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8 In general, when electromagnetic radiation (i.e. laser or LED) is applied to tissues, it is reflected, transmitted, scattered and/or absorbed. This can result in different effects, e.g., photochemical (photonically induced chemical reactions), photothermal (alterations to chemical bonds resulting in heat, ablation and/or coagulation) and photomechanical or photoionizing (cell damage resulting from destruction of cell membranes, proteins and/or DNA) effects (Graber et al., 2012; Norton et al., 2008). The term “laser” (Light Amplification by Stimulated Emission of Radiation) was first proposed by Gordon Gould in 1957 and describes a form of electromagnetic radiation with a very narrow wavelength and focus. In comparison to sunlight, laser light is coherent in nature, with a fixed relationship between the electric field values at different locations (spatial coherence) or at different times (temporal coherence). It is generated in a resonator and emitted continuously or in a pulsed manner, such that they are propagated over long lengths without divergence (Paschotta, 2008). Lasers are classified according to their ability to produce tissue damage. The classification of a laser is dependent upon the characteristics of power, wavelength, exposure and cross-sectional area of the laser beam. In accordance with the International Electrotechnical Commission, the safety of lasers is expressed in terms of maximum permissible exposure, describing the highest power that tissues can be exposed to without damage. The safety ratings range from 1 which is safe under all conditions to the maximum 4 which is always hazardous to view and has devastating damage to the eye and skin and with an ability to ignite material. Additionally, within the first and second safety ratings is a sub-category denoted as “M”, referring to the power not exceeding a certain limit as measured through a 7mm aperture at a distance of 10cm from the source (Graber et al., 2012).PDF Image | Effect of LED Phototherapy on the Rate of Orthodontic Tooth Movement
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