Why Watch Dials Glow: A Guide to Luminescent Materials in Horology

Have you ever wondered what makes a watch dial glow in the dark? That soft, luminous shine isn’t just for aesthetics - it’s the result of decades of innovation in materials science and watchmaking. From the dangerously radioactive radium of the early 20th century to the safe and long-lasting Super-LumiNova and Chromalight used today, the evolution of luminescent materials tells a fascinating story. Whether you're a vintage watch collector or a modern enthusiast, understanding what’s behind the glow can help you identify, date, and appreciate your timepiece on a whole new level.

What Is Luminescent Material?

The glowing substance used on watch dials is called luminescent material, but that term is just a general label. In reality, it refers to a variety of different compounds developed over time, each with its own characteristics, safety profile, and glow behavior.
Here’s a breakdown of the most commonly used lume materials in watchmaking history:

1. Radium (Used until the 1960s)

Radium was the first material used to make watch dials glow, popular from the 1910s to the early 1960s. It glowed continuously without needing light - but it was dangerously radioactive. Workers who painted radium on dials, like the famous Radium Girls, suffered severe health issues, leading to its eventual ban. Rolex stopped using radium in 1963.

• Glow Type: Self-luminous (radioactive decay)
• Pros: Constant glow without light exposure
• Cons: Highly radioactive and hazardous
• Identification: Often unmarked or labeled "Radium"; glows continuously

2. Tritium (1960s–1990s)

Tritium replaced radium as a safer alternative. It glows continuously but emits far less radiation. With a half-life of about 12 years, its brightness fades over time. Tritium dials are usually marked with "T Swiss T" or "T<25" near the 6 o’clock position.

• Glow Type: Self-luminous (low-level radioactivity)
• Pros: Safer than radium, glows for years
• Cons: Dims over time, eventually stops glowing
• Identification: Marked “T Swiss T” or “T<25”

3. Luminova (Late 1990s)

Luminova introduced a non-radioactive revolution in watch lume. It uses photoluminescence, meaning it absorbs light and re-emits it in the dark. It doesn’t degrade over time and can be recharged indefinitely. Dials are typically marked “Swiss” or “Swiss Made” without the “T”.

• Glow Type: Photoluminescent (requires light charging)
• Pros: Safe, long-lasting, non-radioactive
• Cons: Needs light to charge
• Identification: Marked “Swiss” without “T”

4. Super-LumiNova (Modern Standard)

An enhanced version of Luminova, Super-LumiNova offers brighter and longer-lasting glow. It’s widely used today across many brands and comes in various colors. Like Luminova, it’s non-radioactive and safe.

• Glow Type: Advanced photoluminescent
• Pros: Brighter, longer-lasting, widely adopted
• Cons: Requires light exposure
• Identification: Marked “Swiss Made” without “T”

5. Chromalight (Rolex Exclusive, since ~2008)

Chromalight is Rolex’s proprietary lume, introduced around 2008. It emits a distinctive blue glow and lasts longer than Super-LumiNova. Used in modern Rolex models like the Submariner and Explorer, it’s exclusive to Rolex and not available for aftermarket use.

• Glow Type: Photoluminescent
• Pros: Long-lasting blue glow, Rolex-exclusive
• Cons: Not available outside Rolex
• Identification: Found on post-2008 Rolex models

Lume in Watch Repair: What’s Available Today

In the world of watch repair, radium and tritium are no longer available for obvious safety and legal reasons. You can’t just buy “a little bit” of radioactive material and no one would sell it to you.

Instead, watchmakers use lume paste, which varies in quality. Even reputable suppliers like Bergeon offer lume kits, and you’ll find many options on platforms like AliExpress. These are all non-radioactive, but none match the performance of factory-applied lume - especially Rolex’s Chromalight.

From my experience, restored dials rarely glow as well or as long as originals. If I were reluming my own watch, I’d keep expectations low. Some specialists claim their lume paste is exceptional, but in reality, it’s never quite as good as it should be. Keep in mind, the reluming process is done by hand - not with the precision of Rolex’s specialized machinery - so the result won’t be as crisp or flawless as a factory-original dial. I would only do if there is a visible damage on a dial and there is no other option left, but to relume it.

I’ve even experimented with replacing Rolex hour markers with Super-LumiNova markers from other brands. They do look more "factory", but they don't glow as well as authentic Rolex. Once again I have to admit - nothing compares to Rolex’s factory lume.