Radium: 1910s to the 1960s
The first luminous material used in watches was radium — specifically radium-226 mixed with a zinc sulphide phosphor. Radium is radioactive, and it was this radioactivity that excited the phosphor and caused it to glow continuously without any need for a light source. The glow was permanent, self-sustaining, and visible even in complete darkness.
The military demand for legible watches and instrument panels in low-light conditions drove radium's adoption rapidly after the First World War. By the 1920s, radium dial painting was an established industry, and the workers — mostly young women known as the Radium Girls — were painting dials in factories across the United States and Europe.
The tragedy of the Radium Girls is now well documented. Factory supervisors encouraged workers to point their brushes with their lips to achieve precision on fine dial work. Radium ingested or inhaled accumulates in bone (the body treats it similarly to calcium) and causes radiation damage from the inside. Many workers developed jaw necrosis, bone cancers, and anaemia. The legal battles brought by the Radium Girls in the late 1920s were among the first occupational safety cases in US history and established foundational principles of employer liability for workplace health hazards.
Radium-226 has a half-life of approximately 1,600 years. Watches painted with radium between 1910 and the 1960s still contain active radium today. The quantity on a watch dial is small enough that wearing such a watch is not considered a significant health risk to a normal person — but opening the case, disturbing the lume, or storing multiple such watches in an enclosed space concentrates the exposure.
If you own a pre-1960s watch with original lume: do not attempt to clean or disturb the dial. Do not store it in a sealed box with other watches. If you're opening the case for service, inform the watchmaker that it may contain radium — handling protocols differ.
Tritium: 1960s to the 1990s
Radium was replaced progressively from the 1960s onwards by tritium — a radioactive isotope of hydrogen. Tritium's radiation is much weaker (it cannot penetrate human skin) and its half-life is only 12.3 years, meaning a tritium-painted watch from 1975 retains very little activity today. Watches painted with tritium are generally considered safe to wear and handle.
Tritium was used as a paint mixed with phosphor (as radium had been) and also, from the 1990s, as a gas sealed inside glass tubes. Tritium gas tubes (sold under trade names like Trigalight, used under the H3 designation) contain small amounts of tritium sealed hermetically; the beta radiation excites a phosphor coating on the inside of the tube, producing continuous glow without any external energy source.
Tritium tubes remain in use on some contemporary watches (particularly military-specification pieces) and offer true darkness glow — they don't require charging with light. Their limitation is that after approximately 10–12 years, the gas pressure decreases and the glow fades. At 25 years, they're typically dim. Replacement is possible but requires specialist workshop work.
On specification sheets, tritium content was historically designated by the "T" mark followed by a number indicating the activity level: T25 for 25 millicuries (common in consumer watches), T100 for higher-activity military pieces.
Super-LumiNova: The Modern Standard
Super-LumiNova (manufactured by RC Tritec, Switzerland) is the dominant luminous material in watches made from the mid-1990s to the present. It is entirely non-radioactive — a strontium aluminate compound that absorbs visible and UV light and re-emits it slowly as glow. The mechanism is photoluminescence: the material must be charged by light exposure before it glows.
Super-LumiNova requires approximately 1–2 minutes of light exposure to produce a meaningful glow and typically glows brightly for 30–60 minutes after full charge, then fades gradually. It does not glow continuously in complete darkness the way radium or tritium-tube pieces do — if you put it in a completely dark room and come back hours later, it will be dim or dark.
Super-LumiNova is produced in multiple grades. The A-grade (BGW9/C1/C3) is the highest performance and produces the characteristic blue-green glow familiar from modern dive watches. C3 is the brightest grade currently available. The lume colour (green, blue-green, or off-white) depends on the grade used.
Unlike radium and tritium, Super-LumiNova does not degrade with radioactive decay. It does fade over decades due to UV exposure and oxidation, but replacement — relume — is a standard watchmaker service on older watches.
Identifying the type: A vintage watch glowing in complete darkness without recent light exposure almost certainly contains radium (pre-1960) or tritium tubes (post-1990). Painted tritium (1960s–90s) and Super-LumiNova require charging and will be dim or dark without recent exposure.
Have a Vintage Watch With Faded Lume?
We can relume dials and hands with modern Super-LumiNova, matching the original colour and application style. Bring it to our workshop in Durrës for an assessment. Walk in, no appointment needed.
Rruga Aleksander Goga · Durrës 2001 · Albania · +355 67 636 0510
Published by Iglisi Watch · Durrës, Albania · April 2026. This article covers the history of luminous watch dial materials as general watch knowledge.