Method for determination of realistic UV protection factors or broad spectrum indices

Method for determination of realistic UV protection factors or bread spectrum indices of a sunscreen preparation, which comprises:

• (1) Determination of absorptivities of pure known UV filter substances in a pure solvent or a mixture thereof, at wavelengths from 290 to 400 nm, in wavelength steps in the range of 1-10 nm;
• (2) Calculation of the theoretical UV absorbances A(λ)th, from 290 to 400 nm, in wavelength steps in the range of 1-10 nm, of some pure UV filter substances or mixtures thereof which are contained in different amounts in a defined basic formulation, according to application of Beer-Lambert law to the absorptivities previously determined in step (1) by using the following equation:
• 15 A ( λ ) ⁢ th = w * 1 / 100 * ∑ n = 1 n = p ⁢ K λ ⁡ ( n ) * a ( n ) [ 1 ]
• wherein n is the number of UV filter substances from n=l to n=p, w is the initial surface density of the sunscreen preparation immediately after deposition on an uneven transparent substrate in mg/cm2, Kλ(n) is the absorptivity of the number n of UV filter substances at a wavelength λ and a(n) is the amount of the UV filter substance in the sunscreen preparation in % by weight;
• (3) Determination of the experimental UV absorbances A(λ)exp, from 290 to 400 nm, in wavelength steps in the range of 1-10 nm, of the UV filter substance or mixtures of substances of step (2), which has the same amount and is in the same basic formulation such as in step (2), which formulation is selected from the group consisting of the type O/W, W/O, oil, gel, stick, mousse, aerosol and ointment and wherein the formulation is deposited on an uneven transparent substrate, to achieve an irregular film of a surface density of w mg/cm2;
• (4) Applying of pairs of experimental absorbance data A(λ)exp of step (3) and theoretical absorbance data A(λ)th of step (2) of the same wavelength in a graph;
• (5) Determination of a mathematical function in correlation with the graph of step (4) by incorporation of parameters of an uneven film, which film represents a mathematical model of the application of a sunscreen preparation on a transparent substrate such as in step (3) and wherein by optimizing the fit to the pairs of absorbance data, A(λ)exp versus A(λ)th, through an error assessment, different area fractions f(l) to f(i) with different surface densities w(l) to w(i) are calculated, whereby applies:
• 16 ∑ n = 1 n = i ⁢ f ⁡ ( n ) = 1 ⁢ ⁢ and ∑ n = 1 n = i ⁢ f ⁡ ( n ) · w ⁡ ( n ) = w
• wherein w is the initial surface density of the sunscreen preparation immediately after deposition on the transparent substrate, and wherein the mathematical function is:
• 17 A ( λ ) ⁢ exp = - log ⁡ [ ∑ n = 1 n = i ⁢ f ⁡ ( n ) · 10 - w ⁡ ( n ) · A ⁡ ( λ ) ⁢ th / w ] , [ 2 ]
• wherein A(λ)th is the calculated theoretical absorbance according to step (2) and A(λ)exp is the experimental absorbance according to step (3);
• (6) Application of the method described in steps 1 to 5 to the prediction of realistic sun protection factors of a sunscreen preparation, previously to achieve any experimental in vitro or in vivo determination, wherein the following steps are involved:
• (7) Calculation of the theoretical UV absorbances A(λ)th, from 290 to 400 nm, in wavelength steps in the range of 1-10 nm, of an UV filter substance or mixture of substances which are contained in wt % amounts a1 to an in the sunscreen preparation of step (6), according to equation [1] of step (2);
• (8) Calculation of realistic UV absorbances A(λ)real, from 290 to 400 nm, in wavelength steps in the range of 1-10 nm, according to the theoretical UV absorbances calculated in step (7) and the mathematical function [2] of step (5), in which the area fractions f(l) to f(i) and the surface densities w(l) to w(i) were previously determined in step (5) for a basic formulation of the same type as the sunscreen preparation under study, by equation
• 18 A ( λ ) ⁢ real = - log ⁡ [ ∑ n = 1 n = i ⁢ f ⁡ ( n ) · 10 - w ⁡ ( n ) · A ⁡ ( λ ) ⁢ th / w ] [2a]
• (9) Calculation of realistic sun protection factors(SPF) or bread spectrum indices on basis of the realistic UV absorbance A(λ)real determined in step (8), with known equations.