A light based
skin treatment device (10, 20) is provided, comprising a
light source (18) for providing an incident
light beam (21) for treating a
skin (30) by
laser induced
optical breakdown (LIOB) of hair or
skin tissue, a transparent exit window (14) for allowing the incident
light beam (21) to exit the device (10, 20), and an optical
system for focusing the incident
light beam (21) into a focal spot (221, 222) in the hair or
skin tissue outside the skin treatment device (10, 20). The exit window (14) comprises an outer surface (41, 42, 43, 44) having optical scattering properties such that, for a predetermined power and
pulse duration of the incident light beam (21), when the outer surface (41, 42, 43, 44) is in contact with a medium having a
refractive index equal to a
refractive index (n1) of the exit window (14), a dimension of the focal spot is sufficiently small for a
power density of the incident light beam (21) in the focal spot to exceed a threshold value for inducing a LIOB phenomenon in the focal spot, and when the outer surface (41, 42, 43, 44) is in contact with a medium having a
refractive index equal to a refractive index (n2) of air, a dimension of the focal spot is sufficiently large for a
power density of the incident light beam (21) in the focal spot
not to exceed the threshold value for inducing a LIOB phenomenon in the focal spot.