What Makes Watch Dials Glow?

After years of wearing and admiring watches, you start to develop an eye for the parts that will light up in the dark. The familiar coating of Super-LumiNova or other luminous materials on the hands and indexes becomes almost second nature to spot. It’s always a surprise when an unexpected part of the watch face lights up. Often, you can tell where the lume is—typically on the hands and markers. But sometimes, there’s a hidden detail that only reveals itself under a UV light.

Nowadays, you can find all sorts of glowing details on a watch or in some cases on the entire watch, from the strap to the dial, like in the case of the IWC Ceralume concept watch. However, a few decades ago, luminescence was applied more sparingly or sometimes too generously, given the hazardous materials used at the time. 

There are countless ways to make a watch face glow in the dark, from different materials to unique application methods. Each approach brings its own set of results and the quality can vary significantly. Some watches might have subtle illumination while others shine brightly. 

Evolution of Luminous Materials

Consistency among all luminous elements of the watch really comes down to the lume’s background. Even the best lume needs a good foundation to shine properly. Take the hands of a watch, for example. The luminous compound is often suspended within a solid frame, with nothing behind it. It’s fixed in place, allowing light or heat to charge it from all directions. This setup can make the lume glow incredibly bright, almost as if it’s floating and radiating light from within.

Hour markers are a different story. Pad-printed markers on a dark dial need several layers of lume to achieve the right thickness and combat the dulling effect of the background. Adding a white or reflective base first helps, and placing lume into polished, applied markers is even better. However, there’s no substitute for the best luminous compounds paired with these foundational techniques. Here are some materials that have been used to make watch dials glow in the dark, from then to now.

1. Radium

The discovery that revolutionised night-time visibility in watches came from Marie and Pierre Curie in 1898. Radium, a fascinating element, always feels warmer than its surroundings due to its radiation. Its real charm for watchmakers, however, was its natural glow. This allowed watch and clock hands and dials to be coated with radium, making them shine in the dark. In 1902, William J. Hammer invented luminous paint by mixing radium with zinc sulphide. Unfortunately for Hammer, he didn’t patent his invention, and George Kunz from Tiffany & Co. seized the opportunity and did.

Radium has the unique ability to glow without any external light source. However, it comes with a significant downside. The emissions of alpha particles, gamma rays, and beta particles from its decay products make radium highly unstable and a serious radiological hazard.

At first, radium dials were quite costly, so they were mainly used by the military, adorning the clocks and gauges in aircraft cockpits and other military timepieces like pocket and wristwatches. Collectors don’t typically seek out watches with radium dials and hands, but an interest in vintage pieces from 1910 to 1960 will inevitably lead to encountering such timepieces. One such example is the 1952 Rolex Oyster Perpetual 14ct Model Ref. 6084, which features distinctive blackened lume dots and a starburst dial ‘burn’. 

2. Radiomir 

Panerai, a Florentine brand renowned for supplying instruments to the Royal Italian Navy, was a trailblazer in enhancing diver’s watches. One of the biggest challenges for these watches was readability in the dark, murky depths. In 1916, Panerai tackled this issue head-on by creating Radiomir, a radium-based powder that made their dials brilliantly luminous. This Radiomir paint was among the brightest at the time, leading to the creation of an entire collection named after it.

However, when the dangers of radium—high radioactivity and severe health risks—became clear, even Panerai ceased using it. Despite this, when Panerai watches became available to the public, they kept the iconic Radiomir design. Today, these watches still carry the legacy of their luminous past, but without the hazardous radium.

3. Radium Alternatives 

Radium turned out to be a terrible idea for many reasons. In the early 1900s, women working in U.S. watch factories—known as the ‘radium girls’—suffered severe radiation poisoning from painting dials with radium paint. Despite the dangers, people were mesmerized by radium’s glow, and it wasn’t banned until 1968.

The search for safer alternatives led to other substances. Promethium was one option. Although radioactive, it only emits beta particles and has a half-life of less than three years. Seiko experimented with promethium, but its short-lived glow faded quickly. Tritium, with its low beta emission and a half-life of 12.32 years, proved to be a better choice. Panerai used tritium in their patented luminescent substance called Luminor, which inspired the name of their famous Luminor collection. Patented in 1949, the tritium-based Luminor led to the iconic Luminor case design and collection. However, Panerai, like others, eventually stopped using tritium. By the late 90s, tritium was banned, with Omega ceasing its use in 1997 and Rolex following suit in 1998.

A few watch manufacturers are now bringing tritium back, but not in the form of paint or paste. Instead, tritium gas, a radioactive form of hydrogen, is sealed inside borosilicate glass capsules coated with phosphor. As the tritium undergoes beta decay, it releases electrons that make the phosphor glow. While the gas is still radioactive, it’s considered safer than tritium-based paints because it’s contained within these tubes. Brands like Luminox and Ball prominently use these tritium gas tubes on their watch dials. 

4. Super-LumiNova

After years of grappling with radioactive substances, there was finally a breakthrough that made glowing watches both effective and safe. In 1993, Japan’s Nemoto & Co, a company already experienced in providing luminous paint since 1941, introduced LumiNova. This new material, made from strontium aluminate and europium—a safe chemical that enhances its glow—proved to be a major leap forward. LumiNova could shine far brighter and longer than the old zinc sulphide paints, without the hazards associated with radioactive elements. While it didn’t offer the perpetual glow of its radioactive predecessors, it came without the associated risks, which was a huge relief.

Eventually, Nemoto teamed up with the Swiss company RC-Tritec to create LumiNova AG, which began producing Super-LumiNova—a Swiss version of the same strontium aluminate pigment. This is now the standard for most glowing watch elements. Over time, various brands have customized this super pigment to develop their own proprietary versions, enhancing performance and durability while leaving the dangers of the past behind.

5. Other brands’ “Super-LumiNova”

Rolex made a significant leap in 2000 when they began using Super-LumiNova, but it was in 2008 that they unveiled their own special version, Chromalight, with the launch of the Oyster Perpetual Sea-Dweller Deepsea. This watch wasn’t just built to withstand depths of up to 3,900 meters—it also featured a glow that could shine brilliantly even in the darkest underwater environments. The blue hue of Chromalight, compared to the more common green of Super-LumiNova, offers a steady glow for up to eight hours. Seiko’s LumiBrite is another contender in the luminous watch world, with a similar promise of intense brightness that lasts up to five hours after exposure to light. 

A blue glow isn’t exclusive to Chromalight. Super-LumiNova, too, can shine in various hues, including blue, though it might not be as intense as Chromalight. Take, for example, the 2020 Breitling SuperOcean Heritage 57. This watch showcases hour markers in a vibrant spectrum of colors—yellow, green, blue, indigo, violet, red, and orange—demonstrating the range of hues Super-LumiNova can offer.

6. Stellar Super-LumiNova:

In 2020, Panerai took a nostalgic trip back to their roots with the Luminor 70th anniversary edition. Drawing inspiration from their early days of luminous innovation, they introduced three new special edition Luminors. These watches go beyond the usual dial markers and hands, featuring Super-LumiNova on the inner bezel, the distinctive crown protector, the crown itself, and even the stitching on the straps. The latest Super-LumiNova X1 used in these models offers an advanced glow, surpassing even the brightness of their previous tritium-based Luminor.