An illumination device is manufactured by providing (101) a substrate (1) having a first side (1a), sealingly coupling (106) an at least partially light transmitting cover (2) to the substrate (1) such that an enclosed space (3) is defined by at least the first side (1a) of the substrate (1) and the cover (2), providing a through-hole (4) into the enclosed space (3), and introducing (107) a luminescent material into the enclosed space (3) via the through-hole (4). By hermetically sealing (108) the through-hole (4) after the introduction of the luminescent material, the enclosed space (3) becomes sealed and hence luminescent materials being relatively sensitive to e.g. water and/or oxygen become protected from exposure to the environment.