Optical Properties of the Skin

Skin consists of three main layers—the epidermis, separated from the underlying dermis by a basement membrane; the dermis, with collagen and elastic fibers produced by fibroblasts, blood and lymph vessels, hair follicles, sweat and sebaceous glands, erector pilae smooth muscles, and nerves; and the sub epidermal tissue, consisting of a fat layer. The epidermis of the skin, further consists of four layers—the stratum germinativum, the stratum spinosum, the stratum granulosum, and the stratum corneum. When radiation strikes the skin, part is absorbed, part is remitted, and part is transmitted through the successive layers of skin, until the energy of the incident beam is dissipated. The transmittance of a planar sample is defined as that fraction of the radiation incident on one side of the sample that passes through and emerges from the other side of the sample. Remittance is defined as that fraction of the radiation incident on one side of a sample that returns from or through the same side. The term diffuse reflectance is spatially isotropic remitted radiation. The regular reflectance of an incident beam normal to skin is 4–7% over the spectrum from 250 to 3000 nm. The action spectrum wavelengths of photobiologic responses are those absorbed by specific chromophores that initiate the photochemical reactions. The remittance of incident radiation with respect to skin color and the Kubelka-Munk model for radiation transfer in a scattering, absorbing medium is also described in the chapter.