Lume is the photoluminescent material applied to indices and hands to ensure hour readability in the absence of light. It is not just any phosphorescent paint: it absorbs luminous energy — sunlight or artificial light — and releases it progressively as visible light in the dark. The duration and intensity of the glow depend on the chemical formula, the exposure time to light, and the thickness of the applied layer.
The most widespread compound today is Super-LumiNova, produced by Swiss company RC Tritec, based on strontium aluminate doped with rare earths. It is not radioactive — a fundamental distinction from the radium and tritium used for decades until the 1990s. It exists in many variants: BGW9 emits white by day and green in the dark, it is the brightest and most used on tool watches. SLN-C1 emits blue, has longer glow but less intensity at the start. X1 tends toward warm beige, often chosen for dials with vintage inspiration. X2, used by Panerai among others, allows differentiation of color between hands and indices on the same dial. Grade A simply indicates high quality of charge; it is not a separate formula.
The application technique changes everything. On entry and mid-range models, lume is printed flat. On higher-level watches it is applied by hand, filling recessed indices on the dial or carved into the hand with a spatula. The so-called solid lume — a mixture of luminescent and ceramic — allows for significant thicknesses: by day the index appears compact and three-dimensional, in the dark the mass of material ensures longer-lasting glow. The sandwich dial takes the concept further: the lume is spread on a separate lower plate, visible through the cutouts of the upper dial. Panerai has built an entire aesthetic identity on this system.
Some manufactures produce proprietary formulas. Rolex calls theirs Chromalight: it emits blue, with claimed duration double that of standard green. Seiko uses Lumibrite. In both cases the chemical base remains strontium aluminate; the proportions and dopants change to optimize color, duration, and stability over time.
On vintage collecting, a separate note applies. The 1993 Panerai Luminor watches with so-called non-matching dials are a textbook case: the lacquer used to fix the tritium reacted chemically with the radioactive material, yellowing the numerals toward orange while the hands remained light green. Panerai eliminated the lacquer, but those irregular examples are today among the most sought after. It is the paradox of collecting: a production defect becomes a historical document.