Abstract
BACKGROUND: The sun protection factor (SPF) of sunscreens is evaluated using standardized protocols based on the application of 2 mg/cm(2) of product. However, the amount of product applied by sunscreen users in real life is likely to be much lower. OBJECTIVES: To evaluate a new multispectral imaging approach for determining the actual quantity of sunscreen applied by users and to assess the benefits of an application guide for the use of an SPF 50+ sunscreen. MATERIALS AND METHODS: Analyses of the reflectance spectra obtained from multispectral images were used to determine the actual dose of sunscreen that 26 healthy volunteers applied to their face following three application modalities: a single application, reapplication after 30 min, and application according to an instruction guide. RESULTS: Without the application guide, volunteers applied an average of 1.04 mg/cm(2) of sunscreen during the single application and 1.23 mg/cm(2) during the repeated application. With the application guide, the amount of sunscreen applied was 1.45 mg/cm(2) : around 40% higher than during the single application. Spreading of the sunscreen was also less uniform with the unguided single application than with the other application modalities. CONCLUSIONS: This study showed that the multispectral imaging approach can be used to measure the amount of sunscreen applied in vivo. Our findings confirmed that the standard dose used for SPF measurements and other sunscreen tests is far higher than that applied by users in practice. Providing users with precise guidelines could increase the amount of sunscreen applied, resulting in more adequate photoprotection.