Light based permanent hair reduction can only target hair that is in the growth (or Anagen) phase. Hair grows in waves so treatments are generally spaced 4-6 weeks apart to ensure that the next wave of hair is in the growth phase. To effectively treat all of the waves of hair while they are in the growth phase a series of treatments will be required to complete the desired result.
Like Your Look treatments such as the Energist VPL (variable pulsed light) use light to target the melanin in the hair follicle and heat it rapidly to a target temperature. When it hits this temperature the hair follicle no longer produces hair. Hair grows in cycles so repeating this process a number of times creates the overall permanent hair reduction.