The history of watch luminescence

Last Updated on February 22, 2025 by Jason

One of the limitations of early watches was that it was impossible to read the time without a reliable light source. In darkness, you would need to strike a match or light a candle to check the time. There were solutions, such as the braille watch or “montre a tact” developed by Abraham-Louis Breguet around 1800 or repeater watches that chimed the hours and minutes. Neither of these solutions was widely adopted. A solution needed to be found to illuminate the hands and the hour markers four users to be able to read the time in the dark. In this article, we will explore the history of watch luminescence from the early 20th century until the present day.

Radium

Marie and Pierre Curie discovered Radium in 1898. Radium (chemical symbol Ra) is a naturally occurring radioactive metal. Radium was officially named in 1899, after a French word derived from the Latin radius, meaning ray, a fitting name for an element that continually emits radioactive energy. However, radium doesn’t emit visible light on its own. However, it was soon discovered that by mixing it with a phosphor, a material that glows when exposed to radiation, a light source can be formed. In 1908, the Austrian scientist, Dr Sabin Arnold von Sochocky (1883 – 1928), developed a recipe for luminous paint using radium. On a sad note, Dr Sabin Arnold von Sochocky’s early death at the age of 45 was attributed to his exposure to radium. The luminous paint invented by Dr Sabin Arnold von Sochocky was soon applied to watch dials.

During the First World War (1914-1918), the use of luminous paint became widespread with the arrival of trench watches. A luminous dial was critical in the dim conditions of trench life and time became critical when coordinating attacks. Watchmakers had factories full of dedicated employees applying luminous radium paint mixed with zinc sulphide phosphor to watch dials and hands. Typically, women were employed under the belief that their smaller hands and deft touch were more suited to the delicate work.

WW1 trench watch with radium-based lume.

Radium Girls

The women hired to paint the watch dials came to be known as the “ghost girls”. This is because the radium dust in their working environment made their clothes, hair, and skin glow. Many of the women would wear their best dresses on the job so the fabric would shimmer when they went dancing after work. Some even applied the paint to their fingernails, faces and teeth. Even their work practices encouraged the ingestion of radium because they were instructed to use their lips to bring their paint brushes to a fine point to work on the tiny dials. All the while, the women were assured by their managers that the radium was perfectly safe to work with. In the post-war years, the adverse effects of radioactive radium were beginning to become more evident.

In the early 1920s, the “Radium Girls” began to experience the physical symptoms of their exposure to radium. The first symptoms were dental pain, loose teeth, lesions, and ulcers. Many of the “Radium Girls” women later began to develop anaemia, bone fractures, and necrosis of the jaw, a condition now known as radium jaw. In 1923, the first of the Radium Girls died as a result of her exposure to radium on the job. By 1927, more than 50 women had died because of radium paint poisoning. Once the public became aware of the danger of radium, it was common to see the lume removed from watch dials.

Mappin Campaign watch with the radium lume removed.

Legal action

In 1927, in the US, the surviving “Radium Girls” launched a legal case against their employers. The trial didn’t start until 1928 and some of the women were too ill to raise their hands to take their oath. Fearing that they would not survive a long trial, the women and their lawyers made the decision to settle the case out of court in June 1928. Each “Radium Girl” was awarded damages and an annual payment for the rest of their lives. The “Radium Girls” case led to safer working practices for workers across the radium industry. As a result of the legal action, the use of radium in consumer products was gradually phased out and banned completely in 1968. Radium paint was still being used on watch dials after the legal action, but in significantly smaller quantities.

How does radium paint work?

Radium dials are coated with a mixture of radium-226 (a radioactive isotope) and a radioluminescent phosphor chemical (zinc sulphide phosphor). The radioisotope continually decays, releasing radiation particles. These radiation particles strike the phosphor molecules, exciting them. As the excited phosphor molecules relax to their ground state, they emit visible light, creating a glowing effect on the dial.

Tudor cushion cased watch with radium paint on the hands.

Radium-226 has a half-life of approximately 1,600 years. This means that over time, half of the radium atoms decay into other elements. As a result, the luminosity of radium paint gradually diminishes. This would not be noticeable for centuries. However, the zinc sulphide phosphor breaks down much more quickly due to the constant bombardment of radiation. Its luminescence is measurable in a matter of years. It is quite possible that early trench watches may have needed to be repainted before the end of the war.

