OptLab-SPX   In vitro Determination of the Solar Protection Factor (SPF)

With regard to the growing ozone hole in the earth’s atmosphere, the relevance and consumption of skin UV protection products are increasing. Skin protection products are classified according to the increase of acceptable exposure time for protected skin compared to that of unprotected skin. For example, a solar protection factor (SPF) of 8 signifies a possible 8-fold increase in the exposure time. However, the stated values of SPFs have proven to be rather unreliable. This is possibly due to the in-vivo determinations on a small number of test persons using simulated solar illumination.

The action of solar protection products mainly derives from the absorption of UV-A and UV-B radiation. An in-vitro determination can thus be based on the measurement of the absorption spectrum of a thin film of the product in the UV spectral range. With this sample spectrum a convolution calculation using a standardized solar energy distribution and a standardized skin sensitivity distribution will result directly in the SPF of the measured sample. The relevant standards, such as COLIPA or AS/NZS standards, only differ slightly in the data used for the calculations and the wavelength range. OptLab-SPX includes the calculation algorithms for both standards.

While calculations using OptLab-SPX are straightforward and clearly defined, the main problem lies in the sample introduction procedure and the measurement technique employed. The skin protection product cannot be measured in solution, since typically the spectrum looks quite different to that of the undissolved sample. Therefore a uniform thin film of a defined thickness must be measured. The thin film should have a structure similar to the film on the skin. Obviously. the most generally accepted procedure is to put the product on a roughened quartz plate. Since this arrangement scatters light strongly, an integrating sphere must be employed. However, the use of an integrating sphere does not allow angle-dependant weighting and it is sensitive to fluorescing constituents in the sample. The fluorescent interference can be minimized by setting the sample back from the sphere and by applying appropriate cutoff filters.

Considering these problems in the measurement technique, it is surely necessary to discuss and refine the respective standards. Only then will it be possible to replace in-vivo measurements with in-vitro measurements. However, for the quality control of UV protection products the technique is certainly applicable. The automated calculation of the SPF by OptLab greatly facilitates the procedure.