Optimizing Chroma and Lightness of OGV Inks for Expanded Color Gamut

摘要

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 6.0, 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 in an attempt to optimize 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 at 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 consistent values. The resulting profiles for the orange, green and violet sectors were evaluated for gamut volume expressed in cubic AE and the number of Pantone colors each could produce.