Tritium is a rare radioactive form of hydrogen that powers the self-glowing lights on some watch dials. It became a watch lume after radium was banned from wristwatches in 1968. Its half-life is about 12.3 years, so its glow fades steadily over time.
How does tritium make a watch glow?
In the gas-tube form, tritium gas is sealed inside small hollow glass tubes. The inside of each tube is coated with a layer of zinc sulfide phosphor. As the tritium decays, it gives off electrons. These strike the coating and make it light up. The light comes from the tritium itself, not from stored daylight. This is why it is called radioluminescent: the glow is driven by radioactive decay.
Why does tritium lume fade?
Tritium’s half-life is about 12.3 years. After each half-life, its light output halves. So tritium lume grows steadily dimmer as the years pass, and an older dial will not glow as brightly as it once did. Even so, tritium tubes keep glowing longer than Super-LumiNova. That is why military watches often use them. The Swiss firm mb-microtec, a maker of tritium light sources, was founded in 1969.
Where it comes from
The name comes from the Ancient Greek word for ‘third’, and the isotope is also called hydrogen-3. Edward Shapiro patented the idea of using tritium in self-luminous compounds in 1953. Before tritium, watchmakers used radium for self-glowing paint, from the early twentieth century onwards. Once radium was banned from wristwatches in 1968, tritium took its place. Dials with tritium lume were marked to show it, usually with T, TT or H3. Omega is among the makers whose watches have used it.
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