Photoluminescence in watchmaking is not a single substance: it's a family of materials that absorb photons and return them after the light source disappears. The term you hear most often at the counter is lume, an Anglicism now firmly established in common usage, but the correct technical concept is precisely photoluminescence — to be distinguished from the radioactive phosphorescence used until the 1960s.
The breakthrough came in 1993 with Super-LumiNova, developed by Japanese company Nemoto & Co. and distributed in Europe by RC Tritec. The basic formula is strontium aluminate doped with europium: no radium, no tritium, no inherent energy source. The material works only when charged — sunlight, artificial light, even a UV torch needs just a few seconds. The emission then decays over time: the first half hour is the most intense, then the glow progressively fades. This is the physical limit of the system, and anyone telling you otherwise is selling you something.
Over time, variants with different performance levels have emerged. C1 emits yellow-green, C3 bright green, BGW9 blue-white — the latter has become the benchmark for tool watches, because the shade reads well even at low intensity. X1, introduced more recently, claims 60% higher luminous output compared to the standard version; I've seen it applied to dials, bezels and hands in combination, where the overlap of sources really helps readability. Then there's Super-LumiNova used as a daytime decorative pigment — white, green, orange — with nighttime luminescence as a secondary, not primary function.
Application matters as much as the formula. The lume can be painted directly onto the printed dial, filled inside a hollow applied index, or inserted into a milled hand with a dedicated housing. The thickness of the layer affects emission: too thin and the photon reservoir is meager; too thick and the paste may crack over time, especially on hands that undergo vibrations. On galvanized or enameled dials, the lume goes into special recesses, because the surface doesn't absorb the binder.
One thing I often see misunderstood: the color you see during the day and the color emitted in the dark don't necessarily coincide. A hand with white lume during the day can emit intense green at night. Anyone choosing a watch also for its nighttime aesthetics should know this in advance.
