TRISTIMULUS COLOR MEASUREMENTS IN FADING STUDIES 375 ioo 8o • 60 nr _ •: 40 2o o o4 0.5 0,6 0,7 o 8 o 9 WAVELENGTH, MICRONS SPECTRAL RESPONSE OF LIGHT DEPENDENT RESISTOR (LDR-Ci) Figure 6. Spectral response of light dependent resistor tor. Barium sulfate was used as the white standard, the illuminant was Source C, and the specular component was excluded. In the next several figures are the spectrophotometric curves made of the compacts prior to exposure and then after the indicated number of Langley units of direct sunlight exposure. Figure 7 shows the curves of the white controls. The curves exhibit the very slightest change at the low end, perhaps ultimately fading to- ward yellowness if the test had been carried out for a much longer period of time. Figure 8 contains the curves of the compacts colored with a green in- soluble colorant. These compacts showed very little change during the period in which they were evaluated. The curves attest to the same fact. They show very little change, regardless of what criterion is used. Figure 9 shows the curves of compacts colored with a combination of water-soluble blue and yellow FD&C dyes. The compacts exhibited a continuous fading as well as a loss in saturation as they were exposed to the direct sunlight. The curves show the same things qualitatively. The compacts are getting lighter, as indicated by the greater reflectance there is a slight shift in hue and the tendency of the slopes to become less steep shows a loss in saturation.

376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS i co - WHITE 8o 60 40 20 CONTROL ......... 596 I I I I I I I I I I I I I I I 460 540 620 700 WAVELENGTH (MILLIMICRONS) Figure 7. Reflectance measurements of white con- trols. The numerals refer to the number of Langley units of direct sunlight to which the specimen has been exposed The curves of compacts colored with an aluminum lake are shown in Fig. 10. These compacts appeared to have suffered the greatest degree of color change of this series. The curves indicate essentially the same characteristics as those of the FD&C dye combination. In this particular study the use of color difference formulas obviously is not necessary the pigment is the most stable colorant. However, to illustrate the point, the different methods for following the fading of colorants are used to see what results. Table II contains a summary of the data for this first series of com- pacts. The first column identifies the compacts and the next column the number of Langley units to which each was exposed. Next are the chromaticity coordinates and the Y, or lightness value. Following these are the Z•,e, c, and L values which indicate change in color, chromaticity, and lightness, respectively. These were computed using a color dif- ference computer programmed on the basis of the previously mentioned MacAdam color difference formula.