A hair treatment (40) device is provided comprising a light-based detector (10) for detecting a hair (22) near a skin surface (21) the detector (10) comprising a light source (11), optical elements (14, 16, 17, 18) and a polarization-sensitive light sensor (12, 13). The light source (11) is provided for generating a light beam (31) with an incident polarization. The optical elements (14, 16,17, 18) are provided for focusing the light beam (31) at the hair (22) near the skin surface (21). The polarization-sensitive light sensor (12, 13) is provided for detecting light interacted with the hair (22) or the skin surface (21) and having a predefined linear polarization. The light source (11) and/or the optical elements (14, 16, 17, 18) are arranged to cause the light beam (31) to simultaneously have multiple spatially separated foci (51, 52, 53) at or near the hair (22) or the skin surface (21).
