The performance of sunscreens has taken on an ever greater importance in recent years as a result of increasing health concerns arising from excessive exposure to sunlight. Sunscreen manufacturers have focussed on developing new and improved formulations that provide the necessary high levels of protection as well as photo-stability.
The performance testing of sunscreens was originally performed on human volunteers. So-called "in-vivo” testing involved the determination of the SPF (sun protection factor) based upon the delay in the onset of erythema (sun burn) on protected versus unprotected skin. Supplementary, predictive (“in-vitro”) SPF testing was developed in the early 1990s based upon the measurement of the spectral absorbance of a sunscreen applied to a transparent substrate material (originally Transpore Tape, more recently roughened plates of polymethyl methacrylate – PMMA).
It is important to understand that SPF only defines the protection that the sunscreen affords in the UVB part of the spectrum (290-320nm). It doesn't address protection in the UVA band (320-400nm). Our understanding of the damage sunlight can cause to our skins has developed and it is now accepted that in the long term UVA as well as UVB radiation is potentially damaging. Clearly, a method to measure the protection provided by sunscreens in both the UVB and UVA bands is required.
Various metrics have been developed to indicate the degree of broadband protection given by a sunscreen. Critical wavelength is the shortest wavelength at which the sunscreen absorbs 90% of the sun’s energy. If the critical wavelength is 370nm or greater, the formulation can be described as having broad spectrum protection. In the UK, Boots developed its Star Rating system that awarded one to four (subsequently up to five) stars dependent upon the ratio of mean UVA to UVB absorbance. The higher the UVA absorption, the higher the ratio and the greater the number of stars a product will earn. Elsewhere, researchers have developed an in-vivo method for UVA protection based upon the observable end-point of persistent pigment darkening (PPD).
In 2007, the European Cosmetic Industry Association (COLIPA) published its new UVA protection factor (UVAPF) test method that defined an in-vitro test based upon a spectral transmittance measurement scaled to the PPD action spectrum. This test method is essentially the same as that used to perform in-vitro SPF testing, but with some important differences. First, the sunscreen must maintain its protection after exposure to sunlight. The photo-stability of the formulation is determined by measuring the absorbance before and after exposure to a solar simulator. Second, the action spectrum invoked in the calculations is that for PPD rather than erythema. Third, COLIPA required that the spectrophotometer intended to be used for the test should possess a high degree of sensitivity in order to accurately report the ratio of UVB to UVA absorbance and to cope with higher protection factor formulations. The absorption sensitivity required by the COLIPA method is 2.2 at all wavelengths in the 280-400nm band.
Labsphere developed its original sunscreen analyser – the UV-1000S – in the mid 1990’s in order to provide sunscreen formulators with an affordable tool that helped them to bring new formulations to market more quickly and at reduced cost. In 2008, Labsphere launched the UV-2000S which was designed to fully comply with all current and proposed industry requirements, including the COLIPA 2007 UVAPF method, the 2008 update to the Boots Star Rating system as well as the proposed United States FDA protocol. The UV-2000S has a minimum specified absorption sensitivity of 2.7 and remains the only affordable and simple-to-use instrument to fully satisfy the requirements of the COLIPA guidelines.
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