where E(l) is the spectral irradiance of the ‘‘standard sun’’ [21],
B(l) is the erythema action spectrum. The final in vitro SPF values
have been calculated as the mean SPF values computed with the
different A(l) curves measured, while the error bars associated
represent the standard deviation from the mean value.
3. Results and discussion
3.1. Transmittance and reflectance measurements on substrates
Regular and diffuse total transmittance and reflectance have
been measured for all substrates (Fig. 2). Vitro-Skin not hydrated
and PMMA covered by glycerine have also been investigated.
All tested materials had a certain amount of total reflectance; it
is important to considered that the back reflected beam interacts
again with the sunscreen when applied. In particular, in case of
PTFE the reflectance is higher than the transmittance. The
substrates presented different diffuse properties. While quartz
and PMMA diffuse components were due only to the superficial
roughness, in case of the other materials the main contribution was
due to a volume scattering. PTFE was the only material with a
negligible regular component. Transpore diffuse transmission
represented about 73% of the total transmission in the spectral
range considered, Vitro-Skin about 70%, roughened quartz plate
about 82% and PMMA plate about 80%. These optical properties
were responsible of a very complex optical path of light in the
sunscreen–substrate coupled system, which does not allow to
recover the absolute transmittance curve of the sunscreen alone.
As a matter of fact, if the absorbance signal of the reference
substrate is subtracted from the absorbance signal of the
sunscreen–substrate, the results do not correspond to the
transmittance of the sunscreen layer; this would be true only in
case of a smooth transparent substrate with negligible surface/
volume scattering, such as a polished plate of quartz. However, this
is an unrealistic situation because a smooth surface would not
allow sunscreen adherence.