Determining the light-protection factor
Where there’s light, there is also shade. And where there’s sun, there lurks the risk of sunburn.
This is why it is so important to reliably determine the light-protection factor.
The light- or sun-protection factor (SPF), which is determined in vivo, is an indicator of the sun-protection product’s effectiveness.
It is calculated from the results of meticulous tests designed to observe the formation of erythema, or rashes, on the skin caused by increasing exposure to UV rays. The SPF is the energy ratio required for the skin to generate a minimal erythemal response both with and without sunscreen. To trigger these UV-induced erythema, the spectrum of a xenon-arc lamp is mapped to the effective range for erythema (COLIPA spectrum) by means of special filters and applied to the skin. Varying UV dosages can be administered with the help of a shut-off mechanism that can be adjusted to the individual radiation level required, thus controlling the incremental UV-exposure treatment. The SPF is ascertained by measuring a treated area of skin (sunscreen applied) and an untreated area (unprotected skin).
Perfect protection calls for perfect testing methods.
The main requirement of a sun-care product is to be safe for the consumer to use and to fulfil the specified effects. Sunlight contains a significantly higher percentage of UVA rays than UVB rays. They can penetrate much deeper into the skin and are accordingly responsible for damage to the structure of the skin’s connective tissue, which may manifest itself in the form of a more pronounced, premature ageing of the skin. A good sunscreen must therefore provide well-balanced protection against UVA and UVB rays. This test serves to determine the correlation between the in vitro UVA protection factor (UVAPF) and the advertised UVB protection factor. It is based on the principle of measuring the transmission of UV light through a thin film of the sun-protection product previously applied to a special glass slide. Transmission is measured before and after radiation with a specific dose of UV rays and the level of UVA protection is calculated accordingly.