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Lume (luminescent material) is applied to watch hands and dial indices to enable reading in low-light or dark conditions. Modern watches use Super-LumiNova, a photoluminescent material that absorbs light energy (from any light source) and re-emits it as a visible glow. It charges quickly under light and glows for hours afterward. The brightness and duration depend on the amount of material applied and the quality of light exposure during charging.
Super-LumiNova (SLN) is photoluminescent—it requires external light to charge and gradually dims in the dark. Tritium is a radioactive isotope that produces continuous self-powered glow without needing light exposure, maintaining constant (if dim) luminosity. Tritium was standard until the 1990s when radiation concerns led to replacement by SLN. Some watches—notably Ball Watch Company and select tactical watches—still use tritium gas tubes (T-Swiss Made designation), which glow constantly for 20+ years.
Super-LumiNova comes in multiple colors—the most common is green-blue BGW9 (used by most brands for maximum brightness), white C3 (slightly less bright), and various custom colors. Green BGW9 offers the highest luminosity. Some brands apply different lume colors to hours and minutes hands for distinction in the dark (e.g., green vs. blue). The underlying pigment color affects both day appearance and night glow intensity—yellow-green lume generally outperforms white or blue.
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