What does “T Swiss Made T” or “Swiss T<25” mean on a watch dial?

Look under six o'clock on a Swiss watch from the late 1960s to the late 1990s and you will often find a T on each side of "SWISS MADE", or "SWISS T<25" printed in one line. Collectors' sites, dealer journals and forum threads all give the same two numbers, 7.5 and 25 millicuries, and all name ISO 3157 as the source. None reproduces the standard: it was sold, not published, and ISO has since withdrawn it. Below: the limits from a regulation that copies them, the US and German rules beside them, every lume mark you are likely to find, and what is left of the tritium in a dial from each decade.

Where the 7.5 and 25 come from

ISO 3157, Radioluminescence for time measurement instruments: Specifications, was the international standard for radioactive lume. ISO's own catalogue entry says the 1991 second edition "specifies requirements and test methods for the optical, mechanical and radioactive characteristics of radioluminescent deposits fixed on instruments". It was six pages long, it replaced the 1975 first edition, and it was withdrawn on 3 January 2018 with no replacement listed. The text is still sold through national standards shops, but it cannot be read for free.

The free copy of its numbers is a UK regulation. The Radioactive Substances (Basic Safety Standards) (Scotland) Regulations 2000, SSI 2000/100, made on 31 March 2000, has a Table B of luminised timepieces. A note under it says the levels "are those allowed by" ISO 3157:1991(E). These are the per-watch limits it lists:

The two dial figures are the first and third tritium rows, written to two significant figures. The forum posts that quote ISO 3157 directly give 277 MBq (7.5 mCi) and 925 MBq (25 mCi). 7.5 mCi is exactly 277.5 MBq and 25 mCi is exactly 925 MBq, which round to the regulation's 2.8 and 9.3 × 10⁸. So two independent copies, one a regulation and one a quotation, agree to the becquerel.

The regulation gives classes, not dial wording. The link between a mark and a row comes from the figures that Chronext and the forums attribute to ISO 3157: under 7.5 mCi for T SWISS T, under 25 mCi for SWISS T<25. Those land on the worn-on-the-person row and the special row, and on no other row. What ISO counts as a "special" instrument is in the paid text; the free sources do not say.

Every lume mark at six o'clock

The Swiss origin words sit in the middle and the lume letter goes either side of them or after them. This is what each combination declares, with the source for each line:

The marks are a convention for the luminous deposit, separate from the origin claim. "SWISS T<25" is still the protected word "Swiss", with the same legal weight as described in what Swiss Made actually means. The tritium letter adds nothing to that claim and takes nothing away.

Is T<25 the US limit?

No, though it is easy to see why people think so. The US Nuclear Regulatory Commission's exemption for timepieces, 10 CFR 30.15(a)(1), lets anyone own or pass on a timepiece holding up to "25 millicuries of tritium per timepiece", with no more than 5 mCi per hand and 15 mCi per dial, bezel included. That is the same 25. But the same 25 mCi (925 MBq) is in the ISO-derived row for special instruments, in a regulation that never mentions the US. A T SWISS MADE T watch sits well inside the US figure; a T<25 watch sits at it.

The two rule sets part company on promethium. The US allows 100 µCi per watch, with 20 per hand and 60 per dial, and 200 µCi for any other timepiece. The ISO-derived limit for a watch worn on the person is 5.5 × 10⁶ Bq, about 149 µCi, so here the US figure is the stricter one. For a timepiece not worn, both come to 200 µCi.

The German figures come from the Federal Office for Radiation Protection (BfS), in a page dated 31 July 2018: watches above "the exemption limit of 1 GBq" of the German Radiation Protection Ordinance cannot be sold there. The same page says tritium paint watches "show a Tritium activity of about 0.2-0.3 GBq on average". That is 5.4 to 8.1 mCi, the same range as the T SWISS MADE T class. In other words, a typical painted dial was made close to the ordinary ISO ceiling, not far below it.

What is left of the tritium today

Tritium "is a beta-emitter with low energy of up to 19 keV and a half-life of 12.3 years", the BfS says. The share left after t years is 0.5 to the power t/12.3. Our comparison of Super-LumiNova, tritium and LumiBrite uses the same half-life. Worked through for dials made in each period and measured in 2026, and assuming a dial filled to its class ceiling:

A 1990 dial is at about 13 percent; 6 percent is where a dial from about 1975 stands now. The BfS dates the end of tritium paint to "the middle of the 90ties", so 1995 is about the newest row a painted dial can sit on. The glow falls faster than these figures, because the phosphor the tritium excites also wears out, as our guide to watch lume explains. That is why a period-correct T dial barely shows in a dark room. One that glows brightly has probably been relumed with a photoluminescent pigment, which is one of the checks in spotting a redialed watch.

What the mark means when you own or buy one

1. It dates the dial. A T mark puts the dial in the tritium paint era, from the mid-1960s to the mid-1990s on most Swiss pieces. That is a useful check in the vintage buying guide, because refinishers often leave the letters off.
2. It tells you not to expect much glow. At 3 to 17 percent of the original tritium, faint or no lume is normal. It is not a fault to fix; see why lume fades for modern pigments.
3. It is not a radium warning. On the BfS figures, a painted tritium watch gives a dose "mostly below 20 µSv per year". The radium case is different, and our radium answer puts the two US limits side by side.
4. It affects relume decisions. Replacing tritium plots with modern pigment ends the radioactivity but also changes what the dial is. Ask about this at service before the dial goes out, and before the watch is valued.

A T<25 ceiling is about three times a T dial's, but tritium's beta radiation does not get through the crystal and case, so the difference is one of bookkeeping, not of wearing.

T25 and T100 on gas tube watches

Modern self-lit watches use "tiny, narrow glass tubes filled with tritium gas (GTLS = gaseous tritium light sources)", in the BfS's words. Their rating counts the whole watch: "All tritium watches have a rating on them which typically states T25 or T100", WatchGecko writes, "a measurement of total millicuries". NITE describes T100 as "between 25-100" units and says it gives "roughly four times more light". Both ratings are far under the 200 mCi ISO-derived row for GTLS watches. A T100 watch is over the 25 mCi in the US 30.15 timepiece figure, and at 100 mCi (3.7 GBq) it is over the German 1 GBq limit too. A T25 watch fits all three. For the tube versus paint trade-offs, see the dive watch guide and the guide to dial legibility.

When this answer stops applying

• The dial is not Swiss. The BfS's "H3, T or T25" describes common practice; it is not a marking rule.
• It was made before 1991. The figures here are from the 1991 edition. The 1975 first edition's limits were not read; we do not know if they differ.
• The law, not the mark, is the question. SSI 2000/100 was read as made in 2000, as a copy of ISO's figures, not as current Scottish law. For any jurisdiction, check the regulator's current text.
• The dial has been restored. A relumed or reprinted dial may keep the T and lose the tritium, or the other way round. The pre-owned buying guide covers what to ask.

Otherwise: T SWISS MADE T means tritium under 7.5 mCi, SWISS T<25 means tritium under 25 mCi, both are ISO 3157 classes, and a 1990 example now holds about an eighth of what it started with.