Measuring the Sun Protection Factor (SPF) of Sunscreens

Sun protection products, including sunscreen lotions, are designed to absorb or reflect the sun’s UV radiation to protect the skin from damage. The rating system for sunscreens specifies a Sun Protection Factor (SPF) value, which can be thought of as a time factor for the protection of skin compared to exposure without any protection. For example, if a person would show visible erythema (sunburn) after five minutes of exposure, application of a sunscreen with SPF eight extends that time to five minutes times the protection factor of eight, or forty minutes. Scientific methods of evaluating the SPF of sunscreens have been developed and specified according to Australia/New Zealand (AS/NZ) standard 2604:1998. Other nations and regions have produced their own standards modeled after this original work, for example the COLIPA International Sun Protection Factor Test Method, most recently updated in 2006. These standards rely on diffuse transmittance measurements obtained using a UV-Visible spectrophotometer equipped with a diffuse reflectance accessory. This application note describes how sunscreen transmission measurements can be carried out using a Thermo Scientific EvolutionTM 600 spectrophotometer, a Diffuse Reflectance Accessory (DRA-EV-600) with 0.008mm pathlength quartz demountable cell, and VISIONliteTM (VL) MaterialsCalc software.