Watch lume: how it works and why it fades

Every glowing watch dial works in one of two ways. Either the material soaks up light and releases it slowly afterwards, which is photoluminescence, or it contains a radioactive isotope whose decay continuously excites a phosphor, which is radioluminescence. Almost everything people find confusing about lume follows from not knowing which one they are looking at.

Photoluminescent lume is bright, harmless and temporary: it needs charging and fades over a night. Radioluminescent lume is dim, constant and self-powered: it never needs charging and dies over decades. Modern watches are almost entirely the first kind, the exceptions being the few using sealed tritium gas tubes.

Modern lume: strontium aluminate

The material in essentially every modern luminous dial is strontium aluminate doped with small amounts of europium and dysprosium. It is a phosphor: light falling on it promotes electrons to a higher energy state, the dysprosium provides trapping sites that hold them there, and they leak back down over the following hours, emitting visible light as they go.

It was developed in Japan by Nemoto & Co in the early 1990s and is produced in Switzerland by RC Tritec under the trade name Super-LumiNova. Equivalents exist under other names, Seiko's Lumibrite being the best known, and several brands market their own formulations, such as Rolex's Chromalight. The chemistry is broadly similar across these; what differs is particle size, the dopant recipe, the binder and how thickly it is applied.

The step change over the older copper-doped zinc sulphide phosphors was enormous: strontium aluminate is far brighter, glows for very much longer, does not degrade quickly in use and is completely non-radioactive. It swept the industry within a few years of arriving.

Reading the C3, C1 and BGW9 codes

The codes look like grades and are mostly colors. In the common naming convention, the letters and number describe the pigment's appearance in daylight and the color it emits in the dark, not a quality ranking.

• C3 looks pale green in daylight and emits the greenest, brightest glow. Green is where the human eye is most sensitive in low light, so C3 is the brightness champion.
• C1 looks close to white in daylight and emits a slightly cooler green. It is a little dimmer than C3 in exchange for a cleaner dial appearance.
• BGW9 looks white or near-white in daylight and emits blue. Blue reads as dimmer to the eye than green at equal energy, so a BGW9 dial that looks less bright is not necessarily worse material.

Separate designations are used for luminous performance grades, so a specification quoting both a color code and a grade is telling you two different things. Treat claims of "the brightest lume" with the scepticism any unmeasured superlative deserves.

Why it is brightest in the first minutes

A charged phosphor does not glow steadily and then stop. It decays continuously, very fast at first and then progressively more slowly, so brightness plotted against time falls away steeply and then flattens into a long tail.

This is why peak brightness is the wrong measurement. The German standard DIN 67510 for photoluminescent products measures luminance at fixed intervals after a defined charge and, importantly, the time taken to fall to 0.3 millicandela per square meter, the conventional threshold for perception by a dark-adapted eye. Two lumes can be equally dazzling in the first thirty seconds and differ by hours at that threshold.

The radioactive era

Before the 1990s there was no good photoluminescent option, so watchmaking used radioactive materials. The history matters, both because you will meet these dials and because one of them is genuinely hazardous.

Radium

Radium-226 paint arrived around 1910 and dominated for half a century. Its alpha decay excited a zinc sulphide phosphor mixed into the paint, producing a bright glow that needed no charging.

The human cost was severe. Dial painters, mostly young women, were taught to point their brushes with their lips, ingesting radium repeatedly. From the early 1920s, workers at the United States Radium Corporation in New Jersey and later at the Radium Dial Company in Ottawa, Illinois developed anaemia, bone necrosis of the jaw and bone cancers. The resulting litigation, known through the Radium Girls cases, is a landmark in occupational health law. Radium was progressively withdrawn from watch dials through the 1960s.

The cruel irony is that old radium dials mostly do not glow. Decades of alpha bombardment destroy the zinc sulphide phosphor crystal structure long before the radium runs out. A dead-looking radium dial is still fully radioactive.

Promethium and tritium

Promethium-147 saw brief use in the 1960s as a lower-hazard alternative, but a half-life of around 2.6 years made it useless in a watch expected to last.

