Understanding sunscreen SPF performance using cross polarized UVA reflectance photography

Abstract

Objective

Objective methods for understanding sunscreen behaviour in vitro before they are applied to the skin have failed to keep pace with the ever-increasing demands for higher SPF scores where the products are absorbing more and more similar levels of UV. A novel method for visualizing the spreading and location of SPF ingredients based on cross polarized UVA reflectance photography is described here which gives new insights into the formation of final film morphology and how it correlates with in vivo SPF efficacy for a set of test products.

Methods

High resolution UVA based images of sunscreen films spread onto PMMA plates were captured using a modified commercial SLR camera in a custom imaging system. Visual grading and image analysis were used to describe the overall UVA absorbance and streakiness of the resultant films, and the data compared with both in vivo and calculated in vitro SPF scores for the products.

Results

Differences were observed between the products in terms of how they spread during application. A strong correlation was observed between the evenness of the resultant film as determined from the photographs and final in vivo SPF scores.

Conclusions

Cross polarized UVA reflectance photography has been demonstrated to be a valuable new method for assessing sunscreen distribution after spreading, and to differentiate product based on film morphology, as well as strongly correlating with final in vivo behaviour.

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