A light-based skin treatment device comprising: a treatment light source ; a treatment light exit window via which, during operation, treatment light generated by the treatment light source is applied to skin of a user, wherein the treatment light exit window comprises an optically transparent material arranged to contact the skin during operation; and an imaging unit comprising an image sensor arranged to generate an image of the skin during operation; an optical waveguide comprising a treatment light receiving surface , an imaging light exit surface and a main surface , wherein: said treatment light receiving surface is arranged to receive the treatment light so that the treatment light enters the waveguide at the treatment light receiving surface; said main surface comprises the treatment light exit window and is arranged to transmit the treatment light so that the treatment light exits the waveguide at the treatment light exit window; characterized in that said imaging light exit surface is arranged with respect to the main surface to receive light reflected at the main surface by total internal reflection at positions where, during operation, no skin is in contact with the main surface; said image sensor is arranged to receive from the imaging light exit surface light which is guided by total internal reflection from the main surface towards the imaging light exit surface. a treatment light source ; a treatment light exit window via which, during operation, treatment light generated by the treatment light source is applied to skin of a user, wherein the treatment light exit window comprises an optically transparent material arranged to contact the skin during operation; and an imaging unit comprising an image sensor arranged to generate an image of the skin during operation; an optical waveguide comprising a treatment light receiving surface , an imaging light exit surface and a main surface , wherein: said treatment light receiving surface is arranged to receive the treatment light so that the treatment light enters the waveguide at the treatment light receiving surface; said main surface comprises the treatment light exit window and is arranged to transmit the treatment light so that the treatment light exits the waveguide at the treatment light exit window; characterized in that said imaging light exit surface is arranged with respect to the main surface to receive light reflected at the main surface by total internal reflection at positions where, during operation, no skin is in contact with the main surface; said image sensor is arranged to receive from the imaging light exit surface light which is guided by total internal reflection from the main surface towards the imaging light exit surface. said treatment light receiving surface is arranged to receive the treatment light so that the treatment light enters the waveguide at the treatment light receiving surface; said main surface comprises the treatment light exit window and is arranged to transmit the treatment light so that the treatment light exits the waveguide at the treatment light exit window; characterized in that said imaging light exit surface is arranged with respect to the main surface to receive light reflected at the main surface by total internal reflection at positions where, during operation, no skin is in contact with the main surface; said image sensor is arranged to receive from the imaging light exit surface light which is guided by total internal reflection from the main surface towards the imaging light exit surface. <EOS>