The test is based on an assessment of the UV transmittance of a thin film of sun-screen sample spread on a roughened substrate after exposure to a controlled dose of UV radiation from a defined UV source. Due to the current lack of inter-laboratory reproducibility of absolute in vitro UV measurements, each set of sunscreen trans-mission data is adjusted by first converting to absorption data (before and after UV exposure) and then by multiplying by a correction coefficient. This coefficient is de-termined iteratively from the non-exposed sample‟s absorbance data to provide a calculated in vitro SPF value equal to the labelled (in vivo) SPF.