Spectral Gamut Characteristics based on Number of Primaries and Their Characteristics

In applications that require spectral color information, such as fine art printing field, industry has been able to improve reproduction quality substantially in the past decade by increasing the printer gamut space and improving other print quality attributes such as metamerism. As the number of inks used in the printer is increasing and researchers are able to understand the ink and media interaction more in detail, attempting to match a color in spectral space has been possible. Similar trend is happening for electronic displays such that now there are cameras and displays with 4 or 6 color channels or filters available. This paper focuses on the following two fundamental question in spectral color reproduction: 1. What is the optimal number of inks, and which types of inks to use? 2. How to create the ink combination that replicates a given spectral reflectance as closely as possible? Introduction One of the main goals in spectral reproduction is to match each input spectrum as closely as possible. Matching a given color in spectral space, rather than in a lower dimensional space such as CIELAB or CIEXYZ, improves metamerism of the print. To make a good spectral match, the printer gamut is expanded by adjusting the chemistry of the inks, and especially by increasing the number of the inks used in the printer. What is a lower bound on the number of inks needed to do a reasonable job in spectral printing? We test whether the spectral matching improves noticeably as we increase the number of inks from 3 to 6 to 9 and finally 12 inks. To make the study more general, two sets of devices are used. In one device, primaries (inks) interact with each other linearly and in the other device non-linearly. The first part of the paper introduces the data and the models that are used in this paper. The second half of the paper compares performance of gamut coverage for different primary characteristics and interaction between the primaries. To compare performance of each device, an exhaustive hierarchical search algorithm is used. Primary Selection Three types of synthetic primaries were also used. The first type of reflectance used was based on square-wave reflectance as shown in Figure 1 where the edges are sharp, and thus resembles sub-sampling of the spectral reflectance. The 3 inks, as shown in Figure 1, cover the visible wavelengths 380 to 730, and are nonoverlapping. The 6 inks were created by subdividing each ink in the 3-ink model into two separate square waves. The set of 9 and 12 non-overlapping inks were created similarly. The white of the print medium was taken to be the ideal white with 100% reflectance at all wavelengths. Figure 1: 3 Square wave synthetic ink reflectances covering 380 to 730 nm