Generalized Beer-Lambert Law for Evaluating the Absorption of Heterogeneous Sunscreen Films

Area of Science:

• Cosmetic Science
• Materials Science
• Spectroscopy

Background:

• Film heterogeneities in sunscreen impact in vitro sun protection factor (SPF) measurements.
• Current methods struggle to correlate in vivo and in vitro SPF, especially for high SPF values.

Purpose of the Study:

• To investigate the origin and impact of sunscreen film heterogeneities on in vitro SPF.
• To develop a generalized model for sunscreen absorbance and thickness.
• To improve the reproducibility and consistency of in vitro SPF measurements.

Main Methods:

• UV photography and wetting measurements to analyze sunscreen morphology on PMMA plates.
• Generalization of Beer-Lambert's law for nonuniform samples using Gamma distribution for thickness.
• Development of an analytical model based on linear attenuation coefficients.

Main Results:

• An analytical relationship between absorbance, thickness, and homogeneity of sunscreen films was established.
• The model successfully differentiated between spreading quality and absorption potential.
• Experimental validation was performed on three different sunscreen formulations.

Conclusions:

• The developed model explains discrepancies between in vivo and in vitro SPF tests.
• This approach offers a pathway to more accurate and reliable in vitro SPF measurements.
• Findings can contribute to the development of more effective sunscreen products.