A main unit of a lighting device with expandable functionality. The main unit includes at least one light source, driver electronics, and a housing with an at least partially light-permeable housing wall. The main unit further includes a coupling element for forming a wireless coupling interface to an extension unit, wherein the coupling element is located within the housing and is designed such that the wireless coupling between the main unit and the extension unit through the partially light-permeable housing wall is carried out. Furthermore, an extension unit and a lighting device are provided.