Lume

Lume is the luminous material applied to a watch's hands, hour markers and bezel pip so that the time can be read in the dark. Nearly all modern lume is photoluminescent: a strontium aluminate pigment doped with rare earth elements, sold under trade names such as Super-LumiNova and, in Seiko's case, Lumibrite. It has no energy source of its own. It absorbs light, then re-emits it, glowing intensely for the first minutes and fading over the following hours.

The chemistry is specific. The workhorse compound is strontium aluminate doped with europium and dysprosium. Europium does the emitting and dysprosium acts as a trap that releases the stored energy slowly, which is what turns a flash into an afterglow. It was developed by Nemoto and Company in Japan in the early 1990s and reached Swiss watchmaking through the joint venture that gave Super-LumiNova its name in 1998, displacing the zinc sulfide pigments before it.

The decay is not linear, and that is the source of most disappointment. Brightness falls steeply in the first few minutes, then flattens into a long, dim tail. A marker that is dazzling at bedtime and barely visible at three in the morning is behaving as the physics requires, a point why lume does not last all night works through in detail.

The alternatives

Tritium gas tubes are the other live technology: sealed borosilicate glass tubes lined with phosphor and filled with tritium gas, which excites the phosphor continuously. They glow without charging and go on doing so at night, but they are dimmer than a freshly charged pigment and they fade permanently as the tritium decays, with a half-life of about 12.3 years. Makers of tube watches typically quote a useful life of 20 to 25 years.

Two older materials survive on vintage watches. Radium paint, used from about 1910 until the 1960s, is self-luminous and radioactive, and dials painted with it should be left undisturbed rather than cleaned or scraped at home; loose flakes are the hazard, not the sealed dial. Tritium paint replaced it, and dials from roughly the 1960s to the late 1990s often carry a T, a T SWISS MADE T, or a T<25 marking to show it.

Both have usually stopped glowing altogether, and the reason is not that the isotope ran out. Radium-226 has a half-life of about 1,600 years, so a 1950s dial is barely less radioactive than it was. What died is the phosphor, destroyed from the inside by decades of radiation. A dead radium dial is still an active one. Our vintage buying guide covers handling it, and Super-LumiNova compared with tritium and Lumibrite sets the three side by side.

How to read the spec

Pigment grades describe color and brightness. C3 is the familiar bright green emitting at around 520 nanometers, BGW9 the blue that looks white-ish in daylight, and there are brighter and slower-fading formulations besides, marketed with suffixes such as X1. Green typically looks brightest to a dark-adapted eye, whose peak sensitivity sits near 507 nanometers, close to where C3 emits and well away from a deep blue.

The numbers behind those grades come from DIN 67510 part 1. A sample is charged with a standard light source, its luminance is measured in millicandela per square meter after 10 minutes and after 60 minutes, and the afterglow time is recorded as the point where it falls to 0.3 mcd per square meter, roughly a hundred times the threshold of a dark-adapted eye. ISO 6425 asks a blunter question of diver's watches: the time and the running indicator must be readable at 25 cm in complete darkness, as covered in our ISO 6425 guide.

Why application beats grade

Grade matters less than application. A thick, deep block of pigment in a wide handset will out-glow a thin wash of a better grade every time, which is why some dive watches read easily at 3am and some dress watches with the same pigment do not. Depth of fill, marker area and dial contrast all count for more than the letter on the spec sheet, the case made in dials, hands and legibility. Seiko dive watches have their reputation here for exactly this reason: the pigment is unremarkable and the application is generous.

In practice

Charge lume with strong light before you need it. Direct sunlight is on the order of 100,000 lux, an office is 300 to 500, so a minute outdoors is worth more than an evening under a lamp. A few seconds under a UV torch does as much again, since absorption peaks in the ultraviolet and blue; point it at the dial, not at anyone's eyes. Charging does not wear the pigment out.

Fading over the night is normal and not a fault. Lume applied to a dial and to hands from different batches can also age at different rates, which is worth knowing when judging a vintage watch for originality: a relumed dial will often be brighter and cleaner than the hands beside it.

Common confusions

Modern lume is not radioactive. Strontium aluminate is an inert pigment, and a watch made in the last twenty-five years contains no isotope at all unless it has visible glass tubes. The word lume is also not a brand or a material; it is a category, and a listing that says only "lume" tells you nothing about which one.

The last confusion is between initial brightness and duration. They are separate properties, and a formulation tuned to punch hardest in the first minute is not automatically the one still readable at dawn. The fuller treatment is in watch lume explained.