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Glowing in the dark

Watches, especially those made for tasks with low or no levels of natural light, are able to glow in the dark to ensure legibility. It is an often talked about feature, especially on watches for diving. But how does it actually work? As you will see below it is a matter of chemistry. In the following we are examining two different types of materials, or indeed methods, to make a watch glow in the dark. It should be mentioned that this peek at luminous materials is not a complete insight.

We have drawn the line at materials relevant in today's watch manufacturing processes, leaving out old school methods like Radium paint which dominated prior to the nuclear age. 

Some of the first watches with the ability to glow in the dark were based on the use of Tritium. If you have come across an older watch with a small ’T’ in a circle on the dial, you might have experienced it first hand. Tritium or hydrogen-3 is a rare and radioactive isotope of hydrogen.

Naturally occurring tritium is extremely rare on Earth. The atmosphere has only trace amounts, formed by the interaction of its gases with cosmic rays. It can be produced by irradiating lithium metal or lithium-bearing ceramic pebbles in a nuclear reactor.



Tritium is used as the energy source in radio-luminescent lights for everything from watches, instruments and tools, even self-illuminating key chains. It is deployed in a medical and scientific setting as a radioactive tracer. Tritium is also used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and in hydrogen bombs. As you might have figured out by now, Tritium is not your average material.

Being radioactive and rare means that there are some obvious question marks over the safe use of Tritium based materials being worn directly on the skin. However we are talking about fairly low levels of radioactivity, so low that there is no genuine health risk associated with its use in watches. Nevertheless the use of Tritium mixed with paint for decorating a watch dial ceased a number of years ago, and today Tritium is only available in tubes which are then applied directly to a watch dial, or to the hands, or to the hour and minute markers.

The advantage of Tritium is that it glows constantly and doesn’t need to be charged via exposure to light. Although Tritium tubes don’t glow as brightly as other materials they are the best choice for environments with a long absence of natural light. By encapsulating the Tritium in tubes the concern about radiation has been overcome, and with a half life of 12 years it is still the go-to material for really high end watches needed for mission critical projects.

Watches with Tritium tubes often have a radioactivity warning sign on the dial for extra drama, and even some timepieces which pay tribute to watches from the era of the Tritium based paint, have the little ’T’ in a circle logo on the dial, just for kicks.

So with Tritium being radioactive and no longer available as paint, the watch industry has turned its focus onto other solutions.

The leader of the market is Super-LumiNova. Often referred to as Luminova is a strontium aluminate–based, non-radioactive and nontoxic photo-luminescent (afterglow pigments in plain English). 



Luminova was first invented in 1993 in by the Japanese company Nemoto & Co., and in 1998 a joint venture between Nemoto and RC-Tritec AG led to the creation of LumiNova AG Switzerland, with a view to supply the Swiss watch industry. 

Being easier to manage than Tritium, cheaper to source and less dangerous, Luminova has almost entirely taken over the market. You don’t have to encapsulate Luminova to shield the radiation, and its ability to recharge after a period of darkness doesn’t wear off, so it doesn’t age like Tritium does. Its only enemy is water, and with time Luminova parts on a watch exposed to water - typically the bezel, will lose its effect eventually or just wash off.

To offer variation in colour and brightness Luminova is available in a number of different versions. From the ultra bright to the more subtle, giving watch companies the ability to integrate the lumen in the design of a watch dial.

Depending on the application, where something like a diver’s watch will favour as much brightness as possible, a more dressy watch will have less of a desire to light up the entire room it is in. The Enoksen range of watches is making good use of Luminova. From the lushness of the Deep Dive to the discrete lumen of the Fly E03/D, the different ranges have different levels of brightness.


The image shows a number of watches from our catalogue, starting with the Deep Dive, the Dive and the Drive. These three models have the ultra bright C3 Luminova which appears to be plain white in broad daylight, but immediately turns light green in the darkness. Next model is the Fly E03/E which has BG W9 Luminova which is a tad more discrete - still plain white in daylight but with a hint of light blue in darkness. After that comes the Fly E03/D chronograph with its Light Orange, almost parchment daytime colour. It has a very subtle lumen in direct comparison with both C3 and BG W9. Finally, to the far right is a prototype we are currently working on. It is shown here because it has Tritium tubes and offers a very good impression of how Tritium performs next to different Luminova based dials. Less bright but constant and reliable.

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