In vitro measurement of sun protection factor of sunscreens by diffuse transmittance

Abstract The measurement of sunscreens using an in vitro technique that correlates to in vivo measurements has been proposed for many years. In vivo testing, where human volunteers are subjected to potentially damaging and carcinogenic doses of ultraviolet radiation, has been the method of choice by regulatory agencies for determining the efficacy of sunscreens to protect humans from both sunburn (solar erythema) and potential skin cancers related to high UV doses. The problems with in vitro measurements are many fold. A normal spectrophotometer cannot accurately capture the transmitted light from a sunscreen, since both the media and the sunscreen may scatter the incident and transmitted radiation. Secondly, a suitable substrate for dispersing the potential sun protective agent must be found – a material that transmits sufficiently over the range of interest of measurement but also has a texture similar to the human epidermis to allow for proper dispersion of the sunscreen. This paper will discuss the theory of measurement of diffuse transmittance measurements, including the various instrument geometries used to make such measurements. It will also address the calculations required to convert transmittance values to that of sun protection factor. Finally, there is a discussion of substrates for in vitro measurements.