Watch Lume 101: The Complete History and Primer on Reading Time in the Dark

Luminous material on watch dials — referred to in shorthand as “lume” by both industry insiders and avid enthusiasts — is an element taken for granted nowadays. It can be said without hyperbole that a watch dial without lume is a stylistic outlier in this modern era, but this wasn’t always the case. Discerning the time in the dark presented a major problem for watchmakers in the early days before the widespread use of electricity in homes, and at first the only solution that could be offered was an audible rather than visual one: watches that chimed the time on demand, like minute repeaters and sonneries. Only a handful of wealthy individuals could afford these highly complicated timepieces, however, so a more widely accessible technology was needed as wristwatches spread to the general populace. Around the dawn of the 20th Century, watchmakers turned their attention to making watch dials that could be read in the dark, paving the way for the luminous materials that are still used commonly today. The road to perfecting the technology, however, would not be easy, and at times would even be dangerous.

Radium

The first material applied to watch dials for nighttime luminescence was a paint made from radium with zinc sulfide, which, thanks to radium’s half-life of 1,600 years, offered a long-lasting glow during that period before dimming — the catch being that radium, as its name implies, is radioactive. One of its earliest uses can be traced to a pioneer of developing luminescent paint for watches, Guido Panerai, the Florentine watch retailer who made watches, as well as diving gear and other precision instruments, for the Italian navy during the First and Second World Wars. At the request of his military clients for a self-sustaining illumination agent that could render its vital equipment visible in the dark and underwater, Panerai developed a substance that he called “Radiomir,” which was based on radium bromide and also contained zinc sulfide and mesothorium. This highly luminous paint was applied to the dials of the first watches that Panerai made, initially sold exclusively to military divers, and it even gave that model the name that it still uses today, the Panerai Radiomir (modern example pictured above). 

Radium-based luminous substances like Radiomir, however, would prove to be too hazardous for long-term use. As detailed in the book and movie, Radium Girls, the predominantly female watch factory workers of the 1920s (above), who painted the watch dials with the radium compounds and even licked their brushes to keep the tiny bristles at a fine tip, started falling ill and dying at alarming rates. The scandal led to lawsuits against the companies that produced the material and eventually, safer working conditions and an abandonment of radium as a luminous substance in watchmaking by the 1960s. The watchmaking industry, accordingly, turned its attention to other methods of illuminating its dials.

Tritium

Replacing radium for a short time as the luminous substance on watch dials was a compound called Promethium (Pm-147), which was less radioactive but had a half-life of only two-and-a-half years and thus a much shorter period of luminescence. The one that followed, which had more staying power, was Tritium H-3, a radioactive isotope of hydrogen with a decent half-life of 12.3 years and, as a low-energy beta emitter, far less hazardous to work with than radium. Tritium, which was used in small amounts to boost the explosive power of the atomic bomb detonated in May 1951 over the Enetawak Atoll, was not entirely without risk, however. Used on surfaces like dials, tritium had a tendency to diffuse, seeping through the case and into the skin of a wearer. Tritium-based paints on dials, which were used by numerous watch brands, were banned in 1998.

A handful of watch brands are still legally, and safely, using tritium today, however, albeit in a different fashion than slathering it directly onto their dials. Tritium does, in fact, boast some advantages over the luminous substances that followed it (which are explored in detail below). Foremost, unlike those substance, it is self-sustaining as a luminous agent and doesn’t require an outside light source to “charge up” for its nighttime glow. The GTLS (gaseous tritium light sources) process, developed in the 1990s, involves inserting small amounts of tritium gas into tiny narrow glass tubes, which are less permeable than a watch crystal over the dial and also have been treated on their inner surfaces with luminous powder that is activated by the electrons in the gas. The tubes are then strategically placed at key parts of a watch’s dial, under the crystal, for maximum lighting effect: usually the hands, hour markers, and numerals. Watch brands that have made this technology part of their stock-in-trade include Ball (whose Engineer III Marvelight Chronomater "Caring Edition" is featured above), Luminox, Marathon, and Traser.

Super-LumiNova

Aside from these outliers, however, tritium has been almost entirely supplanted in the modern era by a non-radioactive luminous substance that traces its origins all the way back to 1941 and a Japanese inventor named Kenzo Nemoto. Contracted by the Japanese military to make luminous paint for the gauges of WWII-era aircraft, Nemoto turned his attention to creating a phosphorescent, non-radioactive type of paint to apply to watch dials after the end of the war, founding the company in 1962 that would eventually develop LumINova in 1993. Nemoto & Co. registered a patent for the substance and licensed it to the Swiss firm RC Tritec AG, which gave it the name Super-LumiNova and remains to this day the only official manufacturer of it in Switzerland. Unlike all the materials that preceded it, LumiNova was different in that it acts as something of a light absorbing, energy-storing battery. The substance, based on a compound called strontium aluminate, doesn’t glow on its own but needs to be activated, or “charged,” by an external light source; in essence, the light your watch dial absorbs during the day is ideally enough to make its luminous elements glow brightly at night, after which it will require charging again the next day.  Super-LumiNova, the undisputed standard in the luxury watch industry, usually glows green when the lights go out, and it’s usually applied not to the entirety of the dial itself but to the elements of the dial that are most important to discern in the dark: the hour and minute hands; one or more of the hour numerals and/or indexes; the tip of the seconds hand so the wearer can see at a glance if the watch is running;  often in the numerals and markers of a bezel, like those used to set immersion times on divers’ watches (like the Blancpain Fifty Fathoms pictured above).

However, watch companies have expanded the scope of Super-LumiNova’s applications and executions over the years. There are now eight different “afterglow” color variations available to watchmakers, from greens and blues to more exotic shades like orange, pink, and violet; and three grades, from standard to “X1,” the latter achieving the longest duration of luminescence after light absorption. Some watchmakers have even chosen to create “fully-lumed” dials rather than using lume for details like hands and markers, like TAG Heuer’s Aquaracer Professional “Night Diver,” Bell & Ross’ BR 03-92 "Full Lum" Limited Edition, and Mühle Glashütte’s S.A.R. Rescue Timer Lumen (pictured above). Despite the ubiquity of Super-LumiNova throughout the industry, a handful of watchmakers have developed their own proprietary technologies and luminous substances (often using the same strontium aluminate compound as their base) to stand out from the pack.

Indiglo

Quintessential American watchmaker Timex, which had been making affordable timepieces for the masses since its founding as the Waterbury Clock Company in 1854, came up with its own non-radioactive solution for illuminating watch dials a year before LumiNova made it to market. Called Indiglo, and debuting on the first Timex Ironman watch, the system consisted of a phosphor-based luminous panel with a clear conductor, between two thin sheets of plastic or glass, linked to an electrode that illuminated the panel (i.e., the dial) with a bright blue-green backlight when the wearer pressed a button. Timex has patented the Indiglo technology and remains the only watch manufacturer that can use it, even creating a separate, fully owned Indiglo Corporation to control its licensing to outside clients and prevent copies from competing watch firms. Though Indiglo is still around, many Timex watches, especially the ones most geared toward enthusiasts and designed to evoke vintage models released before Indiglo’s debut, now use Super-LumiNova for their dial details instead.

LumiBrite

Developed by Seiko, which uses it exclusively, and first applied to a watch dial in 1995, LumiBrite takes the base compound in Super-LumiNova — strontium aluminate — and adds europium and dysprosium for a luminous paint that Seiko claims absorbs light faster and glows for a longer duration after being activated. LumiBrite is basically Seiko’s own version of Super-LumiNova, licensed from fellow Japanese company Nemoto & Co., the firm founded by Kenzo Nemoto that patented the original compound. Just about any watch from Seiko and its more luxurious sister brand, Grand Seiko, will feature the LumiBrite substance somewhere on its dial, often in the hands and markers.

Chromalight

Rolex, a watchmaker renowned for its patented and/or proprietary inventions — like the Oyster case, Glidelock bracelet, and Chronergy escapement, to name just a few — also lays claim to its own luminous substance, called Chromalight. Like most other watch companies, Rolex used first radium, then tritium, then Super-LumiNova on its dials until switching to its own luminous substance in 2008. Chromalight starts out as a metal oxide powder composed of strontium aluminate, europium, and dysprosium, which is then heated to phosphorescent crystalline form. This glowing powder is mixed with a liquid resin to create the luminous paint that Rolex applies to the hands, indexes, and other details of its watch dials. Unlike Super-LumiNova, which in its original form emits a bright green glow in low light, Chromalight always glows a bright blue. Rolex also touts its substance’s longer brightness duration of eight hours, which is about double that of its green-glowing predecessor.