Color matching is a challenging task that requires communication across many different parties; brand owners, printers and ink companies all need to stay in the loop regarding color management procedures.
Color management is particularly important for international brands that print their brand colors at various plants worldwide and expect all of their suppliers to maintain consistent appearance across every pressrun. Color is usually proofed and monitored under the standard illuminant D50, but it is important to realize that the appearance of printed colors will change dramatically when there is a change in viewing conditions. A label seen under D50 light in the printing plant will not look the same under fluorescent lamps at a retail store or in an incandescently lit living room.
When studying the spectral power distribution curves of each illuminant as shown in Figure 1, this is quite apparent. An incandescent bulb produces a majority of long wavelength emissions, resulting in warm red-yellow light. Fluorescent bulbs, on the other hand, emit more in the blue-green, short wavelengths to produce a cooler light, and both of these commonly used lighting conditions emit very different wavelengths than D50 illuminant.
The colors we perceive are dependent on the wavelengths of light that are being reflected and absorbed by the object we are viewing. This is why a red apple will appear black when held under cyan light (see Figure 2).
Matching & Mixing
While incandescent and fluorescent light are not as dramatic as pure cyan light, they will still visibly change the way printed colors appear to the human eye, relative to D50. This concept is connected to what we call metamerism.
Metamerism occurs when two color samples, appearing identical under one illuminant, appear to be different colors under a second illuminant. In the print world, the root cause for metamerism is often a difference in pigment concentrations and selections between two inks that are meant to be the same color. To minimize metamerism, we need to ensure every printer uses the exact same pigment combination when an ink is mixed to match the brand color of a specific product on a specific substrate.
A challenge is that individual printers have their own methods of matching colors and mixing inks. Many printers use on-site ink technicians to handle their ink mixing, while others order their inks externally from facilities with better access to color specialists. Also, there are many ways to communicate ink color values, including L*a*b* values, Pantone ID numbers, L*C*h* values, CxF/X-4 files, etc.
The CxF/X-4 (Color Exchange File version 4) format is gaining popularity among brand owners because it can be fully integrated in their workflows, starting in the design phase (enveloped inside Adobe Swatch Exchange files) and continuing through production. The CxF/X-4 format is a standard tool that includes L*a*b* values and the spectral reflectance data (400-nm. – 700-nm.) of a color; it allows us to digitally communicate color in a way to achieve a visual match across the entire spectral reflectance curve. This functionality is especially critical during the ink formulation process.
While L*a*b* values represent a snapshot of a color under one lighting condition, the spectral data in a CxF/X-4 file provides the full range of reflectance of the color. When used in selecting an ink formula, CxF/X-4 can help reduce metamerism. The data provided in a CxF/X-4 file allows ink technicians to choose pigments that align with the whole spectrum of reflectance. It is important to recognize that many pigment combinations may pass for one set of L*a*b* values while very few pigment combinations will align closely to the entire spectral curve.
Based on this knowledge, I developed a research project to investigate the implications of using a CxF/X-4 workflow for ink mixing. The objective of the project is to numerically measure any reduction in metamerism as a result of using CxF/X-4 spectral data compared to using solely L*a*b* values during ink specification.
Design of Experiment
Consulting with FTA Hall of Fame Member Steve Smiley of SmileyColor & Associates LLC, I chose eight spot colors from the Pantone 2014 library, some highly saturated and some pastel. I communicated with two ink companies that provided me with two sets of inks each. So the inks would not be mixed in reference to a Pantone formula, I renamed the colors in my study. For the first round of orders, I named the colors with the prefix “CPL” and provided the ink companies solely with L*a*b* values, asking them to achieve the lowest Delta E 2000 possible at D50 illuminant and a 2-degree observer angle with a target application volume of 2.88 bcm (see Table 1).
For the second round of ink specification, I named the colors with the prefix “CPX” and provided the suppliers with the spectral reflectance data in CxF/X-4 files. As shown in Figure 3, the CxF/X-4 file is an XML file that can be opened in any text editor and contains key information used in various software applications to describe the reflectance values at 10-nm. increments.
Each ink company was asked to select a formula with the smoothest and closest match to the spectral reflectance curve, using as few pigments as possible. This can be easily done using X-Rite’s InkFormulation Software, or an equivalent tool, which allows ink technicians to plug in the target spectral curve and choose from a list of possible pigment combinations (see Figure 4).
For both trial groups, I provided the ink companies with samples of the substrate (Fasson 60# Semi-Gloss Elite ITC) and asked them to formulate the color using a D50 illuminant, 2-degree observer angle and specifying a 2.88 bcm anilox. In total this experiment tested 32 inks; comparisons were conducted between groups as well as in relation to the reference color.
Analysis, Findings & Takeaways
For each ink, I conducted a drawdown using a Harper Corporation of America QD proofer and recorded color data collected with a Techkon SpectroDens tool. I extracted the spectral reflectance data between 400-nm. and 700-nm. and used the “Metamerism index function” inside the Techkon device for each sample. To compare the reference color data to the experimental data, I entered all of the results into a spreadsheet and conducted statistical analysis using JMP as well as Excel (see Figures 5a and 5b).
As a result, I found there was lower average deviation from the target curve, for each measured wavelength within the curve, in 13 of 16 CxF/X-4 samples, as well as a higher correlation coefficient in 12 of 16 CxF/X-4 samples, in comparison with the L*a*b* samples.There was a reduction in metamerism in the CxF/X-4 samples relative to the L*a*b* samples, particularly in relation to Illuminant A (incandescent light). Overall, the CxF/X-4 samples were more closely matched to the target color spectral reflectance curves than the L*a*b* samples were.
Color is difficult to replicate without numbers. The CxF/X-4 format is a tool that allows us to communicate these numbers to a standard, facilitating consistency across printers and brand owners and pushing us toward gaining a global consensus on how to define color. Segmenting a color into chunks ensures the entire wavelength spectrum is a visual match, which is the most reliable way to specify the characteristics of a color.
Since the observed data set in this project is fairly small, it would be beneficial to expand the study to further investigate the implications of including CxF/X-4 in the ink formulation workflow. Further, I deliberately chose to keep the ink companies in the dark about the objectives of the study, particularly at the beginning, to keep the ink mixing process as standard to their regular procedures as possible. Deeper interaction and coordination with an ink color specialist would likely result in even closer color matches using CxF/X-4, which would be an interesting strategy to use in continued research on this subject.
There is still a lot more to learn about how to best replicate consistent color and I am excited to have participated in the exploration of this fascinating field of study.
This project was a team effort and I want to take a moment to appreciate everyone involved. I would like to thank my two incredible mentors, Steve Smiley and California Polytechnic State University Professor Malcolm Keif, for spending countless hours working with me on this project. It was a pleasure learning both from you and with you. A big shout out to CGS Publishing Technologies, INX International Ink Co and Sun Chemical Corp for their generous partnership in this study. Your contributions are what helped bring this project to life.
My deepest gratitude goes to Mr. and Mrs. Felice Rossini for generously supporting my vision for this project. I am honored to have been selected as an FFTA Rossini North America Flexographic Research Scholarship recipient and I will forever treasure this experience.
Thank you for taking the time to read about my work! If you have any questions about anything written in this article, please feel free to reach out to me.