Optical Principles of Pearlescent and Interference Pigments

In pigments that simulate natural pearl effects, the simplest case is a plateletshaped particle with two phase boundaries Pj and P2 at the upper and lower surfaces of the particles, i.e., a single, thin, transparent layer of a material with a higher refractive index than its surroundings. For small flakes with a thickness of ca. 100 nm, the physical laws of thin, solid, optical films apply. 

Multiple reflection of light on a thin solid film with a high refractive index causes interference effects in the reflected light and in the complementary transmitted light. For the simple case of nearly perpendicular incidence, the intensity of the reflectance (I) depends on the refractive indices (n, Uj), the layer thickness (d), and the wavelength X)  

With given w, and W2 the maximum and minimum intensities of the reflected light - seen as interference colors - can be calculated and agree well with experimental results. Values for the refractive index of the most important materials for pearlescent pigments are shown in Table 7.2. 

In practice, platelet crystals are synthesized with a layer thickness d calculated to produce the desired interference colors (iridescence). Most pearlescent pigments now consist of at least three layers of two materials with different refractive indices. Thin flakes (thickness ca. 500 nm) of a material with a low refractive index (mica, silica, alumina, glass) are coated with a highly refractive metal oxide (Ti 2, FejOj, layer thickness ca. 50-150 nm). This results in particles with four 

Against a black background or in a blend with carbon black, the transmitted light is absorbed and the reflected interference color is seen as the mass tone (i.e., overall color) of the material. In blends of nacreous pigments with absorbing colorants, the particle size of the latter must be well below the scattering limit, i.e., they must be transparent. The nacreous effect or iridescent reflection is otherwise 

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