Quantitative assessment of sunscreen application technique by in vivo fluorescence spectroscopy

For the first time, a method is described for measurement of surface density of sunscreen in vivo. Here, the method is used to measure the uniformity of sunscreen application. The intrinsic fluorescence of a sunscreen (Neutrogena SPF 15®) was quantified by fluorescence spectroscopy, and a dose-response relationship was established with sunscreen density on the skin. Sunscreen was then applied in a crude fashion to one forearm and carefully to the other forearm in five subjects. Fluorescence measurements were taken from 16 sites on each forearm and converted to an equivalent thickness of sunscreen using the dose-response relationship. Whereas the median thicknesses for crude and careful application were approximately the same, the range of thickness was higher after crude application (p <0.007). Hence fluorescence spectroscopy can quantify the adequacy of sunscreen application. This simple and rapid noninvasive in vivo technique for measuring sunscreen thickness could potentially provide a surrogate method for SPF determination in the clinical testing of new products.