Lume: The Illuminating History and Evolution of Luminescent Watches

Have you ever wondered why watches light up in the dark? This is thanks to something called lume, and over the years, watchmakers have experimented with various materials to ensure that watch hands can be read in the dark. Let's delve into this lesser-known area!

The demand for luminescent wristwatches significantly increased during the First World War. Soldiers needed to read the time quickly and efficiently, and pocket watches couldn't facilitate this. Additionally, using flashlights was not a wise decision for soldiers on the front lines. This situation gave rise to the use of lumes in watches, enabling easy, clear reading of time, which remains a significant need today. As a result, most of the watches on the market today "glow in the dark".

Radium

This element was discovered by the famous couple, Pierre and Marie Curie in 1898. They received their Nobel Prize in 1903, and after Pierre's tragic death in 1906, Marie received her second prize in 1911. Early on, radium was believed to possess healing qualities and was therefore used in drinks, toothpaste, and even hair products. Starting around 1915, radium became the go-to material when lume was desired on objects like clocks, watches, and military instruments. The decay process of radium releases electrons, which when combined with zinc sulfide (phosphor), results in a bright glow. The negative impacts of this lume were unknown at the time but were later discovered.

The workers had almost no protective measures against the radioactive material beyond a precision brush. Consequently, health symptoms began appearing a few years later due to the improper use of lume. By 1927, Grace Fryer, a dial painter, and some of her female colleagues, known as the "radium girls", filed a lawsuit in the United States after many of them fell ill or died due to radium exposure. This action led to increased legal protection for workers and stricter industrial safety regulations. From then on, the use of radium drastically declined, leading to its ban in 1968.

Promethium

Having realized how toxic radium was, the watch industry quickly moved away from its use. However, they turned to another radioactive material as an alternative - promethium. This powerful and rare earth metal has a half-life of two and a half years, far less than radium's above 1,500 years. Fortunately, watchmakers only used this material briefly, and most watches made with it no longer exist.

Tritium

By this time, lume had gained a considerable market presence. The industry shifted towards tritium, which you might still encounter today. Also known as hydrogen-3, tritium’s scientific name originates from the Greek word tritos, meaning third. Tritium operates on the same chemical principles as radium: tritium decays, releasing electrons that cause the admixed phosphor to glow.

Watch brands had to indicate when their watch dials contained tritium due to its radioactive nature. The most well-known markings are possibly from Rolex and Tudor, like “T SWISST” and “SWISS T <25”. The latter indicated the presence of tritium but in quantities less than 25 mCi. Rolex's last used tritium was indicated by “T SWISS MADE T”, signifying tritium content was less than 7 mCi. The abbreviation mCi stands for milliCurie, a measurement of radiation named after Marie Curie.

Tritium was king of the lumes until the 1990s when its use was banned in 1998. This ban marked the emergence of early versions of the materials we use today.

Despite its use being heavily restricted in 1998, tritium made a comeback on watch dials. As tritium paint was not allowed due to its radioactive nature, it was instead developed into gas form, encapsulated in glass with an internal coating of phosphor. This reinvented tritium posed a negligible threat due to being safely sealed in glass.

Electroluminescence

Electroluminescence is a phenomenon where light is produced due to an electric current passing through a phosphor. In watches, a lamp made of glass or plastic is coated with an electrical conductor and phosphor and then mounted behind the dial. On pushing a button, an electric current is introduced, causing the phosphor to react and function as a backlight.

While this is common in battery-powered digital watches, the Timex Indiglo is the most famous analog watch featuring this type of luminescence. Affordably priced quartz watches manufactured by Bertucci and Casio also use this type of lume.

Luminova

Luminova was the first non-radioactive luminescent material developed for use in watches. Kenzo Nemoto, moved by the bombing of Hiroshima and Nagasaki by the USA, created a luminescent material that wasn’t radioactive. His aim to produce luminescence from light rays led to the creation of Luminova.

Shortly after Nemoto launched Luminova, Swiss company RC Tritec AG, entered into a deal with Nemoto & Co. in 1998 to mass-produce a Luminova version. RC Tritec AG is currently the leading manufacturer and distributor of phosphorescent pigments in the watch industry.

Super-Luminova®

While Luminova originated from Japan, when watchmakers and aficionados refer to lume now, they think of the Swiss variety, Super-Luminova®. The material operates like a light storage battery, continuously emitting light. Its non-aging feature makes Super-Luminova® the preferred lume for many premium watches, including the Tudor Pelagos FXD and the Christopher Ward C1 Moonglow.

Super-Luminova® comes in three different ‘grades’ – Standard, Grade A, and Grade X1. The Grade X1 level provides up to 60% more brightness two hours later as compared to its Standard counterparts. The Super-Luminova® also comes in eight basic colors, each with its unique code, providing the possibility of countless combinations.

In conclusion, lumes, once exclusive to expensive watches, are now found in all price ranges of watches, with numerous benefits for users. They allow you to read the time even in the dark, function as miniature portable flashlights, and give a unique appearance to your timepiece. As this article shows, the history of luminous watches has been fraught with danger. However, as dark as this history may have been, the future looks bright for luminescent watches, and ominously glowing hands and watch faces remain a staple of modern timekeeping.

Pictures from a friend of mine @gizzhead