Short answer

If a Fifty Fathoms is listed as "tritium," that word describes the luminous paint on the dial and hands, not a model. Tritium was the standard lume from the early 1960s into the late 1990s. It is mildly radioactive, it has a half-life of roughly 12.3 years, and it stops glowing long before it stops looking good. On most listings that use the phrase, the dial is not tritium at all. It is modern Super-LumiNova tinted tan to look old. That tint gets called fauxtina, and it is the first thing I check.

What real tritium does

Tritium was mixed with a phosphor and painted onto the dial plots and hands. The isotope's beta decay excited the phosphor, so the lume glowed with no charging from light. That is also why it died. After two or three half-lives there is not enough radiation left to light the phosphor, and a 1990s dial is now three half-lives in. In practice, real aged tritium barely glows. Hit it with a flashlight in a dark room and you get almost nothing, or a weak flicker that fades in seconds. It also reads dull under UV.

Color is the second signal. Tritium patina runs from cream to sand to deep pumpkin, and it is a surface change in the binder and the phosphor, not a repaint. It reads uneven. Hands usually darken more than the dial plots because the paint sits on metal and cures differently. Two examples of the same reference rarely match each other.

Why the T marking is not proof

Tritium-era dial signatures are easy to spot: T SWISS T, T SWISS MADE T, or a bare SWISS T near the bottom. They mean the dial was made for tritium lume. They are also printed on plenty of redials and replicas, so treat them as one clue among several. Print quality at 10x is the better test. Look at the serifs on the brand name, the spacing in the depth rating, and the edges of the minute track.

Fauxtina: the tell that catches most people

Tinted Super-LumiNova is common now, and it is not dishonest on its own. Several modern reissues ship with aged-looking lume on purpose. The problem is a tinted dial sold as original tritium. One test usually separates the two.

• Charge the dial with a phone light for ten seconds, then cup it in your hands. Tinted Super-LumiNova glows bright and green-blue. Dead tritium does not.
• Compare the hands to the plots. Real patina drifts apart. Fauxtina is uniform because it came out of one batch.
• Look at the lume under UV. Real tritium is flat and dull. Modern lume lights up.
• Check for flaking. Old tritium paint crumbles and leaves crumbs on the dial. Tinted paint sits smooth.
• Sniff the deal, not the watch. A tritium-era price on a dial that glows is the whole answer.

Where replicas give themselves away off the dial

Lume is the easy part to copy, so the rest of the watch matters more. A modern Fifty Fathoms uses a sapphire bezel insert, and the edge of that insert catches light like glass. Plastic or coated inserts look flat and pick up scratches. Caseback engraving on a replica is often shallow, with soft lettering and the wrong reference code for the case size. Inside, Blancpain's in-house calibers such as the 1315 run at a slow beat with a long power reserve and a silicon hairspring, so a look at the balance and the finishing tends to settle it.

Service dials muddy everything

One more thing before you call any watch fake. A genuine vintage Fifty Fathoms that went to a service center in the 2000s may have come home with a fresh Super-LumiNova dial, because tritium is no longer used. That watch is real. It just is not a tritium watch any more. Collectors pay more for original tritium, so dial swaps get hidden, and a bright "tritium" listing is sometimes a service dial rather than a counterfeit.

Safety, briefly

Beta particles from tritium cannot pass through a sapphire crystal or a steel caseback, so a sealed watch is not a radiation concern. Loose flakes are a different story. If paint is lifting off the dial, do not open the case and do not handle the crumbs. Hand it to a watchmaker.

Bottom line

Judge a "tritium" Fifty Fathoms by behavior, not by color. Dead lume that refuses to glow, uneven patina, and hands that aged faster than the dial are the marks of the real thing. Lume that lights up like a new watch, identical color across every plot, and a price that says vintage are the marks of a tinted dial, a service dial, or a replica dressed up as one.