Towards a better understanding of the color shift of effect coatings by densely sampled spectral BRDF measurement

Abstract

To completely characterize the color of effect coatings, the spectral BRDF should be measured for a large number of illumination/detection geometries. This is possible with the GEFE, the gonio-spectrophotometer developed at Instituto de Óptica in CSIC (IO-CSIC). Twenty-four effect coating samples were prepared by AkzoNobel, containing metallic and/or interference flake pigments. The spectral BRDF of all samples was measured by GEFE using a normalized procedure under 448 different geometries. The results are presented in two-dimensional a*-b* diagrams and in three-dimensional CIELAB diagrams to show the color travel. Four different descriptors were used to quantify the color travel of these samples. We show that for many samples and geometries, the resulting colors fall outside the color gamut of a typical display. It was found that the reflection data do not vary considerably at reciprocal geometries, allowing a reduction of geometries to be done. Finally, we show that for 3D rendering applications, the reflection data from BRDF can be strongly reduced. For example, we show that using the concept of flake-based parameters it is possible to only use inplane geometries. The resulting rendered images were shown to be accurate enough for rendering on displays