This test consists of evaluating the Critical Wavelength (CW), which expresses the ultraviolet A (UVA) protection in relation to the ultraviolet B (UVB) protection of a sun protection product over the entire UV spectrum, using a suitable substrate on which the product has been spread and measured using a spectrophotometric method.

(ref. HeCW)

To take into account the photostability of your products, we recommend adding a UV exposure phase to your development tests. The sample is then exposed to UV radiation in a solar simulator at a specific dose before the sun protection is measured.

Taking photostability into account in your screening tests is a very important aspect, as all tests to official standards (ISO, FDA, etc.) include a UV exposure phase.

(réf. UVEXPO)

ISO/DIS 23698 (currently in progress at ISO level) proposes a hybrid approach combining in vitro and in vivo tests to assess the UV protection of sunscreens, in particular their Critical Wavelength (CW).
It uses diffuse reflectance spectroscopy (DRS) to measure UVA absorbance (320-400 nm) on the skin.
To obtain a complete UV absorbance spectrum, the in vitro absorbance values are adjusted according to the data obtained via DRS, enabling the in vitro UVB absorbance to be mathematically linked to the UVA portion measured by DRS.

*Available from WENEOS from January 2025.

ISO 24443:2021 specifies an in vitro procedure for characterizing the UVA protection of sunscreen products in a reproducible way. In order to determine the relevant UVA protection parameters, the method is strictly followed to provide a UV spectral absorbance curve from which a number of calculations can be made, such as the critical wavelength (CW). This method is based on the use of in vivo SPF results to scale the UV absorbance curve.

(ref. HeCW-ISO)

In June 2011, the US Food and Drug Administration (FDA) announced new requirements for sunscreens currently sold over-the-counter (OTC). Sunscreens that pass the FDA's in vitro critical wavelength test procedure, which measures a product's ultraviolet A (UVA) protection relative to its ultraviolet B (UVB) protection, can be labeled as "broad spectrum" with an SPF value on the front label. This test is based on an in vitro spectrophotometric measurement technique involving UV exposure to a representative dose of ultraviolet radiation simulated by the sun, to take account of any photoinstability of the active ingredients contained in the sunscreen product.

(ref. HeCW-FDA)

The UVA measurement method used to assign UVA:UVB ratio protection categories in accordance with the Boots Star Rating system (2011 revision) is based on an in vitro spectrophotometric measurement technique in which the product under test is spread using a suitable substrate and exposed to a representative dose of ultraviolet radiation simulated by the sun, to take account of any photoinstability of the active ingredients contained in the sunscreen product.

(ref. HeUVA:UVB-BOOTS)

The statistical comparison method makes it possible to compare two sun protection products in terms of ageing, batch, benchmark, etc. When checking the absorbance spectrum between a reference and a test product using an in vitro method, two important rules must be observed. First of all, compare the two products at the same time and under the same conditions, to limit measurement variability. Secondly, a three-step verification based on absorbance values, including comparison of curve shape, variability and intensity.

(ref. HeCOMP-1)

Published method : *In vitro comparison - A new accessible and reliable statistical method to compare the global UV protection properties of cosmetics - Lutz and S. Miksa, H&PC Sun Care, July/August 2013 | *Sunscreen products: Some practical applications for a new comparison test method to help Development and ensure Quality Controls - D. Lutz, S. Miksa and C. Guy, H&PC Today, November/December 2014