The present invention relates to a flat lighting device (1) comprising a carrier (2) including a thermally conductive layer (7), at least one solid state light source (3) arranged on a front side of the carrier (2), and an electrically insulating cover member (5a, 5b) in thermal contact with said front side and a back side opposite said front side. The cover member (5a, 5b) is adapted to transfer heat from said carrier (2) out of the lighting device. The cover member (5a, 5b) comprises an optical structure (4) arranged on the carrier (2) in front of the at least one light source (3) and adapted to direct the light emitted by the at least one solid state light source (3).