Luminescent substances

Luminescent materials are among the most fascinating aspects of watchmaking, combining science, history, and aesthetics.

They have evolved over time, from dangerous radioactive substances to safe and sustainable modern compounds.

Here is a comprehensive explanation of their history, how they work, types, and characteristics.

1. What is a luminescent material?

In watchmaking, a luminescent material is a substance applied to the hands, hour markers, or numerals on a dial so that they glow in the dark.

It has two main functions:

• Read the time at night or in low light conditions.
• Give it a visual signature (identity, style, technicality).

2. The historical evolution of luminescent materials

1. The Radium Era (1910–1960)

• Discovery of radium in 1898 by Marie and Pierre Curie.
• As early as the 1910s, radium was mixed with zinc sulfide to create radioactive luminescent paint.
• Widespread use during World War I and World War II:
  • Watches, instruments, compasses...
  • Soldiers could tell the time without an external light source.

Major disadvantages:

• Radium is extremely radioactive (half-life: 1,600 years).
• The “Radium Girls,” female workers who painted watch dials, suffered from serious illnesses and cancers due to accidentally ingesting the material.
• Many antique watches are still radioactive today.

Result: gradual ban on radium in the 1960s.

2. Tritium (1960–1990)

To replace radium, brands are adopting tritium (³H), an isotope of hydrogen:

• Much lower radioactivity (half-life: 12.3 years).
• Significantly improved safety, no danger to the user.
• Used by Rolex, Omega, Seiko, Blancpain, and many military watches.

Tritium is often indicated by:

• The marking “T Swiss Made T,” “T<25,” or simply “H3” on the dial.

How it works:

• Tritium emits electrons (β-).
• These excite a phosphor (ZnS), which emits light.
• The brightness is continuous, even without external light, but decreases over time.

Disadvantages:

• Low and decreasing brightness (after 10-15 years, the watch hardly glows at all).
• Still slightly radioactive, although harmless in a closed watch.

3. Photoluminescent pigments (since 1990)

With the end of tritium, brands are adopting non-radioactive pigments capable of storing light and then releasing it in the dark.

How it works:

• These pigments absorb natural or artificial light.
• They store energy (photons).​
• Then they slowly emit this light in the darkness. Phosphorescence phenomenon.

The most famous:

Composition:

• Strontium aluminate doped with europium (SrAl₂O₄: Eu²⁺).
• Non-radioactive.
• Battery life: 6 to 10 hours, depending on light load.

3. Current and alternative technologies

Tritium microcapsules (GTLS or H₃ tubes)

• Used by Ball, Traser, Luminox, Marathon.
• Small glass tubes containing tritium gas and a phosphorescent coating.
• Shine continuously for 20 to 25 years without needing to be recharged.
• Stable brightness but less intense than freshly loaded pigments.

Modern aesthetic effects

• Various colors available: green, blue, turquoise, orange, white, etc.
• Some brands use them for a design or retro effect:
  • “Old Radium” or “Faux patina” → beige pigments imitating the patina of radium.
  • “Full lume dial” → fully luminescent dial (Seiko, Bell & Ross, Panerai, etc.).

4. Advantages and disadvantages by generation

5. In summary

Luminescent materials tell the story of a century of technical evolution:

From the radioactivity of radium to the safety of tritium, to the eco-friendly and sustainable luminosity of Super-LumiNova.

They are no longer used just to tell the time, but also to assert the character and visual identity of a watch.