The invention provides a hair treatment device (40) comprising a detector (10) for detecting a hair (22) near a skin surface (21). The detector (10) comprises a light source (11) for generating a light beam (31) with an initial polarization direction, a polarization modulator (32) for time-dependent modulation of the polarization direction of the light beam (31) between at least a first polarization direction and a second polarization direction, optical elements (14, 16, 17, 18) for focusing the light beam (31) at the hair (22) near the skin surface (21), and a polarization-sensitive light-based detection unit (13) for detecting light interacted with the hair (22) or the skin surface (21) and for discerning when the interacted light has a polarization-direction component different from the initial polarization direction.
