A device (10) for light based skin treatment is provided. The device (10) comprises a light source (18) for providing an incident light beam (21) for treating a skin (30), optical elements for focusing the incident light beam (21) in a focal point (22) inside the skin (30), and a skin interface element (11) for, during use of the device (10), providing optical coupling of the incident light beam (21) from the device (10) into the skin (30). The skin interface element (11) comprises a transparent exit window (12) for allowing the incident light beam (21) to leave the device (10), on top of the exit window (12), a transparent mixture (13) of a polar substance and an apolar substance, and on top of the transparent mixture (13), a transparent foil (14), the transparent foil (14) being more hydrophobic than the exit window (12).