have been purchased from SCHONBERG GmbH
(Munich, Germany).
The plates have been cut from a large sheet of UV- stabilised Plexiglas1 and roughened on one side by an industrial sandblasting procedure (glasspearls, 90–150 mm, 30 cm, 6 bar). The roughness of our plates, as declared by the manufacturer, was about 2 mm, but PMMA plates with different roughness are also available. Two additional ‘‘reference plates’’ have been purchased from the same manufacturer; the absorbance values measured by our instrumentation of the plates covered with a glycerol film resulted in perfect agreement with the values reported in literature. Since it is known that transparency of PMMA plate increases when a film of product is deposited on the roughened surface, both PMMA plates alone or covered by a glycerol film have been considered as substrates for transmission measurements. - Teflon (PTFE) has been extensively used [18] for UV application due to its good transmittance and its Lambertian properties. Samples of 7 cm2 area have been prepared in our laboratory from a standard sheet (0.8 mm thickness) previously treated with a blast sanding process in order to get a roughened surface. The use of PTFE as a substrates for sunscreen evaluation test has been recently proposed by our group [15].
Microscopic images of the substrates compared with an in vivo skin image are reported in Fig. 1. These images have been realized by illuminating the samples with white light and by acquiring the reflected images. As it can be observed, the morphology of the substrates is very different among the materials. This affects the chemical and physical coupling between sunscreens and substrates. In particular, roughness is partially responsible for the different optical properties. The RMS (roughness measurement system) surface roughness of the different substrates has been measured using a Tencor 40T profilometer. The values are derived from the mean roughness measured on five different samples. The results of these measurements, with the relative error, are reported in Table 1. It is evident that there is a good reproducibility on the measured roughness. The values are compared with the RMS roughness obtained on a silicone mould of in vivo skin.
2.2. Spectro-photometric measurements
All substrates have been characterized by transmittance and reflectance measurements (total and diffuse). Measurements have been performed with a Varian Cary 5000 UV/VIS/NIR (ultraviolet– visible–near infrared) spectrophotometer using a UV/VIS/NIR integrating sphere. The instrument spans a spectral wavelength range from 175 up to 3300 nm. Its optical system ensures a maximum level of light throughput that produces accurate