Lume: how watches glow in the dark

What lume does

Most modern watches use a photoluminescentpigment, typically strontium aluminate sold under names such as Super-LumiNova. It absorbs energy from light, stores it, and slowly releases it as a glow. It isn't radioactive and contains no battery.

Why it fades

The glow is brightest in the first minutes after charging and then dims steadily as the stored energy runs out. Over a night it becomes faint but often stays readable for some hours, depending on how much lume there is, its colour and how well it was charged. Blue-green tints usually glow longest; other colours can be dimmer.

How to charge it

• Daylight works best, because it contains plenty of ultraviolet and blue light. A few minutes outdoors makes a visible difference.
• A bright LED torch or lamp also works; hold it a short distance from the dial for a minute or so.
• Dim indoor light charges lume only weakly, so a watch worn indoors all evening may glow less at night.
• Don't use heat, and don't leave the watch in hot sun on a dashboard to “charge” it.

Other luminous technologies

• Tritium gas tubes glow continuously without charging, using a sealed, low-level radioactive gas. They gradually dim over years.
• Older tritium paint (common until the 1990s) has largely stopped glowing; it often turns a cream colour.
• Radium paint was used on some watches until around the mid-20th century and remains radioactive.