Abstract: The invention relates to light sources and, in particular, to light sources for use in navigational aids such as lighthouses and lighted buoys.In more detail, the light source of the present invention comprises a radio isotope fuel source, thermal insulation against heat loss, a biological shield against the escape of ionizing radiation, a material having a surface which attains incandescence when subject to isotope decay heat and means for transferring energy from the insulated fuel source to produce incandescence of the surface and thereby emit light.