A radiation excited self-luminous light source in which luminescence is provided by the impingement of beta emissions upon a phosphor within the source. A gastight chamber within the source is bounded by a concave phosphor surface and by radiationresistant window. The concave surface is of a predetermined shape designed to maximize the efficiency of the source. In addition, the gastight chamber is designed to withstand pressures of up to 15 atmospheres, thereby enabling gaseous radioisotopes of greater activity to be utilized as the source of beta emissions. In one embodiment, greater efficiency is achieved by utilizing a window having a convex surface which corresponds to the concave phosphor surface and which is positioned a predetermined distance from the phosphor surface equal to the average range of beta emissions within the chamber.