What Makes Luminous Watch Dials Glow?

Sep 24, 2020

Category: Gear

With the widespread adoption of commercial electricity and the increasing prevalence of smartphones, today, luminous watch technology is proving to be far more of a value-added feature than a necessity. As such, it’s easy to overlook this little innovation, for as cutting-edge as it was at the time, it’s often taken for granted. However, it wasn’t always so — prior to these modern conveniences, luminous dials offered a much more convenient way to tell the time in the dark than listening for the toll of a bell tower or hoping to stumble across a street clock.

Moreover, watch lume has been a consistent and appreciated performer in military applications — whether that was in Europe during WW1 or on special ops missions today. In environments when every second counts, you can’t rely on a smartphone to match the precision of a good watch — especially after the sun goes down and a brightly lit screen stands out like a sore thumb.

While we have a slew of lume styles today, it’s been a long time coming. From early missteps to modern makeovers, watch manufacturers have continued to innovate on the luminous dial in the search for the ideal illuminator. Whether you’re a casual enthusiast curious about timepieces or a passionate collector well-steeped in watch know-how, at one point or another, everyone’s probably wondered “What makes luminous watch dials glow?” Well, read on to find out about this revolutionary piece of wrist-based tech and how it came to be.

Radium Paint

Humble Beginnings With Harmful Side Effects

With the discovery of radioactivity around the turn of the 20th century came the first application of radioluminescent elements: radium paint. Because radium paint is self-luminous — meaning it produces light through its own radioactive decay — it quickly became the preferred method for giving watches their glow. Incredibly bright and easy to produce in large quantities, it was considered pretty cutting edge stuff at the time.

The precision necessary to apply luminous material to watch dials required a delicate touch. As such, throughout the 1910s and into the 1920s, thousands of working-class women filled watch factories trying to make a decent living by painting radioactive radium onto watch dials. At the time, it was believed that there were no adverse effects from exposure to the material, so workers didn’t know to beware of the occupation. Furthermore, with the onset of WWI, women could take pride in knowing they were supporting their troops by providing them with legible watches.

With radiation poisoning still very much a distant concept, instructors taught workers to lick to the tips of their brushes in order to get a fine point, after which they’d paint the radium onto the dial. Many women also reportedly painted their nails and their teeth in order to make themselves glow. Over time, workers began suffering from a host of health problems, including anemia, bone fractures, and necrosis of the jaw — also known as “radium jaw.” Confronted with the knowledge that they were making some of their workers sick and others terminally ill, watch manufacturers nonetheless staunchly denied the harmful properties of radium and attempted to cover up its side effects.

Another problem with radium paint was that it would gradually lose its shine due to phosphor deterioration. So while you’d have a watch dial with a half-life of 1600+ years (the time it takes radium to decay by 50%), it would only take a few years to a decade before the glow would be gone. In other words, you’d have an element continue to be radioactive — dangerously so — for far longer than its use justified.

However, factory worker Grace Fryer was determined to seek compensation. With the help of her fellow coworkers, she spearheaded a campaign against U.S. Radium, one of the largest employers of watch dial painters. Dubbed the “radium girls,” Fryer and others like her fought for legal action for nearly 12 years — eventually being heard in the Supreme Court — until finally the watch giants were forced to pay and workers were afforded unprecedented labor rights.

Though watch manufacturers continued using radium paint into the late 1950s and early 1960s (and the military longer still), it was outright banned from commercial production in 1968. Since that time, watch manufacturers have scrambled to find a safer alternative. Several other technologies have taken up the torch, each with its own particular nuances.

Photo: Shinola The Duck

Photoluminescence

A Safer Alternative With Some Compromises

Today’s paints are made using non-radioactive photoluminescent materials. Unlike their radioluminescent counterparts, photoluminescent paints require no dangerous excitants to give them their glow. Rather, they contain phosphors such as strontium aluminate that act as a “light bank” of sorts, taking in ambient light exposure, storing it for later, and then gradually re-emitting it as a bright glow. Compared to radium, photoluminescent paints don’t shine for nearly as long — in the absence of light, only a few hours at most. Though there are several photoluminescent technologies currently in play, Super-LumiNova is the most commonly used variety.

