NACREOUS AND iNTERFERENCE PIGMENTS 175 but somewhat higher than that obtained against the black background however, the increase in reflectance at the minimum is indicative of some small diminution in color intensity. At the specular angle, then, the color is relatively independent of background. Diffuse reflectance curves C (black background) and D (white back- ground) at --15ø/45 ø tell a different story. The reflection against black is still blue, although with much lower color intensity than at --15ø/15 ø The reflection against white is now yellow: highest reflectivity is in the yellow-red region, lowest in blue. The light viewed at a nonspecular angle as in this --15ø/45 ø example is reflected from the white back- ground. It is transmitted through the interference platelets twice: first as the incident beam, then as the reflected beam. The color ob- served is thus the transmission color as opposed to the reflection color which is seen at the specular angle. Interference pigments which have no absorption color thus display two colors which are complementary to one another. The two-color effect of the red interference pigment has been utilized in "iridescent" nail enamel. Coated on the curved fingernail, the pig- ment displays a red highlight against a greenish background, and thus duplicates some of the optical characteristics of natural mother-of- pearl. Curves A and B of Fig. 11 establish the similarity of blue specular reflectance against black and white backgrounds. On casual visual observation, however, the color intensity against white always seems •nuch weaker, mainly because the pigment is not observed at the exact specular angle. It is possible to darken an interference pigment, thereby sacrificing the two-color play, to obtain apparent intensification of the reflection color. For example, a yellow-reflecting TiO2-mica which also has a yellow absorption color displays no blue transmission but instead has a more readily seen golden reflection. Such pigments are the "bril- liant gold" and "dark gold" TiO2-mica pigments. In Fig. 12, an inter- ference gold which has no absorption color is compared with one which has absorption color, at --15ø/15 ø against black and at --15ø/60 ø against white. The two pigments look very much the same in specular reflection (A-S without absorption color, B-S with absorption color). However, they differ markedly in diffuse reflection which reveals the transmission color: the pigment without absorption (A-T) is blue- purple, the pigment with absorption (B-T) is yellow. Only the first displays striking goniochromaticity.

176 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 8 4•0 550 650 W,•,VELENGTH (nm) Figure 12. Spectrophotometric curves for gold interference pigments. A, without ab- sorption color B, with absorption color S, specular reflection T, transmission I/v = -15ø/15 ø PIGMENT CONCENTRATION 450 550 650 WAVELENGTH (nm) Figure 13. Spectrophotometric curves at specular reflection of two gold interference pigments The Trilac goniospectrophotometer has served to demonstrate the essential characteristics of interference pigments. The curves can also be used to determine relative quality of pigments of similar color. In Fig. 13, A and B are, respectively, the yellow-reflecting pigments of Figs. 9 and 12 as compared by specular reflection. A has greater color inten- sity, as seen by the greater sharpness of the minimum and the higher slope. It has a superior luster, as evident from the higher reflectance at the highest part of the curve. It is slightly more orange because its minimum is at a slightly higher wavelength, i.e., displayed in the direc- tion of the red reflecting pigment of Fig. 9. The two pigments could be compared further in diffuse reflectance, in the same manner as for "white" nacreous pigments in the preceding section. The goniospec- trophotometric measurements thus may be used for comparing pigments of similar reflection color as well as for exhibiting overall interference pigment charactertistics. SUMMARY The optical characteristics of "white"- or "pearl"-reflecting nacreous pigments have been demonstrated in terms of specular reflection, diffuse