SUBSTRATE TO MEASURE SPF 131 {EE(k)e(k)/[•E(k)e(k)/PF(k)]2} 2. {E[E(X)E(X). AP(k)/PF(k)2] 2} The variance calculated by this expression assumes no error in either E(k) or E(k). RESULTS The monochromatic protection factors at 5-nm intervals for each of the products, to- gether with the SPF calculated from equation 2, are given in Table III. Also shown in this table are the manufacturers' SPFs determined by in vivo phototesting. DISCUSSION The results (Table III) show that in every case there is close agreement between the in vitro SPF determined by the present method and the published SPF. The SPF deter- mined for product B (DIN reference formulation) was in excellent agreement with the published value of 3.7 despite the fact that the DIN methodology requires the use of an Osram Vitalux lamp (mercury arc lamp) whose ultraviolet spectrum differs significantly from that of sunlight. However, Sayre and Agin (17) showed that for this particular sunscreen, a Vitalux lamp does, in fact, yield an SPF very similar to that obtained with a filtered xenon arc lamp. The low coefficient of variation on the SPFs reflects the good reproducibility of the method the average standard error of the mean SPF of all products tested, expressed as a percentage, was 3%. In well-controlled in vivo studies it is expected (10, 18) that the standard error of the mean SPF should not be greater than 5 to 10% of the SPF. Clearly, the in vitro method described here yields comparable reproducibility. The instrumentation used here consisted of a high quality double-grating spectroradi- ometer. Equipment of this quality is not necessary for the technique but was used simply because it was available in the authors' laboratory. Stray light levels are not a problem with this methodology, and so a simpler and cheaper single monochromator may be used equally well. Since the technique is based upon ratios, absolute calibration of the monochromator spectral responsivity is not required. The spectral transmission approach used here is fundamentally more versatile than techniques that utilize a single detector such as a Berger meter (7), since it allows the monochromatic protection factors to be combined with any chosen source spectrum [E(?O] and photobiological action spectrum [•(2•)], which may be relevant in selecting suitable sunscreens for patients with photodermatoses, for example. All the products tested consisted of lotions and light creams. It was found that the substrate