Luminescence in watches is a technology, not an aesthetic detail. It serves to read the time when light is absent, and in certain contexts — underwater, military, night aviation — this function is as critical as the movement itself.
The principle is photoluminescent: the material absorbs luminous energy (solar or artificial) and returns it as visible light in the dark. It is not fluorescence, which stops as soon as the source is removed. It is not radioactive phosphorescence, which was the standard method until the seventies. Vintage watches treated with radium or tritium emitted light autonomously and continuously, but at the cost of radioactivity. I have dismantled dozens of those dials in my Milan laboratory: the yellow-brown patina on the indices is precisely the decay of those compounds.
The current reference is Super-LumiNova, produced by Swiss company RC Tritec. The basic formula is strontium aluminate doped with europium — a metallic oxide that accumulates photons and releases them slowly. It is not radioactive, does not degrade over time like tritium, and does not yellow the dial. It exists in increasing grades of quality: Grade A is the professional standard, X1 offers a more intense and longer-lasting white-green luminescence, visible for hours after a brief charge.
The color emitted in the dark is not always green. It depends on the formulation: standard Super-LumiNova emits green, but variants in blue, violet, and white exist. Rolex developed its own proprietary version, Chromalight, which emits blue and claims a duration of more than eight hours. Seiko produces LumiBrite internally, applied to indices and hands of Prospex and the entire dive line.
The quantity of material applied directly affects performance. Applied indices — that is, three-dimensional, glued or screwed onto the dial — can contain more luminescent paste than printed indices, and this shows in the dark. Some manufacturers like Arnold & Son have used Super-LumiNova scenographically, revealing urban skyline silhouettes only in the absence of light. It is a decorative use, distant from the material's functional origin, but technically consistent with the principle.
An aspect that magazines overlook: Super-LumiNova applied in beige or cream tones in daylight — the so-called vintage effect — still emits green light in the dark. The daytime appearance and nighttime color are independent. Those who buy a watch expecting a warm emission even in the dark will be disappointed.