There is disclosed a self-luminous source of light preferably as applied to illuminated signs and safety markers which emits a high level of uniform light. The invention utilizes a beta emitter, preferably tritium gas, which is contained within a sealed glass enclosure having an interior coating of a phosphor. The glass enclosure is contained within an outer enclosure formed of a plastic filled with a fluorescent dye. The phosphor coating on the glass enclosure of the tritium gas absorbs beta radiation from the tritium and emits radiation in a wavelength approximately 360 nanometers which excites the fluorescent dye in the plastic outer enclosure.