]. Soc. Cosmet. Chern., 22, 161-177 (March 4, 1971) An Instrumental Study of the Optical Characteristics of Nacreous and Interference Pigments L. M. GREENSTEIN, Ph.D., and R. A. BOLOMEY, Ph.D. •' Presented September 20-25, 1970, Sixth IFSCC Congress, Barcelona, Spain $ynopsis--NACREOUS or PEARLESCENT PIGMENTS are used to impart pearl luster to many items, including cosmetics. The pigment particles are transparent platelets of high refractive index. Although the most significant property of a nacreous pigment is high specular reflectance, the specific OPTICAL BEHAVIOR of a given pigment is determined by the relative magnitudes of specular reflection, diffuse reflection, and transmission. These factors can be compared by measuring reflectance at various angles of illumination and angles of viewing. INTERFERENCE PIGMENTS are nacreous pigments in which the platelet thickness is such that COLOR is produced by light interference phenomena. Interference color is de- pendent on angle of incidence. Furthermore, interference pigments display two colors, a re- flection color and the complementary transmission color. These characteristics have been studied by means of the Leres Trilac GONIOSPECTROPHOTOMETER. The measure- ments make possible the characterization of nacreous and interference pigments, and their comparison in terms of quality. INTRODUCTION Nacreous or pearlescent pigments have long been used in cos- metics, the first large-scale example being natural pearl essence in pearly nail enamels. With the increased variety ot: nacreous pigments which have become available, they are now widely used in lipsticks, eye makeup, * The Henry L. Mattin Laboratories, The Mearl Corp., Ossining, N. Y. 10562. 161

162 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS face powder, and liquid makeup. The newer nacreous pigments create iridescent and gold as well as frosted effects (1). Nacreous pigments have traditionally been evaluated by visual tech- niques. The first nacreous pigment was natural pearl essence in which the pigment particles are guanine-hyl•oxanthine platelets, and its original use was in the preparation of simulated pearls. The initial test for pearl luster, therefore, consisted in visual comparison of simulated pearls which were prepared simultaneously from the test sample and from standard pearl essence samples. The technique is capable of discerning very small differences between samples and is still in use, especially in the evaluation of the more brilliant nacreous pigments, such as natural pearl essence plates. A drawdown method is very effective with the less brilliant nacreous pigments, such as the bismuth oxychloride and titanium dioxide-coated mica pigments which have had extensive applications in cosmetics during recent years. The sample for test is dispersed in a suitable vehicle, such as nitrocellulose lacquer. Using a film applicator, the dispersion of the test sample and a dispersion of a standard are made into adjacent films on a half-black, half-white card. The TiO2-coated micas include nacreous pigments which are also interference pigments, producing color by means of thin film inter- ference phenomena (2, 3). In the case of TiO2-coated mica, a TiO2 layer on each broad face of the mica platelets is of such thickness that a specific color is reflected and its cmnplement is transmitted. Drawdowns very effectively show small differences in color as well as in nacreous lus- ter, but the advent of color in nacreous pigments made an instrumental method of measurement even more urgent than was the case with the white pearlescent pigments. Nacreous luster is a directional effect. The pigment particles are transparent platelets of high refractive index which behave like small mirrors. They are oriented parallel to one another in multiple planes (Fig. 1). Because the reflections arise from many layers of platelets, the eye receives an impression of shimmer or pearly luster as opposed to the sharp, metallic reflection characteristic of a single reflecting surface. The higher the luster of a specific nacreous pigment, the greater is the specular or mirror-like reflection, S, in Fig. 1. The lower the luster, the greater the diffuse reflection, D, which consists of light scattered in nonspecular directions. Finally, a portion of the light, T, is transmitted through the nacreous pigment film. The visual effect of a nacreous pig-