Tritium, hydrogen-3, replaced radium properly. It is a weak beta emitter, and the radiation is stopped by the dial, the case and the crystal, so the external hazard is negligible. Tritium paint dials are marked on the dial itself. T SWISS MADE T, with the T flanking the words at six o'clock, says the luminous material is tritium and that the watch is Swiss made. T<25 and T25 are a different statement: they declare the tritium activity in the watch, expressed in millicuries, with T<25 meaning below the 25 millicurie ceiling. A dial can carry either convention depending on its market and its era.

Tritium's half-life is about 12.3 years, and that number is the whole story of why old dials are disappointing. A dial made in the mid-1980s has been through more than three half-lives, leaving roughly a tenth of the original tritium. The phosphor has also degraded over the same period. In combination, a genuinely period-correct 1980s tritium dial produces a glow you can barely find in a dark room, and that is correct rather than faulty. If a vintage dial marked T SWISS MADE T glows brilliantly, it has been relumed, which is worth knowing when buying a pre-owned watch.

Tritium gas tubes

The other way to use tritium is to stop painting it on. A gaseous tritium light source (GTLS) is a tiny sealed borosilicate glass tube, internally coated with a phosphor and filled with tritium gas. The beta particles strike the coating from inside and the tube glows continuously, with no charging, for its whole life.

The tubes come from a small number of specialists, notably mb-microtec in Switzerland, whose own watch brand is traser and whose tubes appear in watches from Ball, Luminox and others. Because the tritium is sealed in glass rather than exposed as paint, it stays contained even if the watch is opened. Manufacture and sale are regulated, and the rules vary between countries.

In practice a GTLS watch is not as bright as freshly charged Super-LumiNova, and it is far brighter than that same Super-LumiNova at four in the morning. Brightness follows the 12.3 year half-life, so a tube watch is noticeably dimmer after a decade and clearly dim after two, at which point tubes can sometimes be replaced by a specialist. They are also discrete objects, giving a dial of glowing sticks and dots rather than shaped, luminous-filled hands and markers.

The lume types compared

Charging lume properly

Photoluminescent lume absorbs most efficiently at the blue and ultraviolet end of the spectrum, which has three consequences most people get wrong.

1. A short blast of the right light beats a long soak in the wrong light. Twenty or thirty seconds under a UV torch held close will charge a dial far more thoroughly than an hour sitting near a window on a gray day. Point the torch at the dial and never toward anyone's eyes, keep the exposure brief, and be aware that prolonged intense ultraviolet is not kind to some dial lacquers or to strap materials.
2. Warm domestic lighting barely works. Incandescent bulbs and warm-white LEDs emit very little in the range the phosphor wants. Daylight and cool-white LEDs are much better.
3. Charging saturates. Once the traps are full, more light adds nothing, so there is no benefit in leaving a watch under a lamp all afternoon.

What a dive watch needs: duration, not peak

Peak brightness sells watches; duration is what makes them useful. A diver who charges a watch at the surface needs to read it forty minutes later, so a lume with a slightly lower peak and a much longer tail is the better tool.

ISO 6425, the standard for divers' watches, works on this principle. It requires the time to be readable in darkness at 25 cm (10 in), along with clear indication that the watch is running, which is why a dive watch's seconds hand carries a lume dot. That is legibility in real conditions rather than a brightness number, and our guide to dive watches and ISO 6425 sets out the full test regime.

Two design choices follow. Thickness matters, because more phosphor holds more charge, which is why serious dive watches have deep, generously filled markers rather than thin printed ones. And a full lume dial, where the whole dial surface is coated rather than only the markers, maximises emitting area at the cost of a particular daytime appearance, usually a chalky off-white or pale green.

Why old lume turns creamy

Discoloration is almost never the phosphor. It is the binder: lume is a powder suspended in a lacquer, and lacquers yellow with age, ultraviolet exposure, heat and moisture. Tritium and radium plots therefore drift from white toward cream, tan and eventually a deep pumpkin orange.

Even, consistent aging across the plots and the hands is what an untouched dial looks like, and it is prized accordingly. Uneven color, plots that do not match the hands, or crisp bright lume in an otherwise worn watch normally means a relume or replacement parts. Relumes are a legitimate repair rather than fraud, but they are a material fact that should be disclosed.

Modern watches complicate this by using pre-tinted compounds in cream and tan to imitate aged dials from new, so color alone no longer tells you a dial is old.