Based on the principle of selective photothermolysis [23], several lasers have been developed to selectively affect the hair follicle. Eumelanin in the hair follicle acts as a chro – mophore that can absorb the laser radiation of a select wavelength, converting it into thermal energy. Light-based systems for hair removal are either laser-based, a single wavelength coherent beam of light, or are intense pulsed light (IPL) systems based on noncoherent beam of light composed of multiple wavelengths. Four types of lasers, differentiated by the wavelength they produce, are used for hair removal: ruby (694 nm), alexandrite (755 nm), diode (800 nm) and Nd:YAG (1064 nm). There are several IPL systems available with varied range of wavelengths. By appropriately selecting the laser wavelength, pulse duration, and fluence energy, critical follicle components responsible for hair growth, that is, follicle matrix, papilla, and stem cell population can be thermally injured while sparing the surrounding skin tissue. Other factors that can affect the efficacy outcome are hair-growth cycle, the proportion of growing or anagen hair at the time of treatment, the amount of melanin in hair follicle, and the depth of the hair follicle in skin. Generally, the longer wavelengths penetrate deeper into skin than the shorter wavelengths [24]. In addition to the characteristic wavelength, the laser aperture size plays a role in determining the depth of penetration of laser energy. The larger spot size allows for deeper penetration of light into skin.
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