Color and Spectral Mixings in Printed Surfaces

Abstract

International audienceThe present paper discusses the concept of subtractive color mixing widely used in color hardcopy applications and shows that a more realistic concept would be " spectral mixing " : the physical description of the coloration of light by printed surfaces comes from the mixing of light components selectively absorbed by inks or dyes during their patch within the printing materials. Some classical reflectance equations for continuous tone and halftone prints are reviewed and considered as spectral mixing laws. The challenge of extending these models to new inkless printing processes based on laser radiation is also addressed. Color mixing is a key-concept in color reproduction, either by painting, printing, or displaying. It refers to the observation that a large panel of colors (the color gamut) can be achieved by varying the amount of a limited set of base colors, called primaries. With light emitting systems, the primaries are light sources, often with red, green and blue color, that are either superposed or juxtaposed with a shorter period than the visual acuity. Since the tristimulus values of the produced colors is a linear, additive combination of the tristimulus values of the three primaries, this type of color mixing has been called additive color mixing. This concept, based on Grassman's additivity law, enabled the color matching experiments at the basis of colorimetry [1]. In opposition to the light emitting systems, paintings and printed hardcopies selectively attenuate the incident white light in different proportions according to the wavelength. Layers of primaries, paints or inks, are coated on a reflecting support and play a role of spectral filtering of light. This type of color mixing is improperly called subtractive color mixing [2], by reference to the fact that part of the incident light is removed by filtering, but the tristimulus values of paint or ink mixtures cannot be obtained by combining the tristimulus values of the primaries; it is therefore not a color mixing in the sense of colorimetry. However, the subtractive color mixing is also related to a physical experience, which consists in producing many colors by mixing nonscattering dyes, usually of cyan, magenta and yellow color. According to the Beer-Lambert-Bouguer law [1], the spectral absorption coefficient of the dye mixture, () K λ , is a linear, additive combi-The final publication is available at http://link.springer.co

Revisited Yule–Nielsen model without fitting of the n parameter

“…However, we experimentally observe that its optimal value, fitted in order to obtain the best agreement between measured and predicted spectral reflectances for a set of patches, is rather close to 2 for transparent films because of the slight light scattering by the inks [16]. When the scattering in the support increases, the n value generally increases too (the Yule-Nielsen effect is stronger): it can reach 10 for certain paper prints [9], and can even tend asymptotically to infinity [3]. It can also take negative values, especially when the ink penetrates deeply into the paper [17][18][19].…”

“…Let us come back to the Yule-Nielsen model, by starting with the physical interpretation of this model that we introduced in Ref. [3]. We noticed that the Yule-Nielsen equation can be formally derived by modelling the reflectance of the halftone print as the result of the reflection by the support and n events of spectral filtering due to the Neugebauer primaries in respect to their respective surface coverage, these filtering events being separated by n scattering events that make light moving from one primary to any other primary.…”

“…Such multi-view effects can be obtained by using metallic inks [1] or a specular support [2] and observing the print in or out of the specular reflection direction. They can also be obtained by double-side printing with classical inks and supports, by observing one face in reflection mode or both faces simultaneously in transmission mode [3][4][5]. For these printing configurations, color management based on digital methods like ICC profile is almost impossible because the number of needed sample measurements, often beyond one thousand for singlemode observation, exponentially increases with the number of observation modes.…”