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Dovepress Low-level light therapy these commonly known destructive effects of lasers, LLLT is catalogued as “low-level” because the energy content of electromagnetic radiation is inversely proportional to its wavelength. In addition, the target tissue is gener- ally exposed to low irradiances (ie, low Watts per cm2 of tissue), when compared to the energy delivered for ablative applications. Energy doses delivered by LLLT are too low to cause concerns about heating and tissue destruction, yet they are high enough to modulate cell functions. In fact, the typical irradiances used for photobiomodulation applications overlap with those used in topical photodynamic therapy for skin conditions.4 Early experiments demonstrated that photoneuromodulation of electrical activity in neurons can be achieved independently of thermal effects.5 Although cells in vitro are responsive to a variety of wavelengths in the electromagnetic spectrum, beneficial responses in vivo are observed preferentially within a more narrow wavelength range. Obviously, visible light (400–700 nm) penetrates the eyes and activates retinal cells that contain specialized photopigments (rods, cones, and some ganglion cells). But it is unknown whether in vivo exposure to light below 600 nm (such as blue or green light) can have beneficial effects on other nerve cells that are not specialized for photoreception. The red to near-infrared wavelength range has shown to be the most effective at inducing in vivo beneficial effects in cells that do not appear to have specialized photopigments. This is attributed in part to the capacity of different wavelengths to penetrate tissue: lower wavelengths such as violet and ultraviolet appear to penetrate less, whereas those in the red and infrared range have higher penetration. Also, energy at wavelengths shorter than 600 nm is generally scattered in biological tissues in vivo and they tend to be absorbed by melanin, whereas water significantly absorbs energy at wavelengths higher than 1150 nm.6 For clinical purposes, this implies the existence of an in vivo therapeutic “optical window” that corresponds to red and near-infrared wavelengths. As discussed below, this window also matches the ability of luminous energy to excite susceptible intracellular molecules.6 For this reason, LLLT has also been referred to as near-infrared light therapy. LLLT is based on the principle that certain molecules in living systems are able to absorb photons and trigger signaling pathways in response to light.7 This process is termed energy conversion, and implies that the molecule excited by light reaches an electronically excited state that temporarily changes its conformation and function. In turn, this induces activation of signaling pathways that affect cellular metabolism. Eye and Brain 2011:3 Photobiology of LLLT Properties of LLLT The major source of electromagnetic radiation in the environment is sunlight. Solar energy contains a rich combi- nation of waves within the electromagnetic spectrum, includ- ing all wavelengths in the visible spectrum. Solar energy is multidirectional and noncoherent, which means that energy waves are not synchronized in space and time. LLLT differs from solar energy in that it is monochromatic and allows for potential high specificity and targeted molecular biomodula- tion (Figure 1). On the other hand, lasers feature monochro- matic, unidirectional, and coherent electromagnetic radiation, which allows delivery of significant levels of concentrated energy. Because of this, many biomedical applications of lasers are characterized by the destructive effects of energy over very discrete areas of tissues. The advantages of lasers include high tissue penetration, their efficient fiber optic coupling, and high monochromaticity. LLLT can be produced by LED arrays as well as lasers. Both sources have been used for photobiomodulation of the eye and brain. Laser sources produce 100% of coherent light energy in a single wavelength. They allow high tissue Sunlight Laser Light-emitting diode Figure 1 Properties of low-level light. Sunlight is composed of a combination of noncoherent waves with wavelengths spanning the entire visible spectrum. in contrast, lasers emit waves of a single wavelength (monochromatic) that have spatial and temporal synchronization. This high wavelength coherence allows the transmission of energy at a high power density. Finally, low-level light consists of monochromatic or quasimonochromatic waves taking different paths leading to a common target point. while wavelength, radiant exposure, irradiance, and fractionation scheme are relevant for low-level light therapy applications, the authors introduce the possibility that noncoherence may be advantageous for some neurometabolic purposes. Noncoherence allows nervous tissue exposure at “therapeutic” wavelengths at relatively low power densities during the time necessary to modulate neural metabolism in response to activation or injury, even if this time is prolonged. submit your manuscript | www.dovepress.com Dovepress 51PDF Image | Low-level light therapy of the eye and brain
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