Tritium

As radium was slowly phased out, it was replaced by another source of radioluminescence, tritium, which was discovered in 1934. Tritium is a radioactive isotope of hydrogen. It has the same number of protons and electrons as hydrogen. However, it has 2 neutrons, whereas regular hydrogen does not have any. This makes tritium unstable and radioactive. Unlike radium, tritium is a relatively weak source of beta radiation, which itself is too weak to penetrate the skin. However, it can increase the risk of cancer if consumed in extremely large quantities. The half-life of tritium is 12.33 years, meaning that it takes just over 12 years for tritium to decay to half of its original amount. This means that the lifespan of tritium as a source of radioluminescence is limited in comparison to radium. Generally, watch dials with tritium-based luminous paint will only last a couple of decades.

It is quite common to see vintage watches with “T Swiss Made T” wording on the dial. This indicates that the watch was made in Switzerland under strict terms and conditions. The “T” on either side of the marking indicates that the luminescence on the dial is triggered by tritium. The “T Swiss Made T” marking was introduced in the early 1960s. Tritium remained in use until the mid-1990s.

Tritium lume in watches comes in two forms. Firstly, there is the tritium and phosphor mixture which is painted on the dials and hands. Secondly, there was the use of tritium tubes, which are tiny sealed glass tubes with a layer of phosphor on the inside surface and tritium gas inside. This was a more expensive option than the tritium-based paint.

Promethium

Promethium was another radioactive element used in place of radium as a source of radioluminescence. It was discovered in 1945 and used during a similar time period to tritium. Promethium is relatively safe when sealed, but it does produce X-rays as a by-product of beta decay. Unlike radium, promethium lume does not emit gamma rays and is therefore considerably less dangerous. The downside of promethium is that its half-life is only 2.62 years. Long before the final traces of promethium have decayed, the lume in the watch will have become virtually useless. Tritium was used for luminescence from the 1960s until the 1990s.

LumiNova

LumiNova is a brand of photoluminescent paint which relies on its chemical composition to generate light instead of radioactivity. As a result, it is safe in comparison to the radium and tritium-based paints used in the past. It was invented in 1993 by the Japanese company Nemoto & Co., Ltd.

The key element in LumiNova’s composition is strontium aluminate. This compound is capable of absorbing light energy from various sources, including sunlight or artificial lighting. This energy is then slowly released over time as a sustained afterglow. When exposed to light, the electrons in the strontium aluminate absorb photons and become excited. As these excited electrons return to their ground state, they emit light, producing a visible glow. The intensity and duration of the luminescence depend on the amount and duration of the light exposure, as well as the specific formulation of the LumiNova pigment.

LumiNova’s ability to glow brightly for extended periods without the need for an external power source has made it popular not only in watchmaking but also in various other applications. These include emergency signage, safety equipment, and decorative items. Its invention marked a significant improvement in both the performance and safety of photoluminescent materials. Unlike its predecessors, LumiNova is a non-hazardous phosphor that doesn’t decay during the lifetime of the watch, is temperature-resistant and is unaffected by environmental influences.

Super-LumiNova is the registered name under which LumiNova was distributed by LumiNova AG Switzerland. In 1993, a Swiss company called RC Tritec AG was founded and holds the licensing and rights to produce and distribute LumiNova under the registered name of Super-LumiNova. Seiko also licensed the material for their LumiBrite watches.

Caution

The handling and wearing of watches with radio-luminescent paint does need to be carefully considered. In particular, if the paint contains radium, extreme caution should be exercised. Remember that no luminosity in an old watch does not mean that the radioactive part of the lume has disappeared. Radium has a long half-life, and residues of this will still be active in vintage watches. Even if the paint has been removed, there will still be minute traces of radium on the dial. If the watch is dropped and the crystal shatters there is a chance that the particles of radium can be ingested. In the case of tritium, there are no dangers present when merely wearing the watch, because there are no gamma rays. If a vintage watch has promethium lume the amount of radioactive material will be quite small due to the short half-life.