Super-LumiNova: Invented in 1993 in Japan by Nemoto & Co., LumiNova uses strontium aluminate as its phosphor in order to absorb and emit light. In 1998, RC-Tritec AG joined with Nemoto to found LumiNova AG Switzerland, a supplier dedicated to providing luminous tech for the watch industry. Though LumiNova describes the industrial pigment more generally, Super-LumiNova refers specifically to the Swiss-produced variety, guaranteeing both higher quality and watch-specific application. Today, Super-Luminova is available in over 3,000 different colors, with varying brightnesses and shades to boot.

LumiBrite: For a brief time, Seiko had a partnership with Nemoto giving them LumiNova distribution rights. However, after the agreement ended in 1994, Seiko began marketing its own photoluminescent paint technology known as LumiBrite. Much like Super-Luminova, LumiBrite uses a strontium aluminate base for its phosphor, adding in europium and dysprosium in order to make it more effective at emitting light. After just 10 minutes of exposure to light (either natural or artificial), LumiBrite will shine for a solid 3-5 hours in the dark.

Chromalight: Rolex introduced its in-house Chromalight technology with its Deepsea Sea-Dweller watch in 2008. Though there’s some speculation that it’s essentially a rebranded Super-LumiNova with a different color and application method, Rolex markets Chromalight as having superior brightness and longevity compared to other photoluminescent materials. It also comes in a distinctive blue color unique to Rolex. Whether it’s a proprietary paint or just a LumiNova lookalike, one thing’s for sure: Chromalight is found exclusively on Rolex timepieces.

Having armed you with an understanding of the basic photoluminescent paints, here are two solid performers featuring this time-tested tech:

MWC Kampfschwimmer Dive Watch

Crafted for military and law enforcement personnel, the Kampfschwimmer from MWC features a design inspired by the same watches worn by the 1950s German specialist-commando forces. With a 24-jewel automatic movement, a PVD-coated stainless steel case, and a 300m water-resistance rating, this diver is as tough as they come at a price that’s approachable for any. As an added bonus, the Kampfschwimmer comes mil-spec certified with two NATO nylon webbed straps, so you know it can hold up to anything you throw its way.

Purchase: $350

Shinola The Lake Michigan Monster

Named for the great lake, this handsome dive watch is loaded with distinctive American styling, including a stunning midnight blue dial and a matching rubber strap. But that’s not all — powered by an SW200-1 automatic movement, The Monster offers incredible accuracy and reliable performance. And with a 38-hour power reserve and a depth rating of 1,000 feet, you can trust that The Monster is ready for the deepest of dives.

Purchase: $1,250

Photo: Nite Watches Hawk

Promethium and Tritium

A Modern Reboot Of Their Radioactive Forbearer

Having determined that radium was too harmful to continue its use in watch dials, the race was on to find a suitable alternative. One such substitute was promethium, a material explored as a less hazardous excitant to be used in radioluminescent paints. Emitting only beta particles at lower energy than radium, promethium is generally regarded as an option with a much lower risk of dangerous side effects. While promethium doesn’t cause its phosphors to break down as quickly, it has an extremely short half-life of just 2.62 years. In other words, a watch featuring promethium on its dial is only likely to glow very weakly, for a few years. Even still, Seiko used promethium-147 as an excitant for a brief period of time.

Another radioactive lume still found today involves the use of tritium gas. Much like promethium, tritium is low energy beta-only emitter, meaning it’s a far safer material to have on your wrist than radium. What’s more, it features a considerably longer half-life than promethium — 12.32 years — so it’s likely to last a good while longer when it comes to its luminous qualities. Encapsulated in a tube of glass coated with a phosphor layer, the phosphor glows as the tritium undergoes beta decay. Because of the glass tubing, tritium gas is a very low-risk radioactive material. When compared to Super-Luminova, tritium offers the benefit of a constant glow regardless of the ambient light — perfect for applications where you need a watch with dependable, fade-free luminosity. However, it’s important to note that after about 24 years, only a quarter of the tritium gas will be left — little enough that the watch will need servicing should you want it to glow its brightest again. Moreover, even though it’s fairly low-risk compared to radium, many countries have imposed strict bans on the material.

