Details:
Summary:
The use of Metamerism Index (MI) is quite common in the ink industry. It helps us to predict how likely the formulated spot color will still match with the reference under different illuminants and observer angles. Since it is quite certain that most of the printed products will not be viewed by the target audience using standard D50 lighting, MI helps ink manufactures to prevent potential color mismatch due to differences in viewing condition before issue occurs.
Printing with special formulated spot inks could be quite demanding in terms of cost and labor: a custom order needs to be placed between print provider and ink manufacturer, the ink must be delivered and the color verified with drawdowns, and perhaps most significantly, previous spot color inks must be cleaned from the press unit and reloaded before the job. After the job, the leftover spot color ink must be warehoused for future use. Therefore, it makes great sense to replace specially formulated spot color inks with builds of process colors. Since the gamut of the standard process colors does not include many popular spot colors, it is necessary to “extend” the color gamut by adding a few extra “process colors” on top of existing cyan, magenta, yellow, and black to cover more chroma and hue. Most, if not all, Extended Color Gamut (ECG) solutions are based on custom multichannel ICC profiles that use a look-up-table for converting CMYKOVG (or CMYKOV, CMYKOG, CMYKVG, etc.) data to CIE XYZ or CIE L*a*b* back and forth. The ICC profile works quite well if all parties in the workflow agree on a fixed illuminant and observer condition (CIE D50 illuminant and 2-degree observer color matching function).
Besides flexo, inkjet and digital printing have adopted ECG for a long time, where extra separations are added to standard CMYK, such as light cyan, light magenta, light black, orange, green, violet, etc., and they serve the purpose of proofing efficiently. With the proof approved by the print buyer, it will be utilized during the press run to make sure the real production samples match with color expectation under pressroom D50 view booth.
This study compares a set of reference colors with their ECG simulations and show if metamerism will be a potential problem when the retail store’s lighting environment differs from pressroom. This study extends previous work on this topic by including a wider range of illuminants, and by paying close attention to the most popular spot colors.