Impact of deposit homogeneity on sunscreen efficacy: A novel quantitative method using <scp>LC</scp> ‐ <scp>OCT</scp>

Performance of sunscreens is primarily optimized through the development of their organic or mineral filtering systems, that is, the core of their photoprotective action. However, numerous studies have underscored that the homogeneity of the applied deposit is also key to optimize the effectiveness of a given filtering systems. Quantifying the homogeneity of a coating presents a significant challenge, particularly on complex substrates such as skin. Therefore, there is a key challenge in being able to have robust methods to measure deposit homogeneity for both in vitro and in vivo ultraviolet tests. This work focused on developing new quantitative methods for evaluating sunscreen deposit and its homogeneity. Our approach involved adaptation line-field confocal optical coherence tomography, a technique commonly used in biology, to precisely assess the thickness distribution of sunscreen deposit. For this study, we specifically investigated deposit modifications without changing the filtering system, thereby successfully linking variations in deposit thickness and homogeneity directly to filtration performances. This new method not only can be leveraged to advance the development and improvement of new photoprotective technologies but also paves the way for a more comprehensive understanding and characterization of sunscreen applications, both in vivo and in vitro, leading to improved protection.