NACREOUS AND INTERFERENCE PIGMENTS 173 io 5 4 // ,, ,..• /,. ,,. \"\/./ /',, • '-• 2 • //• ½•./ ',,, •-'' 4• 550 6• WAVELENGTH F•u•½ 9. Spectrophotome[ri½ ½u•es of TiO•-mi½a interference pJ•]ents at specular reflec- tion ference. Because of this two-part interference, the color is more intense than that of the preceding pigments. The blue-green curve has a maximum but no minimum within the visual range. Manual operation of the Trilac past the recording range of 400-700 nm shows that there are minima at 395 and 712 nm just be- yond the visible on either side. This green has relatively low visual color intensity because of the absence of a minimum in the visual region. These colors are called "first colors" merely to indicate that they are the colors which first appear when films are thickened sufficiently to produce interference. Further increase in thickness of the TiO2 layer leads into "second colors," beginning with a gTeenish-yellow. This film thickness produces the green-yellow curve of Fig. 9, which once more has both a minimum and a maximum. The green-yellow appears much more lustrous to the eye than the blue-green, even though its maximum reflectance is smaller. The reason is that the green-yellow curve peaks at 550 nm, where the eye is most sensitive, and in general parallels the visual sensitivity curve. The curves of Fig. 9 were obtained at --15ø/15 ø, which is the smallest specular angle attainable with the Trilac. A fundamental character- istic of interference colors is that they change with angle of incidence.

174 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS If it were possible for specular reflectance to be measured at an angle of incidence of 0 ø, all the curves of Fig. 9 would be shifted slightly to the right. Johnston, who has studied the dependence of color on angle of il- lumination and viewing in metallic paint films (7), has applied the term "goniochromaticity" to this property (5), and this term is very apt for interference colors. Goniochromaticity is illustrated by curves A and B of Fig. 10 which is devoted to a blue-reflecting TiO.•-mica. Curve A is analogous to those of Fig. 9, i.e., at --15ø/15 ø Curve B is at --45ø/45 ø. The curve for specular reflectance at the higher angle of incidence moves to the left in agreement with interference theory (2, 3), and the reflection color shifts in the direction blue to purple. Note also that curve B at the higher angle of incidence is higher than curve A, in accordance with Fresnel's formulas for the reflection of light which show that specular reflectance increases with increasing angle of incidence. Figure 11 demonstrates the two-color effect of an interference pig- ment. A is once again the --15ø/15 ø curve for the blue-reflecting pig- ment on a black background. B is the same pigment at --15ø/15 ø against the white part of the card. The curve is essentially the same io 8[.. 3% PIGMENT CONCENTRAoiiIIWhite-15ø/45øonCONCENTRATIONPIGMENT5%•o 450 550 650 WAVELENGTH (nrn) Figure 10. Spectrophotometric curves of blue-reflecting interference pigInent at dif- ferent angles of incidence •6 06 o •4 04• 450 550 650 WAVELENGTH (rim) Figure 11. Spectrophotometric curves dem- onstrating reflection color (A, B) and trans- mission color (D) of blue interference pig- In e n t