NACREOUS AND INTERFERENCE PIGMENTS 167 As for the TiO= curve, it falls below the lacquer blank. In other words, the gloss of the lacquer is diminished by the presence of TiO2 which causes light to be scattered in all directions. The nacreous effect is of course dependent on the concentration of nacreous pigment. Concentration series were run on pearl essence plates, needles, and TiO=. The reflectance at 550 nm is plotted against concentration in Fig. 3. (The center of the visible range was arbi- trarily chosen for comparison of these "white" products.) Reflectance of plates increases up to a concentration of 2.670 crystals and then falls off. A point is reached where increased concentration does not improve specular reflectance for at least two reasons: with increased concentra- tion the smoothness of the surface of the lacquer film is impaired, de- creasing gloss with increased concentration the crystal platelets are more likely to interfere with each other's orientation. The concentration at which these effects occur depends on the particular lacquer being used as a vehicle, since it is a function of concentration in the dry film. The curve for pearl essence plates reaches a maximum relative reflec- tance, Ra.50, of 10.4. In contrast, the pearl essence needles peak at 4.8. In the case of the nonnacreous TiO2, specular reflectance decreases with in- creasing pigment concentration, demonstrating a fundamental difference between nacreous and conventional pigments. Figure 3 also shows that a nacreous pigment of inherently lower luster cannot be used at a higher concentration to duplicate the effects of a pigment o• higher luster. A limiting luster is obtained under given conditions of use.

168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS O.I OOI -15ø/v '•"• 76•o PE Needles '•. '•. lear Lacquer 3•) 40 50 60 70 80 VIEWING ANGLE (v ø) Figure 4. Directional reflectance of pearl essence. Angle of incidence, --15 ø It has been stated that nacreous reflection is directional in character. Figure 4 presents goniophotometric plots for pearl essence plate and pearl essence needle samples which were at or close to optimum concen- trations for specular reflection. Reflection was viewed at varying angles with the angle of incidence kept constant at --15 ø. The clear film and TiO2 samples are once again added for comparison. The first point on each curve is at the viewing angle of 15 ø and represents specular reflec- tance. Diffuse reflectance is measured at all other angles. Each curve is at its maximum at specular reflection. The pearl essence plate sample has the highest specular reflectance, and reflectance falls off rapidly as the viewing angle increases. The less lustrous needle-type pearl essence, on the other hand, has lower specular reflectance than the plate-type and higher diffuse reflectance. Thus, the two curves cross. What is the reason for these relationships? Diffuse reflectance in pearl essence arises mainly from scattering of light by the crystal edges. The total edge effect is very much greater for the narrow needles than for the wider plates. As a consequence, the needles have higher diffuse reflectance and lower specular reflectance, since the scattered light is not available for specular reflection. The curve in Fig. 4 which falls most rapidly is that of the clear lac- quer, an example of a glossy surface with very little diffuseness. The