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,incaseoftheothermaterialsthemaincontributionwas 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/ volumescattering, such as a polished plate of quartz. However, this is an unrealistic situation because a smooth surface would not allow sunscreen adherence.
Fig. 3. Total transmittance measurements on PMMA plate, with or without glycerine applied.
Fig. 4. (a) Transmittance measurements on Transpore 3M attached on polished Quartz and (b) reflectance.
D. Ga
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,incaseoftheothermaterialsthemaincontributionwas 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/ volumescattering, such as a polished plate of quartz. However, this is an unrealistic situation because a smooth surface would not allow sunscreen adherence.
Fig. 3. Total transmittance measurements on PMMA plate, with or without glycerine applied.
Fig. 4. (a) Transmittance measurements on Transpore 3M attached on polished Quartz and (b) reflectance.
D. Ga
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