John Seymour is an applied mathematician and color scientist. He is a professor at Clemson University, teaching color science and process control in the Graphic Communication and Honors schools. He has worked as a consultant since 2012 under the name “John the Math Guy”. John currently holds thirty-one US patents, has authored thirty-four technical papers, has presented at thirty-five conferences, and has keynoted at six conferences. He writes a blog which is described as “applied math and color science with a liberal sprinkling of goofy humor.”

Does Extended Color Gamut Printing Have a Metamerism Challenge?

Jinkai Qian,TECHKON USA; John Seymour, John the Math Guy, LLC

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 an 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 manufacture, 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 angle).

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 bythe 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 will compare a set of reference colors with their ECG simulations and show if metamerism will be a potential problem when the commercial 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 color.

Investigation Of The Relationship Between Spectral Smoothness And Color Constancy

John Seymour, John the Math Guy, LLC, Clemson University

When a spot color ink is initially formulated, the ink technician needs to decide between a large number of potential combinations of base inks that could yield the desired color under the desired illuminant. One of the primary criteria for selection is often the number of base inks that are used. A second criteria which is commonly used is the metameric index. This is the color difference as computed between the spectrum of the target color and predicted spectrum of the recipe under consideration.

If the spectrum of the target color was collected from a previously printed product which is already on the store shelves, this is a very reasonable choice. One wants the new product to match the old under whatever illuminant that happens to be in the stores. On the other hand, it is common for the spectrum of the target color to be based on measurements from a color guide, such as Pantone. Matching the printed product against a Pantone guide in a viewing booth in the pressroom is a useful step in color management. A properly formulated ink must provide an acceptable match under the agreed upon illuminant. But there is no practical benefit to being able to match a Pantone color under store lighting.

A rule-of-thumb method for picking from a list of potential recipes is a subjective judgement about the smoothness of the spectrum. This is summarized in the statement that smooth spectra are less likely to have problems with metamerism [1]. The statement itself is not rigorous, since it does not include a mathematical quantification of smooth and it presumes that there is a defined spectrum to assess metamerism against. But it is a reasonable goal. After all, an ink with a perfectly flat spectrum will not change color under different illuminants.

A careful reading of the previous paragraph will reveal a very subtle change of topic from metamerism to color constancy. The latter is defined as the degree to which an individual object changes color between two illuminants. This is the more appropriate measure when the there is not a firm spectrum for the target color –the brand owner wants the color to “look the same” under any illumination.This paper investigates the premise that a suitably defined measure of spectral smoothness can be used as a proxy for color constancy.

This paper will define a measure of color constancy, will define a measure of spectral smoothness, and will use a previously developed database of metamers [2] to test the correlation between the two measures.

Bibliography
[1] Personal communication with Steve Smiley
[2] Seymour, John, Metameric Proclivity, Technical Association of the Graphic Arts, 2020