Elma Baron (editor) - Light-Based Therapies for Skin of Color

(1)

(2)

(3)

Skin is the largest human organ. It covers between 1.5 and 2 m2, comprising about one-sixth of total body weight. Skin performs a complex role in human physiology. It serves as a barrier to the environment and acts as a channel for communication to the outside world. For example, skin protects us from water loss, ultraviolet (UV) rays of the sun, friction, and impact wounds. It also helps in regulating body temperature and metabolism. All photobiological responses are influenced heavily by the optical properties of skin. Therefore, for the successful development of photomedicine, in-depth knowledge and understanding of light-skin interactions, specifically known as skin optics, is required. The transfer of optical radiation into human skin depends on the absorption and scattering properties of three functional skin layers: epidermis, dermis, and hypodermis. The structures and component chromophores of these layers determine the attenuation of radiation in skin. The enhanced penetration of optical radiation as well as selective targeting of pathology can be achieved by studying and analyzing the wavelength-dependent interactions of light with skin. For example, considering that melanin exhibits maximum absorption in the UV and blue spectral ranges, whereas blood preferentially absorbs blue and yellow light, the treatment protocol have been devised that target pigmented and vascular lesions, respectively. This chapter provides a brief summary and description of the properties of light, its interaction with human skin, the list of the medical light sources, and the diagnostic and/or therapeutic uses of currently available light-based devices within the visible to NIR spectral range in dermatology.

Light can be described as the energy that is carried in the form of traveling wave composed of electric and magnetic fields. Electric and magnetic fields vary in intensity and are at right angles to each other and to the direction that the wave is propagating (Fig. ). Initially, light was defined as the electromagnetic waves in the visible spectral range. At present, however, light is defined as electromagnetic radiation between 100 and 10,000 nm. In photomedicine, this spectral range is subdivided in the following ways: UV light: UVC, 100280 nm; UVB, 280315 nm; and UVA, 315400 nm; visible: 400780 nm (violet, 400450 nm; blue, 450480 nm; green, 510560 nm; yellow, 560590 nm; orange, 590620 nm; and red, 620780 nm); infrared (IR) light: IRA, 0.781.4 m; IRB, 1.43.0 m; and IRC, 31000 m. In physics, the light is classified as UV, 100400 nm; visible, 400800 nm; NIR, 0.82.5 m; middle-IR (MIR), 2.550 m; and far-infrared (FIR), 502000 m.

Simultaneously, light can also be described as a stream of particles, photons, which have a rest mass of zero, are electrically neutral, and carry certain amount of energy. In other words, the energy that is transported by EM wave is not continuously distributed over the wave front defined by crests. Energy is located at discrete points, photons, along the wave front. Photons are created inside the atoms of radiating body from which they receive their energy content. Photons move with velocity of light. When photons are absorbed by atoms of the matter, they lose their identity by transferring their energy to an atom. Energy content in photon is inversely proportional to its wavelength: E photon = hc /, where c is the velocity of light, the photon wavelength, h = 6.626 1034 Js the Plancks constant. Energy is measured in joules (J). Energy characterizes the ability of light to produce some work. Power is the rate of the energy delivery. It is normally measured in watts (W), that is, joules (J) per second (s). The efficiency of lighttissue interaction depends on the energy density, also called fluence. Light fluence is the energy, which propagates through a unit area, which is perpendicular to direction of light propagation. Fluence is measured in J/m2. Power density is power of light wave, which propagates through a unit area which is perpendicular to direction of propagation of light wave. A power density or intensity is measured in W/m2. Fluence ( F1 ) and intensity ( I 1) are proportional: F = I p, where p is the length of pulse (pulse width) or exposure time. Another important parameter that characterizes light is its fluence rate that is defined as the sum of the radiance over all angles at a point