Lume
Lume is the luminescent material applied to a watch's hands and hour markers so the time can be read in the dark.
Lume — short for luminescent material — is a photoluminescent compound applied to a watch's hour markers, hands, and sometimes bezel pip so the watch can be read in low or no light. It charges from ambient light (sunlight, a flashlight, even a phone screen) and emits a soft glow that decays over several hours.
The two dominant lume formulations today are Super-LumiNova (a Swiss-made non-radioactive strontium aluminate developed by RC TRITEC AG) and Lumibrite (Seiko's proprietary equivalent). Both replaced the tritium-painted dials common before the late 1990s and the radium dials of the early-to-mid 20th century, neither of which were photoluminescent — they were radioactive and glowed by their own decay.
Lume quality is a real differentiator between watches. A well-applied modern lume (e.g. Seiko's "high-intensity" Lumibrite on the Marinemaster or Tudor's Black Bay) charges in seconds and reads cleanly for 6-8 hours; a budget lume application is gone within an hour.
Common questions
Is watch lume radioactive?
Modern lume (Super-LumiNova, Lumibrite) is not radioactive — it's a photoluminescent compound that charges from ambient light. Older tritium and radium dials were radioactive but those have not been used in new mainstream watches for decades.
How long does lume last?
Charged lume on a quality modern watch is clearly readable for 6-8 hours, dimming gradually. A full overnight charge with a flashlight produces noticeably stronger and longer-lasting glow than ambient daylight.