Sunscreen in-vitro SPF determination – inter and intra comparison tests between several measurement instruments
Round Robin tests have been conducted to demonstrate the influence of the instrument used on measurements results, SPF and UVAPF. The results of this study show some non-negligible differences in results when using different appliances from different manufacturers. When using the same kind of instruments, the measurement configuration being the same, the repeatability is much better. It has also been demonstrated again that molded substrates ensure better repeatability than sandblasted substrates.
Spreading a sunscreen on a substrate to measure transmittance requires a trained person and good technique. This is one of the main parameters for reproducibility of sunscreens measurements inter and intra laboratories. Another important cause of non-reproducibility comes from the product itself. Some publications (1, 2) have highlighted the importance of the choice of the substrate. This has led to a consensus for the description of a referenced and well defined PMMA plate in the recent ISO 24443 standard (3). However, there has not yet been any study on the influence of the instrument itself.
ROUND ROBIN TEST 1 CONDITIONS
In order to investigate, we need to avoid the influence induced either by the spreading or the product. It was decided to conduct the round robin test using PMMA plates, including filters simulating a spread sunscreen. These PMMA plates were circulated among the different laboratories with different appliances.
The first round-robin test was conducted with 9 different laboratories. The equipment of the 9 labs came from 4 different manufacturers