الفهرس 
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Abstract
Photobiostimulation (biostimulation) is a broadly used therapeutic modality that evolve the application of low energy level to the cell. Photoirradiation at these low energy level could produce significant non thermal photo chemical effects that modulate various biological processes of the mammalian cells without its damage. Tissue biostimulation is only possible if irradiated cells posses molecular photoacceptors (chromophores ) that absorb the light and enter into state of excitation. Some of proposed cellular chromophores responsible for the effect of visible light on mammalian cells are cytochrome c oxidase and photoactive porphyrins. Mitochondria are thought to be a likely site for the initial effects of light, leading to increased ATP production modulation of reactive oxygen species and induction of transcription factors. These effects in turn lead to increased cell proliferation and migration particularly by fibroblasts, modulation in levels of cytokines growth factors and inflammatory mediators, enhanced synthesis of endorphins, bradykinins, decreased c-fibre activity and altered pain threshold.
The field of photobiomodulation is characterized by variety of methodologies and use of various light sources. Originally thought to be a peculiar property of laser light (soft or cold lasers), the subject has broadened to include photobiomodulation and photobiostimulation using non-coherent light(light emitting diode, polarized light ) with different parameters (wavelength, output power, continuous wave or pulsed operation modes, pulse parameters).
Low-level laser therapy is the most widely used name given to this form of photobiomodulation. LLLT irradiation refers to the use of redbeam or near infrared lasers with a wavelength of 600–1100 nm, an output power of 1-500 mW and low energy density (0.04–5 J/cm2). At low doses (2 J/cm2), LLLTstimulates proliferation, while at high doses(16 J/cm2)LLLT is suppressive, pointing to the dose dependence of biological responses after light exposure. He-Ne lasers are the oldest used after the Ruby lasers. The He-Ne laser emits the red laser light at a wavelength of 632.8 nm. Semiconductor diode lasers are the major therapeutic lasers used. Unlike He-Ne lasers, semiconductor laser diodes do not require a high voltage supply and so can be used in portable battery-operated devices. There are three diode types: Indium, Gallium-Aluminum-Phosphide (InGaAlP) laser, Gallium-Aluminum Arsenide GaAlAs) laser and Gallium-Arsenide (GaAs) laser which is unique in that it always is operated in superpulsed mode allowing for deep penetration into body tissues and short treatment time.
Light emitting diode(LEDs) Light emitting diodes are complex semiconductors that convert electrical current into incoherent narrow spectrum light. Light emitted by LEDs (non-coherent, monochromatic light, spontaneous emission) is effective in generating a biomodulatory response within living tissue, and therefore has a therapeutic role if used correctly. LEDs is considered as a viable alternative phototherapy source.
LEDs are much less expensive than even laser diodes. In addition, the solid state nature of LEDs make no filaments to be heated up, and no flashlamps are required to produce light or to pump the laser medium .
As Polarization appeared to be the key factor in biostimulation.
Fenyo (1984) developed a polarised light source and observed a similar stimulation of wound healing as with low energy laser.This light source had several advantages over laser; lower costs, less risks, a larger treatment area, and no complicated user skills.