Existing in vitro methods for the determination of photostability and the antioxidant activities of topical ingredients do not account for diffuse reflectance (DR), which varies with different skin types and impacts measurement parameters. A higher DR increases the probability of more photons being reflected and reacting with topically applied actives on a substrate or skin. This can lead to the generation of reactive oxygen species (ROS), which further contributes to photo-instability and oxidative damage. To address this DR variable, a novel in vitro test method was developed to mimic end-use product conditions and model photodamage processes in different skin types. This approach is used here to examine the efficacy and photostability of a finished sunscreen formulation and to determine the efficacy of a cosmetic ingredient, Camellia sinensis (tea) planta, in protecting against sun-induced ROS in model systems.