Luminescence on a dial is not an aesthetic optional extra: it's a technical choice that determines how long and with what intensity a watch remains readable after ambient light disappears. I've seen three generations of materials pass through my workbench, and the difference between one and the other is measured in minutes of useful reading in total darkness.
The most widespread system today is Super-LumiNova, developed by Nemoto and Co. in Japan and distributed in Europe through RC Tritec. It's a photoluminescent compound based on strontium aluminate: it absorbs photons during exposure to natural or artificial light and gradually releases them in the form of a glow. The most common emission colour is green (classified C3), but there's a C1 variant, white-green, visually less intense but perceived as more neutral on certain light dials. The X1 and X2 versions are next-generation formulations with higher initial luminance and slower decay. Rolex has developed its own proprietary solution, Chromalight, which emits blue light instead of green and claims extended autonomy compared to standard Super-LumiNova. Citizen uses the acronym SLN, short for Super Luminous Noctilucent, to identify an analogous substance applied to its mechanical calibres.
Application varies depending on dial construction. On applied indices, the compound is poured into the internal cavity of the metal piece. On hands, it fills the channel milled in the hand body itself. On sandwich dials, typical of original military Panerai watches, the two-layer overlapping structure exposed the luminescent material through the openings of the upper layer, ensuring high readability with a greater thickness of compound. In the Ming 29.01 Midnight, Super-LumiNova X1 has been integrated into HyCeram ceramic, a more recent solution that solves the problem of compound degradation over time.
The quality of application matters as much as the quality of the material. Non-uniform filling or bubbles create irregular emission points, visible even to the naked eye under UV light. When I receive a vintage watch with poorly re-luminised indices, it's immediately apparent: the compound tends to overflow onto the dial or fall short of the index edge. In original pieces from past decades the material was often tritium, recognisable by the yellow-brown patina it develops with age and the presence of minimal residual radioactivity, now irrelevant but which requires customs declarations in certain countries.
A conceptually different variant is luminescent printing, used on some affordable sports watches like the Swatch ScubAqua: the material is printed directly onto the bezel or dial without engraving, with results sufficient for recreational diving use but less durable over time compared to application on metal.