Modeling the appearance of special effect pigment coatings

Metallic and pearlescent coatings are becoming increasingly important in automotive, currency, and cosmetic applications. These coatings consist of metallic or dielectric platelets suspended in a binder, and are often applied between a pigmented basecoat, and a transparent topcoat. The scattering properties of these composite coatings vary depending upon the incident and viewing directions, as well as the wavelength. The complex nature of the scattering arises from the competition between multiple scattering sources: front surface reflection from the topcoat, reflection from the platelets, diffuse scattering from the pigmented undercoat, and scattering between platelets. The complex interplay between multiple scattering sources affects the ability to achieve quality control during the preparation of these coatings. In addition, the topcoat surface morphology, the properties of the pigmented basecoat, and the intrinsic properties, concentration, and angular distribution of the platelets influence the final appearance of the coating. In this paper, we will present models for light scattering from front surface facet reflection, subsurface flake reflection, and diffuse subsurface scattering. Experimental scattering measurements on pearlescent coatings show that the polarization can be well described in different geometry regimes by these different scattering sources. The models can be used to extract the slope distribution function of the flakes from the intensity data, but some aspects of the results behave inconsistently. Comparison is also made between experimental and calculated diffuse reflectance spectra. These results are intended to enable improved characterization of special effect coatings necessary for quality control and appearance modeling applications.