Angular dependent reflectance spectroscopy of RGBW pigments

Traditional printing relies primarily on subtractive color mixing techniques. In this case, optical color mixing isachieved by one of the established halftoning methods that use Cyan, Magenta, Yellow and Black (CMYK) primaries on a reflective white substrate. The reason behind the subtractive color mixing in printing is the high absorbance of available pigments used in inks. A new type of mica-based pigments that exhibit high reflectivity atRed, Green, Blue and White (RGBW) spectral bands was recently introduced by Merck (SpectravalTM). Printingwith RGBW primaries on black background allows additive color mixing in prints. While offering excellent colordepth, the reflected spectra of such pigments vary with the angles of incidence and observation. As a result,new approaches in modelling the appearance of prints as well as strategies for color separation and halftoningare needed. The prior optical characterization of the reflective inks is an essential first step. For this purpose,we have used SpectravalTM pigments to prepare acrylic based inks, which we applied on glass slides by screenprinting. In this work, we measured the relative spectral bidirectional reflection distribution of Red, Green,Blue and White reflective inks. The measurements were conducted on an experimental set up consisting of agoniometer, spectrometer, and a xenon light source. Based on the measurements, we simulate the reflectancespectra under diffuse illumination and demonstrate ratios of red, green, and blue spectral components for different observation angles of individual inks and their combinations.