To achieve long-lasting or permanent hair reduction, the heat must diffuse, not only to the bulb region, but also to the stem cells of the outer root sheath and the bulb region [60]. To ensure maximum safety and efficacy, the wavelength, pulse duration, and fluence of the laser must be selected in such a way that the thermal injury is confined to the hair follicle [61,62]. The removal of unwanted hair by lasers or other high-energy light sources is based on the process of photothermolysis [55]. To ensure specificity that is necessary for this treatment, melanin of the hair follicle has been selected as the target chromophore [52,53]. The absorption spectrum of melanin is between 250-1200 nm [63]. The reduction in the absorption of the light by melanin with increasing wavelength is offset by the greater depth of penetration of light at higher wavelengths [56]. In the range of wavelengths used for laser hair removal, 650-1064 nm, a balance between specificity and penetration has been achieved [52]. To ensure sufficient thermal destruction and confinement to the hair follicle, the pulse duration should be equal to the thermal relaxation time of the hair shaft (10-100 ms) [55]. Using a pulse width in this range has worked well for subjects who are fair-skinned with dark hair, because the heat generated in the relatively fewer number of melanin granules in the epidermal layer of the skin is relatively low and can easily be removed by cooling the stratum corneum surface [64-67]. However, the absorption of the light within the pigmented epidermis of individuals with higher melanin content has required the modification of the laser parameters to minimize unwanted side effects due to the greater absorption and diffusion into heat in the epidermis [64]. This is achieved in darker skinned subjects by depositing the energy more slowly at longer pulse duration and/or higher wavelengths [5]. The longer duration of energy delivery allows the smaller granules of the epidermis to lose more of this heat [65]. To take full advantage of the adjustments in the laser-treatment parameters for dark-skinned individuals, it is imperative to efficiently cool the skin [69,70].
The FDA has approved the long-pulsed diode and the Nd:YAG systems for permanent hair reduction in those individuals with darker skin [8,55]. While the diode laser may be more effective for hair reduction, the Nd:YAG may have fewer negative side effects on the skin [5]. Pulse durations of 400 ms or longer with the Nd:YAG laser have been used to safely treat individuals with darker skin types [64].