2.4. Total absorbance/transmittance measurements on sunscreens
Seven sunscreen products produced by the same manufacturer
labelled with different SPF protection level (from 4 up to 30). The
method could be in principle extended to whatever SPF; it is
however important to have instruments with high signal-to-noise
ratio, because transmission of sunscreens with SPF50+ is really low.
This measurement should be performed with a double grating
spectrophotometer (similar to the spectrophotometer used for this
work), and not with a single grating spectrophotometer. The main
ingredients of the sunscreens, as derived from the label, are reported
inTable 2. Only one sunscreen formulation (i.e. lait) was used. Each
product has been applied on each substrate material. Only one
sunscreen formulation from one producer has been applied to each
substrate at a time, in order to maintain the same chemical
composition and remove then the effects of possible different
chemical reactions. Sunscreen formulations have been applied
without UV pre-irradiation. The application of the product has been
realized by spotting the sunscreen on several points of the substrate
with a pipette, and by uniformly distributing it with a finger glove.
After the application, the volatile compounds the product has been
allowed to evaporate. The film of product applied on the substrate
has been weighed with an analytical balance (max error< 0.2 mg)
after the drying process; the weight has been recorded in order to
evaluate possible dependences of the quantity evaporated from the
substrate used. In all cases during the drying process the sunscreen
lost about 40–60% of its weight, and meaningful differences have
been registered among different substrates. The amount of product
initially applied on the substrates corresponded to 1 mg/cm2which
has been demonstrated to allow a good correlation between SPF
values evaluatedin vivoandin vitro. Total absorbance/transmittance
measurements have been carried out using the apparatus already
described. The curves A(l) = Log T(l) =Log[1/PF(l)] [16] have
been obtained by the ratio between the transmittance curves
measured with and without the sunscreen applied respectively. In
order to increase statistic, for each sunscreen formulation several
samples (about 10 for each formula) have been prepared and each
sample has been tested in three different areas to overcome possible
error due to non-uniformity of the product application.
2.5. In vitro Sun Protection Factor computation and correlation with
value reported on labels
The absorbance curves have been used to compute the SPF
according to the following definition: