For each set of x and y values in Figure 4, we may compute a power curve fit equation of the form y = ax? (9.3)
Based on measurements of a large number of products, we have arbitrarily defined a sunscreen with a ? value of less than 1.1 as photostable .
The UV source is a Solar Light Company Model 16S solar simulator, equipped with a WG320 UVC-blocking filter, a UG-11 filter that blocks visible and infrared light, and a dichroic mirror. Approximately 90% of the effective radiation is in the UVB range.
We measure spectral irradiance from 290 to 400 nm, at 1 nm intervals, using an Optronic Laboratories Model OL754 spectroradiometer equipped with a 6 in. integrating sphere. First we measure the spectral irradiance of the lamp and the spectral irradiance transmitted by a collagen substrate alone, which are both assumed constant. Then we measure the spectral irradiance transmitted by the substrate with sunscreen, at one-minute intervals.
To obtain effective UV doses, we multiply the spectral irradiance value at each wavelength for the lamp by the appropriate erythemal effectiveness factor (see Fig. 2 above)  and integrate over wavelength and time. Effective doses are expressed in MEDs.
Next, we apply a thin film of the sunscreen formula (approximately 1 mg/cm2 before evaporation) to the substrate and allow it to dry for at least 15 min.
Finally, we irradiate the sunscreen film and measure the effective spectral irradiance transmitted through the sunscreen film and substrate, from 290 to 400 nm, at one-minute intervals. Sunscreen photostability is evaluated by calculating the cumulative effective UV dose in MEDs transmitted by the sunscreen product vs the cumulative effective UV dose applied to the surface of the film, until the transmitted dose reaches one MED.
The value of ? is obtained from the power curve fit equation, and the estimated product SPF is calculated as the applied UV dose in MEDs for which the transmitted UV dose reaches 1 MED.
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