A lot of brands like to boast about how their watches light up at night. But what exactly makes this happen? We talk about phosphorescence, radium and all the technicalities behind the glow. Science doesn’t have to be boring at all: Explore the much-hyped world of watch lume.
Ever since WWI, when commanders wanted their men to be able to tell the time at night, watchmakers have been exploring ways to make this possible. Since traditional watches, including today's top luxury timepieces, do not use electricity, the primary way to light up a watch has been certain kinds of chemicals that are painted onto the dial and hands. The result is an amazing glow-in-the-dark-phenomenon. And while it was initially reserved for soldiers and professionals, lumed watches have gone on to become mainstream, and a much discussed feature among collectors. But how does it all work?
Luminous designs are not restricted to just the watch industry and can be found in various branches with a notable example being in cars. Lighting technology plays a crucial role in motor vehicles that goes beyond the usual safety aspects. Much of that wow factor that you get in cars is generally a reaction when you see some rather fancy lighting. Furthermore, you see light designs being extended to computers, decor and even furniture. It just so happens that watches have more years behind the technology.
A light in the darkness
Phosphorescence is what happens when certain chemicals can absorb light and then emit it. Essentially, the energy transmitted by light photons "excite" the receptive atoms, charging them up and causing them to subsequently emit their own photons. Some chemicals do this over the space of milliseconds - they glow brightly while the light is on them, which is called fluorescence. But other chemicals continue to emit light for much longer periods after the light source is removed. This is phosphorescence, and chemicals that can do it are called phosphors.
The atomic age
Strange to think, but the dangers of radioactivity were very poorly understood by the general public in the first half of the twentieth century. Not long after Marie Curie discovered radium, but before it killed her, the highly radioactive element had been made into paint and was applied to watches by factory workers who were told the substance was perfectly safe.
Radium paint is an excellent phosphor - the radiation itself is an energy source that greatly improves the potency of the zinc sulfide phosphor that is typically added. Radium-painted watches were worn for many decades until the dangers became widely known, and they were banned outright until 1968. Today they continue to be highly sought by vintage collectors - while the glow has faded, they can be identified by Geiger counters, since they will remain radioactive for thousands of years.
Various alternatives to radium have been attempted over the years. Electroluminescence was developed and is used in battery-powered watches - it lights up a phosphor when an electric current is applied with a button press. Such technologies are seen as unsuitable for mechanical watches.
Some manufacturers replaced radium with tritium, another radioactive chemical, but much less powerful and posing no health risk to the wearer, since it is stored as a gas in little tubes. Tritium shines brightly and cleanly, but the downside is its half-life of about 12 years, after which it fades away. Still, it is used with some success today by brands such as Ball and Luminox.
A new light
By the late 20th century, a new solution had been found in the unassuming form of strontium aluminate. In many ways, it was a miracle. Unlike other phosphors, strontium aluminate does not require an energy source to work (such as radiation or electricity). It has the capacity to simply absorb and emit a large amount of light. It literally glows in the dark by itself.
Even better, it continues to do this as long as new light is supplied. No one knows how long it will take for it to stop working, since it's a relatively recent invention, but it clearly lasts several decades at the very least. Indeed, strontium aluminate's only limitation is the necessity for new light. Where radium and tritium dials glow permanently at all times, this non-radioactive, non-toxic lume fades over the course of about seven hours.
The substance was discovered in 1993 in Japan, and in 1998 it was rebranded with the somewhat more glamorous name of Super-LumiNova. Soon, this brand of lume was supplying virtually the entire Swiss watch industry, and it remains the industry standard today.
A few notable watches using quality lume:
Panerai Radiomir 8 Days Ceramica
Panerai has a special connection to watch lume, since it was one of the first to invent radium-based watch paint. It filed the patent for this in 1916, long before it even produced its own watches. The Radiomir series is named after this original lume, although today it uses Super-LumiNova, as in this gorgeous ceramic example.SHOP: Panerai Radiomir 8 Days Ceramica
TAG Heuer Formula 1 Calibre 16
This is an excellent new model in TAG Heuer's ever-popular Formula 1 series. With a Swiss-made mechanical movement, it's a superb sports watch even beyond its sleek good looks. Try it out in the dark to see the lume really shine.SHOP: TAG Heuer Formula 1 Calibre 16
Omega Speedmaster Dark Side of the Moon
The Dark Side of the Moon is a twist on the beloved Speedy known for its high-tech materials - including a zirconium oxide ceramic case and chromium nitrate tachymeter. Omega fans also appreciate the excellent quality LumiNova that perfectly fits the space theme.SHOP: Omega Speedmaster Dark Side of the Moon
Rolex Submariner Date
The Sub is of course probably the most famous and desirable diving watch in the world. And diving watches are perhaps the foremost cases for watches where the lume really matters. Rolex uses its own branded lume, "Chromalight", which is comparable in quality to Super-LumiNova (some speculate it's the same thing).SHOP: Rolex Submariner Date
Breitling Superocean 44 Special
Despite being known primarily as an aviation-based brand, the Superocean diving watch has proved highly successful and well-received for Breitling. It's reliable, good-looking and highly legible, especially in the dark where the chunky hands and indexes dazzle.SHOP: Breitling Superocean 44
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