Hair has a growing cycle as anagen (growth phase), catagen (transitional phase) and Telogen (Resting Phase). During the hair removal treatment, thermal energy is delivered to the hair follicle and it destroys the hair follicle.
At the time, the hair on anagen is removed effectively but the hair on catagen and telogen isn’t removed because they do not absorb the laser on the epidermis. So these hairs should be removed when they become anagen by additional treatment.
Therefore, in order to remove all hair, we need hair removal treatment based on the hair cycle repeatedly. It is normally 4 to 6 times of treatment based on skin type and hair type. For thin hair or bright hair which has less melanin, more treatments sessions at shorter pulse duration ( 3-5msec ) can be required