Luminescence is the emission of light by materials that are relatively cool. This is different from the light emitted from incandescent materials, such as burning charcoal, wood, paper etc., as the light in those cases results from heat. Since the emission of light is done by relatively cooler materials, luminescence is also called cold-body radiation.
Gaseous Tritium's light output is known as Radioluminescence. Radioluminescence.is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as beta particles.