TRISTIMULUS COLOR MEASUREMENTS IN FADING STUDIES i oo - RED 8o U 6O •' 4o ,.j 1• •,o CONTROL ./ ...... 28 ,•?' .......... 122 //5?' L ........ 252 , -- I I I I I I I I I I I I I I I :•80 460 540 620 700 WAVELENGTH (MILLIMICRONS) Figure 11. Reflectance measurements of compacts colored with a red aluminum lake. The numerals refer to the number of Langley units of direct sunlight cxposure for each compact 3,81 arbitrary basis, that an apparent 50% fade involves the destruction of 50% of the colorant. It makes the further assumption that the fading is on-hue when the course of the color change is followed at only a single wavelength. Neither of these assumptions may prove to be valid in some studies. Furthermore, even when the assumptions are valid the equation may be linear only over a small range. The third technique for following the change in color utilizes one of the color difference formulas. Of these the MacAdam color difference units appears to be one of the better ones. Mudd (4) stated that color- ant stability can be evaluated either by determining how long it takes to alter a color to a predetermined point or by subjecting samples to definite periods of exposure and measuring the color changes. Using these concepts it is possible to calculate the change in color, zXe/Langley unit, as shown by the figures in the second column next to

382 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ioo 8o 2o YELLOW ONTROL _ /! ......... ! I I i i i i i i i i i i i I I i •0 460 •40 620 700 WAVELENGTH (MILLIMICRONS) Figure 12. Reflectance measurements of compacts colored with a yellow aluminum lake. The numerals refer to the number of Langley units of direct sunlight exposure for each compact the colorant designation in Table III. In the third column are the changes in the K/S values/Langley units. The figures in the last column represent the changes in absorbancy units/Langley units. These figures illustrate that qualitatively all three methods rank the colorant stabilities in the same order. This is due to the fact that all of them were, essentially, on hue and also because the stability of these color- ants was so dissimilar that a method would have to be grossly in error to cause a rank inversion. It should be noted that the Ae and K/S show 3D4 and 5D5 in about the same ratio to each other, while the absorbancy value yields a figure double that. Frequently it may be necessary to find the most stable colorant from a series in which the colors are not the same. The ranking of colorant stabilities has been reported as such in the past. Figure 11 shows the curves of red-colored tablets, prepared with an aluminum lake (in a manner similar to those of the green tablets). Figure 12 illustrates a