Spectrophotometric Determination of SPF

In vitro determination of the SPF via spectrophotometry with integrated sphere was carried out using UV-2000S Labsphere^® equipment, in which a sample is positioned on a quartz plate, functioning as a substrate that is ideally transparent in the UV range, with texture and porosity similar to human skin. The literature recommends alternative complementary substrates to meet all these requirements. Surgical tape (TransporeTM, 3M), a polyvinylidene chloride film (Saran Wrap^®), and collagen membrane covering the quartz plate for meeting the experimental conditions. The quartz plate was prepared with approximately 50 mg of glycerin to obtain a base line in the equipment. The next step was the deposition of approximately 50 mg of the evaluated NE sample on the plate, covered by the film, spread with the help of a latex fingertip, aiming at obtaining the most uniform and homogeneous layer possible for the equipment reading. This reading was performed at nine different points on the quartz plate. The plate was placed on a metal support, which is taken to the equipment and receives the UV light. The SPF readings were made in triplicates for calculating the mean ± relative standard deviation.
The critical wavelength is a spectrophotometrically-determined value based on spectral absorbance and is used to assess whether a photoprotector offers UVA protection. Because it is a relative value–not an absolute value-of spectral absorbance, it is not considered a sensitive measure, such as SPF or that obtained using Boot’s Star Rating (Table 5). For the analysis, the measured spectral transmittance was converted into spectral absorbance, where the ratio (R) was calculated. The critical wavelength is defined as the first value found when the value of R is > 0.9, that is, the wavelength for which the area under the integrated optical density curve, which starts at 290 nm, is equal to 90% of the integrated area between 290 and 400 nm. Therefore, the value of the critical wavelength is related to the level of protection, in which λ_c values between 340 and 370 nm indicate intermediate protection against UVA radiation, and values above 370 nm indicate greater protection over a wide spectrum [37 (link)].
Boots Star rating measured the % of UVA that’s been absorbed compared to UVB rays (source), so in a sense it measured the evenness of the UV protection. The closer the UVA/UVB ratio is to 1, the more stars a sunscreen gets. Five stars on the Boots system means that UVA protection achieved more than 90% UVB protection.