficiently accurate and, above all, not easily printable.

Example: The spot color PANTONE 266 C has an alternate color value of CMYK = 70/81/0/0 in a PDF document. When printed in a high-quality offset process on coated paper in accordance with ISO Coated V2, these CMYK process-color values would produce a dark violet color that is 29.2 Delta E-76 or 8.0 Delta E-2000 off the actual spot color (see screenshot). The color is outside the printable gamut and thus cannot be printed by 4-color offset printing.

On a Multicolor printing system with 7 process colors, which would be capable of wonderfully simulating the spot color, the same spot color with the same PDF alternate color value would likewise only be reproduced with a major color error of approx. 9 Delta E-2000.

Using this example, conversion via a stored CMYK alternate color value will result in highly unsatisfactory color reproduction in both printing processes.

Other types of approaches on the market

Lab alternate color values have been stored for spot colors in the PDF document since the launch of Adobe InDesign CS6. At first glance, Lab alternate color values appear to be a better alternative than CMYK alternate color values, but it creates other, sometimes even more serious problems when converting spot colors.

The screenshots (image) show how differently a spot color can be converted to CMYK with a Lab alternate color value. The spot color PANTONE Cool Gray 7C is highlighted in red in the original file in the screenshot on the left. The screenshot in the middle shows that conversion of this spot color by means of a standard tool leads to four process colors, which is highly unsuitable for printing. In contrast, the screenshot on the bottom right shows that conversion using ZePrA not only achieves the most accurate color reproduction, but also manages with the minimum number of channels (only the process color Black is used). Moreover, the screenshots (image)show that the spot color gradients converted with ZePrA have a far purer color appearance than the gradients converted using the standard tool.

Instead of alternate color values, other color server solutions on the market use a color library that contains the color values of the solid tones of the spot colors, measured in Lab. Rather than using the PDF alternate color value when converting spot colors to the target profile, these solutions take the Lab value of the solid tone from the color library and convert it to the target color space by the absolute colorimetric method. If the gamut of the target profile is large enough to be able to map the spot color, a good colorimetric match with the solid-tone value is obtained.

In our example using PANTONE 266 C, (image) this would be the case with our Multicolor printing process, and the spot color could be reproduced quite well with 1.9 Delta E-2000 (the screenshot on the right shows the color conversion that would be obtained with standard tools).

However, should the gamut of the target profile not be large enough and the spot color be out-of-gamut, only rarely would the best possible color value with the smallest Delta E be calculated, depending on the target profile used.

This is due to the inaccuracy of profiles and the Gamut Mapping used. The biggest drawback of simple colorimetric conversion is, however, that unwanted process color components can emerge (in our Multicolor process, for example, CMYK +Orange+Green+Violet = 3/3/6/0/5/0/87), impairing the quality of the printed image.

Just think of a text or a barcode that would thus be composed of several colors and be unsharp as a result of register problems when printed. At the same time, problems can arise when overprinting what were previously spot colors with process colors or other spot colors. Furthermore, a library that contains merely the measured solid tones permits only an incomplete prediction regarding the appearance of graduations of the spot colors. So, this approach is bound to fail and may, from the printing point of view, produce poorer results than converting the alternate color value by means of DeviceLink profiles, as previously done in ZePrA.

ZePrA’s Spot Color module features

Many users would like exact colorimetric conversion of spot colors. This results in a need for both, for color libraries with the correct, measured spectral color values and also for additional Delta E minimization. Delta E minimization would even make it possible to reproduce our specimen color PANTONE 266 C exactly and without a color error in the 7C Multicolor printing process (see screenshot below). (image)

Users would like to create and use several libraries, if necessary, for the same spot color because they need separate and optimized color values for different substrates and printing processes.

Similarly, the color libraries must work with measured graduations of the spot colors, so that the 50% value of a spot color, for example, can also be reproduced optimally.

To accurately calculate overprinting simulations, the measured values should additionally be stored in spectral form. Additionally, an intelligent, spectral color mixing model should be used.

It is also desirable if the color server is able to preserve overprinting properties to the greatest possible extent.

When dealing with elements consisting of a mixture of spot colors and other process colors, the color appearance should be simulated as accurately as possible following resolution of the spot color.

When converting to the target color space, the fewest possible process colors should be used, but the result should still yield the smallest possible visual color difference. In our example, the optimum solution is to use only the Violet process color (see screenshot below) and achieve a color error of just 0.8 Delta E-2000.

Some spot colors need to be converted to a specified target value (e.g. to a process color), regardless of the smallest possible Delta E. This is necessary for texts or barcodes, for example.

Similarly, some spot colors have to be excluded from conversion, e.g. cutting marks created as spot colors, varnishes or braille.

Application of Photoshop color corrections to PDF files

Colorimetrically generated DeviceLink profiles cannot be used in some applications. This is the case if, for example, a customer provides printing data and, instead of a correct proof, a print that needs to be matched as accurately as possible in production printing. If you open the printing data on a monitor with soft proofing based on the printing standard for production printing, there may be pronounced differences compared to the print provided by the customer.

Options for solving the problem: either to print outside the standard and try to adjust the specifications by using the gradation corrections in ZePrA, adjustments on the printing device or to apply Photoshop corrections to the PDF file.

Photoshop approach: has the advantage of applying standards to proof the color corrected data and have them approved by the customer. This method requires an person with Photoshop experience and CoPrA’s Edit module.(link)

The demo version of the programs allows a single test run with production data. The operator compiles color relevant objects, taken from the PDF data to be corrected in a file in Photoshop and combines the test data with the CoPrA Edit Chart. Then correct the test file on the monitor under soft proofing conditions. The edited image is loaded into CoPrA and the corrections are saved as a DeviceLink profile.

The DeviceLink profile can subsequently be used in ZePrA to correct the colors of the original PDF data. If necessary, these data can be approved by the customer on a proof before printing.

The printer can work according to a standard in the accustomed manner and reliably match the proof.

The individual steps for converting Photoshop corrections into DeviceLink profiles are described in the tutorial manual (link) for CoPrA.