An isotope of hydrogen that is used to make watch hands and indices glow in the dark. Until a few years ago, hands and indices were often painted with tritium paint. But because tritium is mildly radioactive, and feared by some to be a health hazard, tritium paint is no longer used on dials. (The consensus among scientists is that, despite consumers’ apprehensions, tritium paint on watch dials presents no danger.) Instead, some watch companies affix tritium-gas-filled tubes to their dials’ hands and indices. The tubes are more acceptable commercially because the gas is contained and hence emits even less radiation than tritium paint does. Tritium is unlike other luminous substances used on watch dials (Super-LumiNova is the most common of these) because it does not require exposure to light to make it glow: it will do so for years without fading appreciably.
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