Diwali Special: The Origin, Evolution And Chemistry Of Watch Lume

Generally speaking, if you want to read the time off your watch in absolute darkness, one way to do so is by means of an auditory mechanical party-trick, aka the minute repeater complication. Usually, the said is a reserved contraption for watches of very high monetary echelons from manufacturers of the highest grade.

The only possible means to read time off your watch in dark.

Well, not all have the means or the motivation to make a hundred-thousand-dollar purchase to traditionalize an act that can be easily achieved just by looking at your smartphone. Here, if you want to throw the argument of “art is valued simply because it is” at my face, I can state the existence of lumed watches as my humble counterpoint and as means to tell the time when it’s dark without burning through your inheritance.

That’s probably the only other way to know what’s-past-what if you’re Chuck McGill from Better Call Saul or simply when the lights go out.

Chuck Mc Gill from Better Call Saul (left). Watch lume is a topic fit to discuss around Diwali (right).

Also, it just makes sense to me - a watch nerd, to talk about glowing watches in anticipation for Diwali, the day when an entire nation illuminates.

Why Do Watches Need To Glow?

To tell the time when it’s dark. Obviously! But what makes it so important to know the time when, logically speaking, you should be dreaming about that Ref. 126500LN allocation. It has got something to do with war-time utility and also the genesis of the category of “tool watches.” Some watches glow because they’re designed to be used in low-light conditions, like diver’s watches, pilot’s watches or military watches.

What Makes Watches Glow?

Luminescence is the emission of “cold light” from a substance, where light is produced by energy conversion from a source other than high temperature, unlike incandescence. The main causes for this effect include electrical currents, chemical reactions, or radiation. Now, usually when light hits most materials, some of it is absorbed and some gets reflected. However, certain materials (phosphors) have the ability to “store” the energy from light, and then release it. This is known as photoluminescence.

Photoluminescent materials can either be fluorescent or phosphorescent. The former emits all absorbed photons (light energy) very rapidly and the latter does so very gradually over a period. So, phosphorescence is the preferred principle behind the luminescence of most common lumed watches.

The Evolution Of Luminescent Materials

In the history of watchmaking materials, photoluminescence came after radioluminescence. Materials such as radium, promethium and tritium are self-luminous and don’t actually require any external light source to cause particle excitation, unlike photoluminescent materials. The use of radioactive materials for luminescence on watch dials started in the early 1900s.

Pierre and Marie Curie discovered the element radium (left). Zinc sulfide phosphor on a radium dial clock (right).

A mixture of zinc sulfide (a phosphor) and radium (for exciting the phosphor) was common in watchmaking applications. Keep in mind, the toxic effects of radioactive materials were very apparent and also attributed to many tragic tales eventually leading to the ban on use of radium on watch dials in 1968.

Soon, tritium - another radioactive material, but mildly radioactive, took over. It’s very common for Swiss watches with tritium as lume to have the marking “T SWISS T” or “T<25” on their dials.

"Radium Girls" were hired to paint dials (left). Rolex dials with tritium “T SWISS T” and “T<25” markings.

To completely get away with the detrimental consequences of using radioactive excitation of phosphors to make watch dials glow, innovation came in the form of photoluminescent materials. One such material that gradually emits light after exposure to light is Luminova. It was invented in 1993, by a Japanese company Nemoto & Co. which in 1998, collaborated with RC-Tritec AG to establish LumiNova AG Switzerland. The latter became the principal supplier of Super-LumiNova for the Swiss watch industry.

LumiNova uses a very efficient phosphor - strontium aluminate which is non-radioactive and non-toxic. A single exposure to light charges the phosphor which glows with a bright intensity and continues to glow for several hours, although with gradually diminishing intensity. The phosphor strontium aluminate is the reason why most lumed watches radiate a green or blue glow.

As a non-toxic, non-radioactive phosphor that glows ten times brighter and ten times longer than zinc sulfide, strontium aluminate is the near-ideal remedy to nocturnal constraints of telling time. Its only disadvantage in comparison to radioluminescent materials is that its intensity gradually fades until again exposed to light.

Legal And Safe Radioactive Lume

Very few watch manufacturers (including Ball and Luminox) still use radioactive tritium in their lume - but safely. Rather than its direct application on the watch dial, tritium gas is filled in tiny glass tubes which are less permeable than watch crystals. The inner surfaces of the gas tubes are treated with a luminous powder which gets activated by the electrons in the gas. This is known as GTLS (gaseous tritium light sources) and was developed in the 1990s.

Despite being a self-luminous material, tritium poses one disadvantage - it has a half-life of approximately 12.5 years, so it eventually fades permanently.

Brand Proprietary Lume Recipes

While the principles of underlying chemistry are more or less similar, certain brands have innovated their own alterations to the development of luminescent materials. Panerai’s “Radiomir” luminescent substance, patented in 1915, was based on radium bromide and also contained zinc sulfide and mesothorium. In 1992, Timex innovated “Indiglo,” an electroluminescent substance which relies on electric current to glow the phosphor. Seiko pioneered “LumiBrite” in 1995 as a modified version of the Swiss Super-LumiNova. It uses europium and dysprosium as a luminous paint in addition to strontium aluminate.

Many brands like Seiko and Rolex use proprietary luminous materials on their watches.

Rolex too has its own proprietary luminescent material which it calls “Chromalight.” In 2008, Rolex graduated from radium, tritium, and Super-LumiNova to Chromalight which glows a bright blue.

Can Every Day Be Diwali?

Just like you can blow candles all year long, the luminous décor of Diwali too can be a perpetual thing. But I don’t attest to either as it’s better to save such actions for their reserved occasions. However, with a watch that literally serves Diwali’s luminous vibes as a utilitarian rationale, there’s very little that could prevent me from savoring it for the remaining 364 days as well.

Well, what about you?