NEW EVALUATION TECHNIQUES FOR SUNSCREENS ,587 direct spectrophotometric transmission measurement. Figure 4 indi- cates the arrangement of the various components for this measurement. Only light passing straight through the thin film reaches the photocell and is evaluated as transmitted light. This light will be defined as direct transmitted light. Light scattered by the film as shown in Fig. ,5 cannot be measured by this technique moreover, measurements on substances that scatter light are likely to indicate erroneously that significantly less light is passing through the specimen. The second measurement utilized the integrating spheres in con- junction with the spectrophotometer. An integrating sphere is es- sentially a hollow metal sphere, coated internally with barium sulfate, placed in the light beam of both the sample and reference compartment of the Cary spectrophotometer. The sample slide is placed directly in front of the integrating sphere. The light is reflected as it enters the sphere from a mirror to a standard reference surface in the sphere. The standard reference surface was prepared with magnesium oxide, having a reflectivity of essentially unity. Now the light is diffusely reflected about the interior of the sphere by its barium sulfate coated interior until it is reflected through the exit to the photocell. Differences be- tween spheres and the greater amplification of light required are elec- tronically compensated. The forward scattered light from the thin film is collected by the integrating sphere and evaluated by the photo- cell as the total transmission of the film. The arrangement of the components for these measurements is shown in Fig. 6. By replacing the reference surface of the sample compartment sphere with the sample, the reflection or backward scattered light can be measured. Figure 7 illustrates the arrangement for these reflection measurements. All the light, no matter how it is reflected from the surface of the sample, is part of the flux in the sphere and is measured by the photocell. Thus the optical properties of the thin film can be measured or calculated. Comparison of the first and second types of measurements (direct transmission and total transmission) indicated if the film behaves as just a chemical sunscreen (i.e., obeys the Lambert-Beer Law) or if it has both the chemical and physical sunscreen attributes. The forward scattering (FS) measurement indicates the percent total transmitted light. The backward scattering or percent reflection (BS)measurement de- termines the total light reflected by the specimen. The percent ab- sorption (Abs.) of the sunscreen product can then be calculated: Abs.: 100 -- BS -- FS.

588 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Measurements on Thin Film Specimens Figure 3 illustrates the difference between total transmission and the monochromator direct transmission measurements on two physical sunscreen products and a chemicalf physical sunscreen combination product. Figure 2 compares the results for the total transmission, direct transmission, and monochromator direct transmission for a product with only a chemical absorber. All three measurements readily agree in this instance. Hence only the total transmission of the thin film need be •---"• MIRROR J SAMPLE • INTEGRATING SPHERE PHOTOCELt Diffuse scattering with integrating spheres i •.' ' SAMPLE INTEGRATING SPHERE PHOTOCELL Diffuse reflectance with integrating spheres determined, regardless of the type of protection offered by the film, since the major protective value of each sunscreen is determined by its total transmission in the sunburn region of the spectrum. Little varia- tion was obtained in total transmission in a thin film after drying. Ob- viously, if the film separates mechanically, physically, or chemically upon drying, its optical characteristics will change. In general, the total transmission values agree with the direct trans- mission values from the monochromator for each sample. Usually the monochromator values show less transmission than those from the inte- grating spheres total transmission values. This difference was antici- pated since the monochromator with the photocell did not collect all available light from each sample. The most comparable values ob- tained were those in the region where the chemical absorber absorbed