Several manufacturers — such as Ball, Luminox, and Marathon — use tritium gas today. Because of the nature of the tech, it’s typically limited to higher-priced options, but it’s ever-expanding in usage. Here are two of our picks for the best tritium watches:

Nite Watches Hawk

Built to be life-proof, this watch features a reinforced polycarbonate case, a Swiss-made Ronda quartz movement, and a sapphire crystal. It also comes in a range of colorways with either a PVD bracelet or a polymer strap, so you can customize your look and your finish to your heart’s delight. With aggressive styling and a durable construction, the Hawk tritium watch is an adventure-ready accessory providing constant, bright illumination.

Purchase: $510

Ball Engineer Hydrocarbon Original

From the Ohio-based Ball comes the Engineer Hydrocarbon Original, a handsome, hard-wearing watch designed to stand up to the daily rigors faced by the most demanding of technical pros. Built around a 40mm stainless steel case, this watch comes with an Amortiser anti-shock system and an automatic RR1102-CSL movement rated for 80,000A/m anti-magnetic resistance, 200m of water-resistance, and 7,500Gs of shock resistance. But the most striking quality of the Engineer Hydrocarbon Original would undoubtedly be its use of 30 tritium gas tubes to illuminate the dial.

Purchase: $3,199

Photo: Timex Fairfield

Electroluminescence

Light At The Push Of A Button

Electroluminescence (EL) describes the process of charging a phosphor with an electric current in order to produce light. It relies on the use of powders and thin films as semiconductors, with the addition of active materials such as copper to give the EL panel its color. First adopted for commercial use during the 1980s, thin-film phosphor electroluminescence offers even illumination with little battery drain. As such, it’s proven to be a popular option for providing backlight in liquid crystal displays (LCD) and watch dials. Although Timex is one of the most steadfast supporters of EL lume, others such as Casio have also taken to the technology over time. Compared to Super-Luminova and tritium gas, EL has the benefit of being battery powered. In other words, it’s long-lasting, easily serviceable, and harmless. However, because it runs off your watch’s battery, overuse does accelerate its discharge.

Indiglo: First introduced by Timex in 1992 through the Ironman watch line, Indiglo is now featured on some 70% of the Timex lineup. The name originally comes from the word ‘indigo’ because Indiglo backlights feature a characteristic blue-green tint. Offering an even glow across the whole watch face, Indiglo can be used on the entire dial or in the digits only as per negative LCDs.

Super Illuminator LED : Though Casio released an electroluminescent technology similar to Indiglo back in 1995, it’s more recently switched over to an LED backlight. Much like an electroluminescent dial, LED backlights require an electrical current for illumination. They require even less energy than EL, are more efficient for solar charging applications, and are extremely cheap to produce. However, it’s worth noting that an LED backlight doesn’t hold a candle to EL when it comes to even, full-dial illumination.

Though electroluminescence is a pretty reliable piece of tech, you won’t find it on top-tier timepieces. Even when Timex was producing its premium TX Watch Company line, it used Super-Luminova on the dials. Nonetheless, EL watches are solid performers and you can’t go wrong with either of our picks.

Timex Waterbury

Created in celebration of the brand’s 160th anniversary as an American watch brand, the Timex Waterbury offers an affordable chronograph with timeless styling. Sporting a Red Wing Shoe leather strap and a solid steel construction, this classy piece punches well above its weight class when it comes to style and quality. And with Indiglo Night-Light and 50m water-resistance, this watch is as functional as it is good looking.

Purchase: $175

G-SHOCK GMWB5000d

Inspired by the first G-SHOCK model, the GMWB500d sports a full-metal construction and some serious technical chops. For instance, its solar battery allows for convenient self-charging and its Atomic Timekeeping technology provides unrivaled accuracy no matter where you are. Additionally, it comes with BlueTooth connectivity so you can sync time two ways via the G-SHOCK app. Top things off with a super illuminator LED light featuring full auto LED, selectable illumination duration, and afterglow, and you have a rugged watch that more than lives up to the G-SHOCK name.

Purchase: $550

The 40 Best Men’s Watches For Any Budget

Now that you’re an expert on all things lume, check out our guide to the best men’s watches for any budget to see how this piece of tech is put into practice.

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