Measuring Sunscreen Reflectance and White Cast Using an <i>In Vitro</i> Skin Tone Model

Abstract Sunscreen white cast is a visible whitening on skin caused by light scattering from sunscreen films and is one of the primary reasons consumers underapply sunscreen below the 2 mg/cm2 thickness required to achieve labeled SPF. Existing methods for quantifying white cast rely on in vivo scoring by human participants, and standard in vitro SPF measurements consider only the transmission of UV light, therefore conflating absorption and reflectance. Using integrating-sphere reflectance spectroscopy, we measure three organic-filter and three inorganic-filter sunscreens with similar UV-filter compositions. We find that reflectance accounts for up to 30% of UV light partitioning away from the skin, a contribution that is not explicitly captured by standard transmission-based SPF measurements. We then design a quartz skin construct (QSC) that approximates Fitzpatrick Phototype (FP) II and V skin tones that enables contextual colorimetric analysis. We show that white cast is twice as perceptible on QSC-FPV than on QSC-FPII, which is directly attributable to the increase in diffuse reflectance in the visible region. By comparing sunscreens with similar UV filter compositions, we attribute differences in reflectance in the visible region to formulation variables, in addition to the UV filters. In this work, we establish reflectance as a key contributor to sun protection and offer a translational in vitro method for evaluating cosmetic outcomes across skin tones.