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.