A rapid single session in vivo method to quantify blue light protection using immediate pigmentation darkening

Blue light has been implicated in oxidative stress and pigmentary changes, yet no standardized method exists to quantify cosmetic protection in this spectral range.

To develop a single session in vivo method that quantifies the protection of topical products against blue light-induced immediate pigmentation darkening (IPD) and to compare its outcomes with a multi-day protocol from the literature.

IPD was assessed using the Individual Typology Angle (ITA°) measured before (ITA₀) and after exposure (ITA_imm) to a 420 nm LED source delivering 80 J cm^-2. Twenty-two cosmetic formulations (mineral and organic filters, nano and non-nano, with or without additional blue light actives) were applied at 2 mg cm^-2 to the backs of volunteers spanning ITA₀ -5 to 45°. For each product, 24 paired (ITA₀, ITA_imm) values were modelled by linear regression and compared with a fixed no product control. Protective efficacy was summarized as a Blue Light Protection Index (BLPI), defined as the mean percentage reduction in immediate darkening relative to the control over ITA₀ = 10-40°, where immediate darkening corresponds to the difference between ITA₀ and ITA_imm.

BLPI values ranged from 14.6 to 79.2 (median 34.0), demonstrating a wide dynamic range. Mineral filter formulations predominantly occupied the upper BLPI range, whereas organic-only products were mainly in the lower to intermediate range. Non-nano mineral filters generally outperformed their nano equivalents, and mixed TiO₂ + ZnO systems yielded the highest BLPI values within mineral products. A flat increase in BLPI was observed for formulations containing 2%, 4% and 6% benzotriazole, consistent with a concentration effect, although pairwise differences did not reach statistical significance. For four products, rankings obtained with BLPI were concordant with those obtained using a multi-day Blue Light Protection Factor method from the literature.

The proposed single session IPD-based method provides a rapid, reproducible, and biologically relevant assessment of blue light protection. It discriminates effectively between formulations, reflects expected optical behaviour and shows good agreement with a multi-day protocol. This approach is well-suited for formulation screening and could contribute to future methodological harmonization in the visible light domain.