Viewing angle, pearlescent pigments

Color effects depend on the viewing angle. Pearlescent pigment platelets split white light into two complementary colors that depend on the platelet thickness. 

The development of the mica-based pigments started with pearlescent colors (Fig. 76 A, TiOz - mica). It was followed by brilliant, mass-tone-colored combination pigments (i.e., mica, Ti02, and another metal oxide) with one color (interference color same as mass tone) or two colors (interference and mass tone different) that depend on composition and viewing angle (Fig. 76 B). In the 1980s further development was made by coating mica particles with transparent layers of iron(III) oxide (Fig. 76 C) [5.222]. 

Pearlescent pigments are used to obtain pearl, iridescent (rainbow), or metallic effects, and in transparent color formulations to obtain brilliance or two-tone color, luster flops and color travel effects (changing with viewing angle). The most important applications are plastics, industrial coatings, printing inks, cosmetics, and automotive paints. 

Nacreous (pearlescent) pigments are thin transparent platelets of high refractive index which partially transmit and partially reflect light. The pearlescent effect is due to specular reflection from the broad surfaces of the transparent platelets, and the parallel orientation of those platelets at various depths within the binder film. Light transmitted through platelets near the film surface is partially reflected by deeper platelets. The dependence of reflection on viewing angle and the sense of depth created by reflection from many layers produces the pearly luster. 

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