Our skin color depends on its melanin concentration : the more melanin there is, the darker the skin will be. The Fitzpatrick classification made it possible to determine 6 different phototypes according to their reaction to sun exposure. For phototypes 4 ( dark skin ), 5 (dark skin) and 6 ( very dark or black skin ), the hairs are usually very dark and will capture the laser beam which targets the melanin.
When the skin is dark and its melanin concentration is therefore high, it becomes difficult to differentiate between the pigmentation of the skin and that of the hair. If the laser wavelength is not adapted to the skin tone, there is a risk of burns and skin damage.
The Alexandrite laser has a single wavelength of 755nm and risks confusing the melanin of the skin with that of the hair. This is why this type of laser is used for the permanent hair removal of fair skin. Laser hair removal for dull and dark skin uses the Nd:Yag laser , whose wavelength of 1064 nm is able to act in depth on the nourishing structures of the hair bulb.