Activation of a zinc sulphide phosphor with b-radiation will provide such a self-sustaining light source. Tritium in the form of HT gas is an ideal radionuclide for this purpose because, owing to its relativeley low radiotoxicity, it can be used in a high specific activity form with resonable safety, resulting in a high light output per unit volume. Gaseous tritium light sources usually consist of borosilicate glass capsules or tubes in a variety of shapes and size, internally coated with a zinc sulphide phosphor. After coating the capsule is evacuated, backfilled with HTx sealed. The phosphor then becomes activated by the b-radiation from the tritium and emits light. Depending on the type of phosphor used, the emitted light varies from ultraviolet to low infrared. The internal pressure of the HT within the capsule can range from about 10 kPa up to about 250 kPa, depending on the brightness requirement.