Spectral uniformity: a new index of broad spectrum (UVA) protection.

Despite the wide acceptance of in vitro assay for the determination of the spectral absorbance of a sunscreen product, we find that different authorities are recommending different summary metrics of this spectral profile to express the UVA or broad spectrum, protection offered by topical sunscreens. Regrettably, the situation that now prevails is not ideal and the option for rationalizing the situation in the interests of consumer-focussed international harmonization is either the adoption of one of the existing metrics or universal acceptance of a new metric for expressing broad spectrum (UVA) protection. There may be greater harmony in adopting the second proposal and so I introduce here a new metric, which I term the spectral uniformity index, for consideration by the international sunscreen community. It is shown that this new index is aligned to the fundamental requirement of a topical sunscreen to provide a flat spectral profile, is simple to calculate and conceptually easy to understand, and correlates strongly with existing indices.