NEW EVALUATION TECHNIQUES FOR SUNSCREENS 583 mental sunscreen products by this solution-dilution method on a Beck- man Model DB spectrophotometer. Disadvantages of this method are: (a) Only the chemical sunscreen and isopropanol-soluble components in the product are extracted from the finished product the insoluble components of the base are left be- hind. (b) Since the physical sunscreen is usually insoluble in the sol- vents used, it is not extracted and therefore cannot be evaluated. Thus, WAVELENGTH rnp Figure 1. Ultraviolet transmission of sunscreen products using solution-dilution method and spectrophotometer. - - - 7t•o p-aminobenzoic acid in clear viscous solution (0.069672 g/1 of isopropanol) -- 5% amyl-p-dimcthylaminobenzoatc -}- 10% talc in opaque cream base (0.12746 g/1 of isopropanol) ..... 10% talc in opaque cream base (0.09929 g/1 of iso- propanol) and --- opaque cream base (0.10914 g/1 of isopropanol) in case of a chemical/physical sunscreen combination, the total sun- screening capacity of the product cannot be determined. (c) The product is evaluated in a condition and concentration entirely different from those used on the skin. (d) The procedure is quantitative but involved and time consuming. THIN FILM TECHNIQUE This study, designed to avoid the disadvantages of the solution-dilu- tion method, has resulted in the development of the Thin Film Tech- nique. it consists essentially of sandwiching a thin film of the sun- screen product between two quartz slides separated by means of spacers of known thickness. The quartz slides* are ground and polished on * Arthur H. Thomas Co., Philadelphia 5, Pa.

584 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS both sides, measure 2.54 X 5.08 cm, and have a thickness of i mm with a tolerance of 0.000-0.001 min. The spacers consist of undistorted 0.0254 ñ 0.00254 mm thick No. 1100-0 alloy aluminum foil* cut into strips 3.2 mm wide X 25.4 mm long. The complete sunscreen product was used as such without any processing. A clean, quartz slide was placed on a hard, flat surface. Two strips of foil spacers were cut care- fully to avoid distortion and placed on the slide at either end. A small amount of product was placed in the center of the slide. Another quartz slide was aligned and placed on this slide carefully, starting from the center and working away from it. Both ends of the slide were then firmly pressed simultaneously. A uniform film of the product showed in the center of the slide sandwich. While pressure was maintained, the slides were then taped t to one another at both ends above the foil loca- tions. This thin film specimen was then ready for spectrophotometric, monochromatic, reflection, and other measurements. A blank was pre- pared in the same manner without the product. In the studies that follow, thin film specimens prepared as above were used. Monochromator Studies A high intensity xenon arc ultraviolet light monochromator system described by Sayre et al. (7) was used in conjunction with an RCA 935 photocell and specially constructed quartz cuvette/quartz slide holder for measurement of the transmission spectra of the sunscreen products. The technique utilized was to measure the output of the source from 250-400 m/• wavelength in 10 m/• increments, with blank quartz slides in the slide holder. Next, quartz slides with the thin film of the sun- screen product sandwiched between, as described above, replaced the blanks in the holder, and the light transmitted was measured by the photocell. The output of the source was again measured with the blank slides in the holder. Any significant change in the intensity of the source would thereby be detected during the experiments the intensity, caused by wandering of the plasma in the arc, was found to vary up to 10•. The values of the light transmitted by the blank and sample are both expressed in volts thus the ratio of sample to blank in volts gives the light transmitted by the sample. Transmission measurements were made for all the experimental sunscreen products. These are shown in Figs. 2 and 3 as "monochromator direct transmissions." * Alcoa Research Laboratories, Foil & Packaging Division, New Kensington, Pa. I Scotch Brand, Minnesota Mining and Manufacturing Co., St. Paul, Minn.