NACREOUS AND INTERFERENCE PIGMENTS 171 0.1 o ol - 15'/v VIEWING ANGLE (v*) Figure 7. Directional reflectance of plate pearl essence and BiOCI-II 0.8 • o.6 0.2 -15'/60' g g% p[ plO• 450 550 WAVELENGTH (rim) 6•o Spectrophotometric curves of Figure 8. white background reflected through plate pearl essence and BiOCI-II --15ø/60 ø which is evident in Fig. 7. Figures 6, 7, and 8 demonstrate, respectively, S, D, and T of Fig. 1. They indicate why the two pigments which might appear to be equivalent in nacreous luster because of their similar specular reflectance actually look quite different: diffuse reflec- tance and opacity must be considered as well. The pearl dipping test shows that, of two samples with the same specular reflectance, the greater pearl luster is given by the one with lower diffuse reflectance and lower opacity. These data demonstrate that two chemically different pigments are very unlikely to give the identical nacreous appearance. The character- istics of the reflection are established by refractive index, platelet dimen- sions, and the smoothness of the platelet surface. Different chemical substances will generally differ in refractive index, making a perfect match between two different nacreous pigments impossible. Where given chemical components are involved, as in the two pearl essence sam- ples of Fig. 3 and the BiOC1 and TiO2-mica pairs of Fig. 5, differences in nacreous luster depend on platelet dimensions or platelet smoothness or both. BiOCl-II has a higher refractive index than pearl essence plates, as was noted above. BiOCl-II therefore reflects more of the incident

172 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS light and conversely transmits less. The crystals (average diameter about 15 •) are very fiat bipyramids rather than perfect platelets, and their aver- age thickness is greater than that of pearl essence plates. Hence, maxi- mum reflectance occurs at a higher wavelength. The optical differences between the two pigments can thus be correlated with the properties of the pigment platelets. Which is the "best" nacreous pigment? The single criterion most useful for defining the luster of a nacreous pigment is the R550-concentra- tion curve (Fig. 5), but the differences inherent in the various aspects of the pigment as exhibited in Figs. 6, 7, and 8 all contribute to the optical effect. Accordingly, the choice of a nacreous pigment for a particular cosmetic application lies with the cosmetic stylist. Interference Pigments The titanium dioxide-coated mica pigments make up a series which include "white"-refiecting platelets ("pearl") and color-reflecting plate- lets (interference pigments). The mica platelets generally average about 20 • in length and about 0.3 • in thickness. The color depends on the optical thickness of the TiO2 (anatase) layers: pearl, approxi- mately 140 nm yellow, 210 red or magenta, 265 blue, 330 green, 395. Further increase in TiO,• thickness produces a second yellow and a repetition of the color cycle. Spectrophotometric curves at specular re- flection are given for several colors in Fig. 9. These measurements were made as usual on the black portion of the drawdown card. The reflection curve for yellow has a minimum in the blue portion of the spectrum. The interference films are of such thickness that blue is eliminated from the reflection. The residual components of the white light, which are reflected, create an impression of the complementary color, or yellow, on the eye. It is seen from the curve that the residual light includes violet, yellow, and red. The combination of this spectro- photometric curve with the pigment's directional reflectance has the visual impact of "gold." The light which is eliminated from the reflec- tion is actually transmitted, as will appear below. The red curve shows the reflection which remains after green is eliminated by interference. The residual light consists primarily of violet and red. The curve for blue is the first which has both a minimum and a dis- tinct maximum. The minimum represents the elimination of yellow- orange by interference, the maximum the reinforcement of blue by inter-