What makes luminous watch dial and hands glow?

Aside from the rotating bezel, another distinct part of a dive watch is its dial with luminous hands and indexes. This characteristic helps divers navigate underwater easily. Due to its luminescent property, divers maintain better visibility even in the dark environments, preventing them from any imminent dangers that may arise while submerged in the water.

While luminous dials are standard in dive watches, there are also casual timepieces with the same property. It is one reason why glowing dials have been an ongoing subject between watch collectors. It has interesting facts that are uncommon to most watch wearers.

It is essential to understand the history of luminous dials and the science behind its concept. For timepieces to have this glowing property, the basic knowledge about chemistry and physics can help us understand how it works and adequately handles such hazardous material.

The Luminous Dial’s Glowing Property

Any materials that can glow in the dark are visible phosphorescence. It is a particular case of photoluminescence or certain elements' ability to emit light after exposure to a light source. As defined, light has packets of energy known as photons, which are quantum particles in an electromagnetic field.

To simplify, if an electron atom in a material absorbs a photon, it gets excited, reaching a higher energy state until it goes back to normal. By this time, the material starts emitting the photon, which we see by our naked eye as visible light.

There are two known types of photoluminescence:

Fluorescence - Fluorescent materials emit absorbed energy very rapidly. They will continuously glow if there is a light source, but the emission of particles occurs on mere nanoseconds. Hence, most of these materials will go dark right away once the light source becomes unavailable. For watch dials, using fluorescent materials is not always viable.

Phosphorescence – Phosphorescent materials also absorb photons from a light source. The only difference is that they emit those photons as light very slowly, which is why phosphorescent paints are usually used for watch dials.

Other Materials Used for Luminous Dials

Radium Paint

This material is self-luminous, only when freshly applied. There is no need for an external energy source because there is already light emitted from the radium particles' radiation in the paint. While this illuminates’ dials in watches, it has disadvantages: 1.) it is chemically unstable, and 2.) it is a radiological hazard.

Back in the old days, some watches had numbers and indexes with radium paint. However, given its dangerous effects in health, it is not almost used today in the modern watchmaking industry.

Promethium and Tritium

Due to the harmful effects of radium paint, promethium and tritium were later discovered as substitutes. Technically, these two substances are considered less radioactive than radium. Promethium emits only beta particles at lower energy and used as an ‘excitant,’ so the dial can emit visible light.

On the other hand, tritium is also a low energy beta emitter with a longer half-life than promethium. Tritium is estimated to have a half-life of 12.32 years, while promethium only has 2.62 years. This makes the former a better ‘excitant’ for achieving luminous dials.

Luminova and Super-LumiNova

As the search to find another substitute material not radioactive at all continues, and this is where the two super players in the modern watchmaking industry come in; the Luminova and Super-Luminova.

The idea for Luminova is to have phosphors that will continue to glow for hours, even without the help of radioactive ‘excitants.’ It was invented in 1993 by Nemoto & Co., a Japanese company. Five years later, RC-Tritec AG partnered with Nemoto in establishing LumiNova AG Switzerland, which supplied the material in the Swiss watch industry.

Super-Luminova, on the other hand, uses no ‘excitant’ at all. Instead, it uses a material known as strontium aluminate – an extremely efficient phosphor that will glow once charged. It shines brightly initially, then slowly diminishes for several hours.

The strontium aluminate needs to be mixed with europium – a non-toxic and non-radioactive chemical element to achieve its perfect glowing characteristic. There is one distinct disadvantage with Super-Luminova compared to radioactive materials: its brightness fades until another exposure to light recharges it. Despite this, it is still considered as an ideal solution for underwater and nocturnal visibility.