Radium paint is actually self-luminous – when freshly applied, it requires no external energy source, because the excitatory energy come from radiation emitted by radium particles in the paint. In order for phosphorescence to take place you need an energy source, and a so-called “phosphor” – a material that will absorb and re-emit light. Despite the name, these transitions still happen, but they’re statistically unlikely and so the trapped light leaks out much more slowly than in fluorescent materials. This is because in phosphorescent materials, the excited electron is in a special energy state, in which the re-emission of a photon would require a “forbidden” energy transition. Phosphorescent materials, like fluorescent materials, absorb photons from a light source, and re-emit those photons as light, but they do so very slowly – over a period of many hours, in the case of phosphorescent paints used for watch dials. These are colloquially known as “black lights,” and black light posters were a dorm room staple in the ’70s (and for all I know might still be).įluorescent materials aren’t much use for watch dials, however for that application you want a phosphorescent material. Anyone who’s played with a UV lamp has seen materials fluoresce. Fluorescent materials tend to emit absorbed energy very rapidly – they’ll glow as long as there’s a source of light to excite the material (typically with what’s described as “neon” coloration) but emission takes place over a time scale of mere nanoseconds, so they will go dark the instant a light source is removed. The two types of photoluminescence familiar to most of us are fluorescence and phosphorescence. While the exact quantum mechanical description of photoluminescence is complicated, the basic idea is simple: if an electron orbiting an atom in a certain material absorbs a photon, it will be excited to a higher energy state, and when the electron “relaxes” to its ground state, it emits a photon which we see as visible light. Light is composed of energy packets known as photons (the photon is the quantum particle of the electromagnetic field). Photoluminescence is the ability of some materials to emit light after exposure to light. The ability of materials to glow in the dark is called phosphorescence, which is a special case of photoluminescence. Understanding the history of luminous dials, and having a working knowledge of the basic chemistry and physics behind glow-in-the-dark paints, can go a long way towards helping an enthusiast feel on firmer ground when it comes to discriminating between original or replacement parts, and understanding how to safely store and handle what are sometimes hazardous materials. The subject is one of the greatest interest to collectors and yet, there is still widespread misunderstanding of how luminous materials actually work. At that point, dials and hands are usually replaced if nocturnal legibility is desired – of course, do that to a vintage watch whose high value rests on originality of all parts, and you may have a watch you can read at night, but you will have destroyed its investment value. On the other hand we would also like our dials to age well, and inherent in the nature of many luminous materials, is that they will sooner or later dim to the point of uselessness. On the one hand, we would like to be able to tell the time in the dark without dropping a hundred large on a minute repeater.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |