Watch Lume - A Brief History of Nighttime Visibility

When I was a kid, there were two ways to see the time at night. Either press a button for a subpar light (cough, Casio digital) or rely on glow-in-the-dark paint on the hands and possibly the indices. What I didn't know is that luminescence in mechanical watches goes back a century and was actually very dangerous in the beginning. It's among the last of the revolutionary advances in watchmaking as it provides legibility in total darkness without an active power source. Most types today need an external light source to "charge," but it's a passive action by the wearer and when nighttime hits, the dial is ready to glow. Back in the day, however, radioactive lume always glowed, but also brought real dangers with it. Let's take a look at how luminescence has evolved in the last 100 years with four primary types.

Radium

As the name suggests, radium is radioactive and consequently dangerous to both the wearer and particularly watchmakers and workers applying the lume. A paintable version of radium was invented in 1908 and used on watch dials into the 1960s. It's very effective as a lume, but emits alpha, beta and gamma radiation and can potentially be inhaled (and swallowed). Radium has a half-life of 1,600 years, so even if old watches no longer glow (due to a breakdown of zinc sulfide within the lume), they're just as radioactive as they were 100 years ago. As radium decays, it also produces the gas radon, which is a major cause of lung cancer. In fact, a study in the UK found that just 30 antique watches with radium dials kept in a small, unventilated room elevated radon levels to 134 times the "safe" level.

In the early 20th century, young women were typically employed to put radium on watch dials and it was common to lick the brushes to a point for accurate applications. This resulted in "radium jaw" that caused distortion and cancer of the lower jaw, while the body recognized radium as calcium, which led to bone cancer. This is just the start of major health issues faced by the Radium Girls as they became known, who sued in 1925 for occupational disease and corporate recklessness. By 1930, the practice of pointing radium brushes by mouth was stopped and radium sickness drastically declined. Health problems persisted, however, until radium was replaced in the 1960s.

Tritium

Tritium is also radioactive, but much less than radium as it only emits low levels of beta radiation. However, it's still problematic in a paint form as it can go from the dial through the case back to a wearer's skin (many case backs at the time were plastic). It was used as an effective lume into the 1990s and is similar to another radioactive lume, promethium. However, promethium has a half-life of only a couple of years, while tritium's half-life is around 12 years. Tritium paint was banned in watchmaking in 1998, but is still used as GTLS (gaseous tritium light sources) in tiny glass tubes like miniature light bulbs. The most well known watchmaker to embrace this is Ball Watch, which has some of the most unique and effective lume in watchmaking.

LumiNova

Although known today as a Swiss lume, LumiNova has its roots in Japan via entrepreneur and lume painter Kenzo Nemoto. He developed a non-radioactive material for lume after the bombing of Hiroshima and Nagasaki, when anything radioactive was considered dangerous and unwanted. He later perfected a phosphorescent material that led to LumiNova in 1993. This safe phosphor-based luminescence doesn't decay like radioactive tritium or promethium, and is very resistant to temperature fluctuations. Nemoto & Co., founded in 1962 by Kenzo, partnered with RC Tritec AG and became LumiNova AG Switzerland in the late 1990s.

The LumiNova paint was strontium aluminate and completely free of radioactivity, up to 10 times brighter than radioactive counterparts and could even glow in two different colors, green and blue. Blue glowed longer than green, but green was brighter. Unlike radioactive lume that always glowed, LumiNova was charge by an external light source, releasing its glow in the dark. It was effective, but had a disadvantage to tritium tubes that always glowed. Consumers, however, embraced the compromise as LumiNova generally glows brighter than tritium tubes and doesn't break down over time.

Super-LumiNova

Super-LumiNova isn't really a superior form of LumiNova, but simply the official name of lume sold by LumiNova AG Switzerland. The "Super" designation means it's 100% Swiss lume. Other brands use LumiNova with their own names attached. For example, Seiko calls its LumiNova, LumiBrite. Super-LumiNova has different grades today and therefore different levels of brightness and longevity. The highest grade is X1, which is used on the latest Horage watch dials. There are also many color options within these grades - C3 is the brightest with a cream color, BGW9 is close to C3 and white, C5 has about 89% of C3's brightness and is mint green, C9 is turquoise and about 83% as bright as C3 and so on. Chromalight is Rolex's form of Super-LumiNova and basically a marketing term, like calling 904L stainless steel "Oystersteel." Chromalight is likely just Super-LumiNova C9, although Rolex may apply it in a specific way to enhance intensity and longevity.

There are other lumes out there, such as Citizen's proprietary Natulite, but this covers the most common throughout history. It's quite interesting that the early days actually brought some dangers to consumers, but today's lume is completely safe and very effective.

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Erik Slaven