quartz plate (Fig. 9c), the radiation at the interface is back- scattered in all directions. A more complicatedsituation happens if both mediaarevolumediffuser,asincase ofsunscreensapplied on PTFE, Vitro-Skin and Transpore (Fig. 9d); in this case, it is very difficult to realize a model to predict the optical path of radiation and the measurement results. As it can be seen in Fig. 8 and in detail for selected sunscreens (SPF 4 and 12) in Fig. 10, the absolute value of the absorbance vary considerably among substrates; this is also because, according to what explained, the amount of back reflected radiation with respect to the transmitted radiation is very different. Besides these physical aspects related to surface/volume scatting of light, another fundamental event that can affect the result is the different chemical interaction of sunscreen with substrate materials. A further possible physical effect that can justify the
differences is the fluorescence. The spectrophotometer used for our absorbance measurements has not filters on the input after the integrating sphere to block out sunscreen product fluorescence. This could alter the absorbance spectrum. In order to check for fluorescence effect, additional measurements of sunscreen absor- bance have been performed by introducing different interferential filters between the sunscreen/substrate and the integrating sphere. In all these cases the results were compatible with those obtained without the filters. This confirms that fluorescence does not affect the measured absorbance values. Moreover, the chemical composition of the sunscreens tested has been analyzed and no fluorescence ingredients have been found. Due to all the effects described, the absorbance curves A(l) recovered by the measurement of the substrates with and without the sunscreen were very different among them both in shape and absolute value. For these reasons, absorbance and reflectance measurement curves are not meaningful by itself for labelling, unless they are used to calculate parameters that correlates to in vivo measurements or they are used only to make a relative comparison between different products. Generally, the trend amongdifferentSPFproductswasmaintained,exceptinthecaseof the product labelled SPF 20 when applied on Transpore (Fig. 8a) and product labelled SPF 15 when applied on PMMA (Fig. 8d). In these cases,thetransmittancecurvesintersectedthose ofproducts with higher or lower SPF values. These curves have been measured differenttimesandresultshavebeenalwaysconfirmed.Inthecase of Transpore, a chemical reaction between sunscreen and the substrate could be the cause of this result. Some measurements have been performed also on Transpore self-standing. As already mentioned, the application of the cream on this substrate is hard to perform due to the fragility of the tape. Results show that, in comparison with Transpore on quartz, the shape of the curves was maintained, but the values of transmit-
Fig.9.Schematicvisualizationoflightinteractionintotwodifferentmaterials.(a)MediumAand mediumBnot-diffusing;(b)mediumAvolumediffusingandmediumBnot- di
quartz plate (Fig. 9c), the radiation at the interface is back- scattered in all directions. A more complicatedsituation happens if both mediaarevolumediffuser,asincase ofsunscreensapplied on PTFE, Vitro-Skin and Transpore (Fig. 9d); in this case, it is very difficult to realize a model to predict the optical path of radiation and the measurement results. As it can be seen in Fig. 8 and in detail for selected sunscreens (SPF 4 and 12) in Fig. 10, the absolute value of the absorbance vary considerably among substrates; this is also because, according to what explained, the amount of back reflected radiation with respect to the transmitted radiation is very different. Besides these physical aspects related to surface/volume scatting of light, another fundamental event that can affect the result is the different chemical interaction of sunscreen with substrate materials. A further possible physical effect that can justify the
differences is the fluorescence. The spectrophotometer used for our absorbance measurements has not filters on the input after the integrating sphere to block out sunscreen product fluorescence. This could alter the absorbance spectrum. In order to check for fluorescence effect, additional measurements of sunscreen absor- bance have been performed by introducing different interferential filters between the sunscreen/substrate and the integrating sphere. In all these cases the results were compatible with those obtained without the filters. This confirms that fluorescence does not affect the measured absorbance values. Moreover, the chemical composition of the sunscreens tested has been analyzed and no fluorescence ingredients have been found. Due to all the effects described, the absorbance curves A(l) recovered by the measurement of the substrates with and without the sunscreen were very different among them both in shape and absolute value. For these reasons, absorbance and reflectance measurement curves are not meaningful by itself for labelling, unless they are used to calculate parameters that correlates to in vivo measurements or they are used only to make a relative comparison between different products. Generally, the trend amongdifferentSPFproductswasmaintained,exceptinthecaseof the product labelled SPF 20 when applied on Transpore (Fig. 8a) and product labelled SPF 15 when applied on PMMA (Fig. 8d). In these cases,thetransmittancecurvesintersectedthose ofproducts with higher or lower SPF values. These curves have been measured differenttimesandresultshavebeenalwaysconfirmed.Inthecase of Transpore, a chemical reaction between sunscreen and the substrate could be the cause of this result. Some measurements have been performed also on Transpore self-standing. As already mentioned, the application of the cream on this substrate is hard to perform due to the fragility of the tape. Results show that, in comparison with Transpore on quartz, the shape of the curves was maintained, but the values of transmit-
Fig.9.Schematicvisualizationoflightinteractionintotwodifferentmaterials.(a)MediumAand mediumBnot-diffusing;(b)mediumAvolumediffusingandmediumBnot- di
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