Optimizing Chroma for Expanded Color Gamut

Liam O’Hara, Bobby Congdon, Kariahlyn Lindsey, and Emily Kalshoven, Clemson University

The use of Orange, Green and Violet inks to augment and expand the reproducible color gamut of CMYK process color sets has been adopted widely in the packaging industry, but the movement towards standardized processes for expanded color gamut (ECG) has only recently gained momentum. To this end, the Flexographic Image Reproduction Specifications and Tolerances document, FIRST 5.0, has specified a base set of pigments and the preferred hue angles for Orange, Green and Violet inks. However, while the hue angles for these inks have been specified, the recommended chroma and lightness of these inks have yet to be established.  In this paper, the authors conduct a series of characterizations for a FIRST-compliant ECG UV ink set staged with varying pigment letdowns and anilox configurations with the goal of optimizing the chroma (and correspondingly, the lightness) for these inks with the goal of creating the largest possible gamut of reproducible colors.

This work was conducted on a seven-color OMET Varyflex flexographic printing press using single-pigment UV inks on coated paper. Three sets of press runs were conducted to attempt to determine a recommended chroma for the OGV inks employed with CMYK. The first of these studies tested each of the OGV inks printed via a banded roll to achieve six different ink film thicknesses in conjunction with CMYK (OMYK, GCYK, VCMK). In addition, each of the inks was let down twice so that the inks were run 100% and approximately 90% and 80% concentration along with unaltered CMYK inks. Chromas ranging from 64.3–100.9 for Orange, 54.9-78.7 for Green, and 61.8–77.4 for Violet were tested using a custom characterization chart designed to fit within six 3-inch bands ranging from 1.03–3.58 bcm.

In order to isolate the pigment concentration from the effects of varying IFTs, a second series of tests were conducted that only employed letdowns of the OGV inks, with chromas ranging from 62.2–102.0 for Orange, 62.1–79.0 for Green, and 56.8–76.8 for Violet, with the CMYK inks held to a constant.

Finally, a third series of experiments was conducted that employed a set of highly pigmented OGV inks and approximately 40% letdowns of the OGV inks in conjunction with a CRPC6-compliant set of CMYK process colors and a set of high-pigment CMYK inks that emulated XCMYK specifications, and the resulting sector gamuts (OMYK, GCYK, VCMK) were analyzed for gamut volume.

Interestingly, while the Orange gamut sector (OMYK) showed a fairly linear relationship of larger gamut volume following increased orange ink chroma, an inverse relationship was indicated for Green and Violet inks. The gamut volume was greater for lower-chroma formulations with both the Green and Violet sectors (GCYK and VCMK, respectively) than it was for higher chromas, with each ink showing a near linear relationship of greater volume following decreased chroma.

The results of the third study employing high- and low-pigmented CMYK and OGV ink sets are currently being analyzed and will be presented at the ATC as well.