Visual Appearance of Printed Special Effect Colors

This paper investigates the visual appearance of printed special effect colors that become increasingly popular in high-quality printing. Since traditional colorimetry is insufficient to assess the unique visual appearance of such prints, improved perceptionbased measures and tolerances are necessary for process control and quality assurance. A prerequisite for developing such measures is a transformation of measurable physical quantities (e.g. Bidirectional Reflectance Distribution Function or Bidirectional Texture Function) into a space of relevant appearance attributes. In this study we determine the latent appearance dimensionality of a selected set of printed special effect colors and analyze suitable attributes for describing its appearance. Samples were produced by screen printing employing 22 special effect inks and two paper grades. A subset of 14 samples was selected showing a high variability with respect to visual appearance, material and spectral reflectance. In a first experiment, subjects rated the difference of the visual appearance between samples. In the second experiment, the magnitudes of twelve reasonable appearance attributes were assigned to each sample. Subsequent statistical evaluations include classical multidimensional scaling and correlation analysis. Our results show that the latent dimensionality of the sample set is at most five. The set of appearance attributes for describing the samples includes the color attributes, one attribute for gloss and one for texture. Introduction Today, special effect pigments can be found in coatings, plastics or cosmetics aiming to enhance their visual appearance. They also become increasingly popular in printing products where the pigments are included into the inks [1]. Special effect pigments may induce a geometry-dependent color, a metal to pearlescent gloss, or a clearly visible texture. Due to this unique visual appearance, they are particularly interesting for high-quality printing applications. Special effect pigments are composed of a transparent, flaky substrate covered with one or more thin coating layers [1]. The goniochromatic appearance is caused by thin-layer interference on the strong refracting coating. The distinct specular reflection at the ink surface as well as multiple inter-reflections in the ink layer induce gloss that is similar to gloss observed on metallic surfaces or pearls. The texture is caused by the large particle size of up to 500 μm. Special effect inks are printable by all conventional printing technologies. For special effect colors, traditional colorimetry is insufficient to predict differences and tolerances that correlate well with our perception. Such perceptually-based measures are a prerequisite for controlling the printing process and for quantifying the quality of the resulting printout without tedious visual experiments. For this purpose, an improved appearance-related measure is required. In this paper, we investigated the number and nature of relevant visual appearance attributes of a selected set of printed special effect colors. In future research, we plan to scale these appearance attributes and to relate them with physically measurable optical properties of the printout, e.g. with the Bidirectional Reflectance Distribution Function (BRDF) or Bidirectional Texture Function (BTF). Such a relationship allows us to measure the corresponding appearance attributes and enables further research on perceptual differences of printed special effect colors. Overview of Visual Appearance Research Eugène [2] subdivided a material’s visual appearance into components for color, gloss, texture and translucency. Since translucency can be neglected for printed special effect colors we confine our overview to color, gloss and texture appearance.