was inappropriate for testing sunscreens in either oil or alcohol vehicles due to absorption of these products into the tape. Similarly, we make no claims as to the value of the substrate in assessing the ability of different sunscreen products to with- stand the stress of sweating and water immersion. This property, sometimes called the "substantivity," is related to the physicochemical processes of diffusion and adhesion and depends upon how the products remain absorbed or chemically conjugated with the proteins of the stratum corneum. We conclude that Transpore tape is a useful medium for rapid screening of sunscreen photoprotection. Unlike either mouse or human epidermis, it requires no preparation,

132 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Monochromatic and Sun Protection Factors Obtained for Each Product Sunscreen product (application thickness, I.tl/cm 2, in parentheses) Wavelength nm A (2.0) B (1.5) C (2.0) D (2.0) E (2.0) F (2.0) G (2.0) 290 4.0 -+ 0 4.5 + 0.9 23.8 -+ 4.3 37.7 -+ 3.5 16.8 -+ 3.5 37.0 -+ 6.9 29.6 -+ 5.3 295 4.8 -+ 0.1 4.7 -+ 0.9 24.6 -+ 4.1 38.3 + 4.8 17.3 + 3.8 39.5 -+ 7.1 32.0 + 6.2 300 5.5 + 0.1 4.7 -+ 0.9 24.7 -+ 3.9 39.4 -+ 4.8 17.5 -+ 3.8 40.9 + 7.5 34.0 -+ 6.5 305 6.2 + 0.2 4.8 _+ 0.9 23.9 + 3.8 39.0 + 4.9 17.4 + 3.8 40.9 + 7.4 34.8 + 6.9 310 6.1 + 0.2 4.8 + 0.9 24.1 _+ 3.8 38.0 ___ 4.2 17.9 --- 4.1 41.5 ___ 7.5 33.1 -+ 6.5 315 5.8 --- 0.2 4.8 ___ 0.8 20.8 + 3.7 30.9 + 3.5 16.6 --- 3.5 33.3 + 4.0 24.1 _+ 3.8 320 4.8 + 0.1 4.0 + 0.7 16.6 --- 3.3 24.7 ___ 2.0 15.0 _+ 2.9 23.3 + 1.9 14.7 + 2.0 325 3.5 -+ 0.1 3.3 + 0.5 11.2 ___ 2.7 18.7 ___ 1.6 12.1 + 2.2 13.2 _+ 1.3 9.9 --- 1.1 330 2.4 + 0.1 2.5 _+ 0.3 6.9 + 1.7 14.3 --- 1.0 9.3 --- 1.4 6.3 _+ 0.7 7.2 + 0.8 335 1.8 ___ 0 1.9 + 0.2 4.2 _+ 1.0 10.6 ___ 0.8 6.9 + 0.7 3.7 + 0.3 5.5 -+ 0.5 340 1.5 + 0 1.5 --- 0.1 2.9 -+ 0.6 8.2 + 0.6 5.0 --- 0.4 2.8 + 0.2 4.5 -+ 0.4 345 1.4 + 0 1.4 + 0.1 2.3 + 0.4 6.4 + 0.5 4.0 + 0.3 2.5 + 0.1 3.7 + 0.3 350 1.4 + 0 1.3 + 0 1.9 --- 0.3 5.0 + 0.4 3.2 + 0.3 2.4 + 0.1 2.9 + 0.2 355 1.4 _+ 0 1.2 _+ 0 1.6 + 0.2 3.8 --- 0.3 2.7 --- 0.3 2.3 -+ 0.1 2.3 --- 0.1 360 1.4 ___ 0 1.1 _+ 0 1.4 _+ 0.2 3.0 --- 0.2 2.4 ___ 0.3 2.3 + 0.1 1.8 + 0.1 365 1.3 --- 0 1.1 _+ 0 1.3 -+ 0. l 2.4 + 0.1 2.2 --- 0.3 2.4 ___ 0.1 1.5 -+ 0 370 1.3 + 0 1.1 _+ 0 1.2 + 0.1 2.0 + 0.1 2.0 -+ 0.2 2.4 + 0.1 1.2 -+ 0 375 1.3 -+ 0 1.1 + 0 1.2 + 0.1 1.8 _+ 0.1 1.9 + 0.2 2.4 -+ 0.1 1.1 -+ 0 380 1.3 + 0 1.1 _+ 0 1.1 + 0.1 1.6 _+ 0.1 1.8 + 0.2 2.3 -+ 0.1 1.0 -+ 0 385 1.3 -+ 0 1.1 + 0 1.1 + 0.1 1.6 + 0.1 1.8 -+ 0.2 2.0 _+ 0 1.0 -+ 0.1 390 1.2 ___ 0 1.1 _+ 0 1.1 + 0.1 1.5 ___ 0.1 1.7 + 0.2 2.0 + 0 1.0 + 0.1 395 1.2 -+ 0 1.1 + 0 1.1 _+ 0.1 1.4 ___ 0.1 1.6 --- 0.2 1.9 + 0 1.0 _+ 0.1 400 1.2 + 0 1.1 ___ 0 1.1 --- 0.1 1.4 + 0.1 1.6 ___ 0.2 1.9 --- 0 1.0 _+ 0.1 SPF 4.4 ___ 0.0 3.7 + 0.2 10.6 ___ 0.4 18.6 _+ 0.5 11.7 + 0.7 15.9 --- 0.5 12.5 + 0.4 Published SPF 4.11 3.7 8 19 10 15 15 Wavelength nm H (2.0) I (2.0) J (1.5) K (1.5) L (1.5) M (1.5) N (1.5) 290 62.2 --- 23.1 18.5 + 9.3 7.5 --- 0.8 17.5 -+ 4.6 5.0 + 1.2 9.6 --- 2.9 48.8 + 11.4 295 65.8 + 23.8 21.0 --- 10.9 8.3 --- 1.0 18.7 _+ 4.4 5.0 + 1.2 9.4 --- 2.8 48.2 --- 10.5 300 66.6 + 23.4 22.1 ___ 11.6 8.8 --- 1.0 19.9 + 4.3 4.9 -+ 1.1 9.3 --- 2.6 47.6 --- 10.5 305 67.1 + 23.3 22.4 _+ 12.0 9.1 + 1.1 20.7 --- 4.6 4.7 + 1.0 9.1 --- 2.4 46.2 --- 10.3 310 67.9 --- 22.7 19.7 -+ 10.4 9.0 + 1.1 20.2 --- 4.5 4.5 + 1.0 8.4 + 2.2 42.7 + 8.5 315 61.0 + 17.6 15.6 --- 7.7 7.1 -+ 0.7 15.9 + 4.2 4.2 --- 0.9 8.0 + 2.0 36.4 -+ 6.3 320 51.3 --- 12.6 11.9 --- 5.2 5.5 + 0.5 12.4 + 3.8 4.0 --- 0.8 7.2 + 1.7 30.8 -+ 4.9 325 39.7 -+ 8.9 6.8 --- 2.5 4.3 + 0.4 9.6 -+ 2.7 3.7 + 0.7 6.4 ___ 1.5 26.2 + 3.3 330 27.8 + 5.7 2.7 --- 0.4 3.5 --- 0.2 7.5 -+ 2.1 3.4 -+ 0.6 5.8 --- 1.3 22.2 + 2.4 335 18.5 --- 3.6 1.8 _+ 0.1 3.0 + 0.2 6.7 --- 1.8 3.1 --- 0.6 5.2 -+ 1.2 18.5 + 1.8 340 12.4 --- 2.2 1.5 -+ 0.1 2.8 -+ 0.2 6.5 --- 1.6 2.8 + 0.5 4.5 + 1.0 15.0 + 1.3 345 8.6 + 1.3 1.4 ___ 0.1 2.8 -+ 0.2 6.8 + 1.7 2.6 --- 0.4 4.0 + 0.8 12.2 -+ 1.0 350 6.0 + 0.7 1.4 + 0.1 2.8 + 0.2 7.0 + 1.7 2.3 -+ 0.3 3.5 -+ 0.7 9.9 -+ 0.7 355 4.5 -+ 0.4 1.3 -+ 0.1 2.9 + 0.2 7.2 _+ 1.8 2.2 -+ 0.2 3.2 -+ 0.6 8.3 + 0.8 360 3.4 -+ 0.3 1.3 -+ 0.1 2.9 -+ 0.2 7.2 + 1.8 2.0 _+ 0.2 3.0 -+ 0.5 6.9 + 0.7 365 2.7 -+ 0.3 1.3 _+ 0.1 2.8 + 0.2 6.9 -+ 1.7 1.9 -+ 0.2 2.7 + 0.4 5.8 + 0.7 370 2.2 + 0.3 1.3 + 0.1 2.6 _+ 0.1 6.1 _+ 1.5 1.8 -+ 0.1 2.5 + 0.4 5.1 _+ 0.5 375 1.9 -+ 0.3 1.2 + 0.1 2.4 _+ 0.1 5.6 _+ 1.4 1.7 -+ 0.1 2.3 + 0.3 4.4 _+ 0.5 380 1.7 -+ 0.3 1.2 _+ 0.1 2.3 + 0.1 5.1 + 1.2 1.6 _+ 0.1 2.2 + 0.2 3.7 + 0.4 385 1.5 + 0.3 1.2 _+ 0.1 1.9 + 0.1 4.1 m 1.0 1.6 + 0.1 2.0 + 0.2 3.2 -+ 0.4 390 1.5 -+ 0.2 1.2 + 0 1.4 -+ 0 2.8 -+ 0.6 1.5 -+ 0.1 1.9 -+ 0.2 2.9 -+ 0.3 395 1.4 + 0.2 1.2 + 0 1.2 + 0 2.0 -+ 0.3 1.4 -+ 0.1 1.8 _+ 0.1 2.6 -+ 0.3 400 1.3 -+ 0.2 1.2 + 0 1.1 + 0 1.6 + 0.2 1.4 -+ 0 1.8 -+ 0.1 2.4 -+ 0.2 SPF 25.7 -+ 1.5 8.4 _+ 0.8 6.6 -+ 0.2 14.7 + 1.1 4.1 -+ 0.3 7.3 -+ 0.7 28.3 -+ 1.6 Published SPF 23 10 6 15 4 10 20