Figure 1. Schematic representation of light reflected and back-scattered from human skin

Visible light returning from normal skin consists almost entirely of two components: the surface reflectance component and the dermal back-scattered light component due to scattering from dermal collagen fibers, shown here. A small fraction of visible light at longer, i.e., red, wavelengths penetrates through the entire skin but in thinner skin, such as under the eyes, the loss of this transmission contributes to a darker, gray-bluish appearance.