Beer’s Law characterizes the absorption of light passing through a sunscreen layer in terms of the properties of the sunscreen. Mathematically, Beer’s Law states that the transmittance of light through the layer (τ) is determined by the absorption coefficient of the sunscreen (α) and layer thickness (t), as illustrated by the following equation:
Here, I0 is the intensity of light incident on the layer and I1 is the intensity of light that passes through the layer. When expressed this way, the transmittance is always on a scale from 0 to 1. In a sunscreen, the absorption coefficient depends on the concentration of UV actives and the wavelength of light.