Photoluminescence

Photoluminescence in watchmaking is not a single substance: it is a family of materials that absorb photons and return them after the light source has disappeared. The term you hear most often at the counter is lume, an anglicism now established in common usage, but the correct technical concept is precisely photoluminescence — to be distinguished from the radioactive phosphorescence used until the nineteen sixties.

The breakthrough came in 1993 with Super-LumiNova, developed by Japanese Nemoto & Co. and distributed in Europe by RC Tritec. The base formula is strontium aluminate doped with europium: no radium, no tritium, no independent energy source. The material works only when charged — sunlight, artificial light, even a UV torch suffices for 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 who tells you otherwise is selling you something.

Over time, variants with different performance characteristics have emerged. C1 emits yellow-green, C3 brilliant green, BGW9 blue-white — the latter has become the reference for tool watches, because the tone reads well even at low intensity. X1, introduced more recently, claims 60% higher luminous output compared to the standard version; I have seen it applied to dials, bezels, and hands in combination, where the overlapping of the sources truly aids readability. Then there is Super-LumiNova used as a decorative pigment by day — white, green, orange — with nighttime luminescence as a secondary function, not primary.

Application matters as much as the formula. 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 influences emission: too thin and the photon reservoir is meager; too thick and the paste can crack over time, especially on hands that undergo vibrations. On galvanic or enameled dials, lume goes into dedicated recesses, because the surface does not absorb the binder.

One thing I see misunderstood often: the color you see by day and the color emitted in darkness do not necessarily coincide. A hand with white lume by day can emit intense green at night. Anyone choosing a watch also for nocturnal aesthetics should know this in advance.