  Sunscreen lotions are used to protect the skin from damage due to solar
ultraviolet (UV) radiation. The active UV blocking components can be organic
molecules or inorganic particles, for example TiO2. While both in vivo and in
vitro methods exist for assessing the protective capacity of sunscreens, their
basic optical properties have received little attention. In this paper we take
the first steps to address this issue by presenting a method to obtain spectra
of absorption and scattering coefficients of a sunscreen by analysis of
experimental transmittance and reflectance spectra. We model the optical
properties of the sunscreen by two flux theory, develop approximate expressions
for interface reflection coefficients and obtain the remaining optical
parameters by fitting the two-flux theory to measurements. We analyze
spectrophotometric reflectance and transmittance spectra of experimental narrow
band UV blocking materials based on mixtures of mesoporous magnesium carbonate
and TiO2, as well as some commercial sunscreens. Our method is demonstrated in
vitro using quartz substrates and is further used to model in vivo measurements
on the upside of the hand of one of the authors. The obtained absorption and
scattering coefficient spectra give information on the relative contributions
of these processes to the UV blocking effect as well as the scattering of
visible light responsible for the whitening of the skin upon application of a
sunscreen lotion. Our analysis method may be useful for computational
optimization of novel sunscreen formulations as well as for the development of
in vivo test methods.
