Coatings that contain effect pigments change appearance as the illuminating and viewing angles change. This phenomenon is called gonioappearance, and the changes in appearance can be very dramatic. While designers love the visual effects achieved by using these effect pigments, the very aspects that make these pigments so visually appealing also make them uniquely difficult to characterize and control. As the development time from concept color to finished product becomes shorter, the challenge to formulate, correct and evaluate gonioapparent pigments grows.
Researchers have found that the angle of illumination is not critical when measuring colors containing only metal-flake and conventional pigments. This has led to the specification of the "aspecular" angle for measuring coatings containing metal flakes. The aspecular angle is the difference between the viewing angle and the specular (mirror-like) reflection angle of the illuminator. Figure 1 illustrates the aspecular angle concept. The first generation of multiangle spectrophotometers was developed to accommodate this specification. Some of these instruments had a single illuminating angle (45°) and from three to five aspecular viewing angles. Others had from three to five illuminating angles with a single viewing angle (0°).