The present invention relates to a new method for determination of realistic UV protection factors or broad spectrum indices of a sunscreen preparation.
Numerous attempts have been made to understand how sunscreen preparations protect against UV radiations e.g. Int.J.Cosmet.Sci., 7, 235-246 (1985). Normally, the final UV protection achieved by a sunscreen product should be the result of each filter UV absorption present in the preparation. To determine it, the simplest way is to add each UV filter Absorbance, measured in diluted solution, wavelength by wavelength, according to Beer-Lambert law""s. However, it is now trivial to remark the poor correlation between the SPF(s) directly calculated from this method and the real SPF(s): several magnitude orders of difference, much too high for the calculation.
Recently, with the generalization of the in vitro SPF method, realistic experimental UV data has been easily obtained on the whole erythemal wavelength range (from 290 to 400 nm). This recent instrumental UV spectroscopy is based on the assessment of diffuse transmission through a film of sunscreen preparation, directly spread on an uneven transparent substrates, chosen to resemble skin topography, for example the surgical Transpore(copyright) tape manufactured by the 3M Company. As a result, a good correlation between the SPF, derived from these experimental in vitro curves, and the in vivo SPF has been successfully obtained. However, although the method was revealed to be a very useful tool for the formulator, there is little logic in the relationship between the amount of active ingredients (UV absorbers) introduced in the sunscreen product and the final UV absorption. The main reason comes from the non-relevance of the Beer-Lambert law for the UV transmission spectroscopy of irregular thin films.
So, until now, it seemed impossible to predict any protection factor from a new blend of UV filters. The only solution was to achieve a complete sunscreen formulation and to test it by the in vitro method. Therefore, formulators were always obliged to numerous fastidious assays to optimize their UV filter combination, according to the SPF or the UVA protection targeted.
Object of the present invention is a new method for determination of realistic UV protection factors or broad spectrum indices of a sunscreen preparation according to the amount of active UV absorbents introduced in a sunscreen formulation.
Surprisingly, it was discovered now that one is able to establish a mathematical relationship between the theoretical Absorbance curve of a sunscreen preparation, calculated by applying the Beer-Lambert law to UV filter composition, and the experimental Absorbance curve obtained through in vitro spectroscopy assays. Very interestingly, this relationship was found to depend on very few parameters, all related to experimental in vitro spectroscopy:
the nature and the roughness of the transparent substrate on which a thin film of sunscreen preparation was spread for assessment of diffuse transmission
the amount of sunscreen deposited
the type of vehicle in which UV filters are incorporated.
Substrates with a roughened upper surface are chosen to simulate the porosity and texture of human skin. As already mentioned in literature, surface irregularities play a major role in influencing the photo-protection of a topical sunscreen product. The amount of deposited sunscreen is carefully determined in order to obtain a good correlation with in vivo SPF. Like in the real SPF, the base in which UV filters are incorporated can also modify the final protection.
One time the previous relationship defined, the sole variable to be considered is the theoretical Absorbance, whatever the wavelength associated with it. So, unrealistic calculated UV data, according to Beer-Lambert""s law, can be easily converted into realistic UV data, from which all kinds of protection factors and broad spectrum indices can be calculated. The following schema can help to understand the different calculation steps: 
Realistic UV Absorbance data means that these calculated data are identical to the ones which could be obtained by achieving in vitro assays.
The mathematical relationship should be previously determined, by plotting experimental in vitro Absorbance data versus theoretical Absorbance data, from selected sunscreen preparations. Specific relationship can be obtained, according to the type of vehicle involved: O/W emulsion, W/O emulsion, Gel emulsion etc. in order to take into account the vehicle effect.