STEP FILM MODEL FOR SPFs 13 Lambda 16 spectrometer. UV transmission spectra of particulate UV absorbers were recorded with the same instrument using an integration sphere attachment (Labsphere B009-4012). The UV spectra of the oil-soluble filters OMC, OC, OT, BEMT, BMDBM, Oxybenzone, and Padimate O were measured quantitatively in highly diluted ethanolic solutions (concentrations typically between 10 -6 and 10 -5 M) with optical cuvettes of 1 cm optical pathlength. The UV spectrum of PBSA was measured in the same way with bidistilled water as solvent. The UV spectra of these UV absorbers are shown in Figures 1 and 2. MBBT is an organic particulate UV absorber delivered as a 50% dispersion in water. The spectra of MBBT and of TiO 2 were measured quantitatively using concentrations of the UV absorber between 0% and 3% and an optical pathlength of 0.0008 cm (optical cells manufactured by Hellma, Germany). In this case, at each concentration ten measure- ments were carried out and the results were averaged. As shown in Figure 3, the Lambert Beer law is obeyed. The spectra of MBBT and TiO 2 are depicted in Figure 4. All spectra shown in Figures 2-4 are given in terms of molar decadic extinction coefficients. Table I summarizes some characteristic features of the spectra, including the extinctions El, 1 at the wavelength X ..... calculated for an optical pathlength of 1 cm and a concentration of 1% (weight per volume). 120000 90000 60000 30000 • OMC --o-- PBSA • MBC • Padimate O --m-- OT 300 320 340 360 380 400 Wavelength / nm Figure 1. UV spectra of some UVB absorbers: PBSA measured with distilled water as solvent OMC, MBC, Padimate O, and OT with ethanol as solvent.

14 JOURNAL OF COSMETIC SCIENCE 50000 E 40000 30000 20000 10000 • BMDBM -0- OC • BEMT -•- Oxybenzone i i i 300 320 340 360 380 400 Wavelength / nm Figure 2. UV spectra of UV absorbers with different spectral characteristics, all measured with ethanol as solvent. SPF DETERMINATION The i, vivo SPF of the sunscreen formulations used in this study were determined according to the European protocol (12). In some cases instead of ten volunteers the studies were carried out with only five volunteers (as indicated in Table I). The formu- lations used in the present study were all of the o/w-type (see Appendix II). In order to increase the significance of this study, data from references 13 and 14 were also taken into consideration. In all cases the amount of sunscreen formulation applied is 2 mg/cm 2, which equals in terms of volume approximately 2 pl/cm 2. This in turn corresponds to an average film thickness of 20 pm. The i, vivo data of reference 13 were measured according to the COLIPA protocol, which implies the use of ten subjects and a confi- dence interval smaller than +20%. The i, vivo data of reference 14 were obtained according to the FDA protocol with a reduced number of volunteers (eight subjects instead of 20). With eight subjects the confidence interval in most cases is smaller than _+25%. SPF STANDARDS As mentioned in the introduction, the step film model was calibrated with the COLIPA standard sunscreen formulations P1 and P3 and a third standard, P4, of higher SPF. The UV absorber contents and i. vivo SPFs of these formulations are: