Using a specific wavelength that will be exclusively absorbed by melanin, the pigment color Brown, accumulated at the base of the epidermis or in the superficial DermIS, sometimes in special cells called melanocytes sometimes in fibroblasts.With a time of very short impact (between the mill and the billionth of a second) the beam of light is very specific to this excess melanin. It will destroy the virtually without damage. The effect is either an explosion of the pigment particles (photoacoustic shock with Q-Switched lasers) or a specific thermal destruction by long-pulsed lasers.