Thursday, January 12, 2012

What is Lume?

Rado's newest 2009 addition, the Sintra Automatic watch is composed of extremely hard ceramic, a steel inner case and titanium screws and clasp. The watch is streamlined, gliding smoothly from watch face to bracelet, widening and narrowing insync with Rado's iconic style. The traditional dial is easy to read, by day and by night as the hands are coated with lume.

What is Lume?
Lume is the substance that allows the hands to glow in the dark. Lume is short for lumious phosphorescent.
What is phosphorescence? A type of photo luminescence.
What is photoluminescence? (abbreviated as PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. (Wikipedia) The photo luminescence is produced when the electrons in a molecule are stimulated by an outside energy source. Either ultraviolet light ("black light"), biochemical reaction (glow-sticks) or radioactivity. Once the electrons are stimulated they quickly return to their original state by emitting radiation, sometimes in the form of visible light.

So we are interested to discover how our lume coated hands work. A phosphorescent substance does not immediately re-emit the radiation it absorbs. The answer is in the involved study of quantum mechanics which I am not an possibly not able to go into at this stage. It is suffice to say that the slower time scales are related to "forbidden energy" state transition in quantum mechanics. Simply put, energy absorbed by a substance is slowly released in the form of light.

What are the most common phosphorescent pigments used today? Zinc sulphide and strontium aluminate.

Most watch manufacturers use strontium aluminate, marketed by the brand name Super-Luminova. This substance is non-radioactive and non-toxic photo luminescence. This technology was developed after the zinc sulphide substance. Being that strontium aluminate is ten times brighter than zinc sulphide, the latter substance was zoned out and currently is something of a novelty.

Watchmakers were on a constant quest to find a substance that allowed time to be displayed in the dark. The problem was initially solved by the chime of a minute repeater or sonnerie, a candle or an open dial. As soldiers of World War One, huddled in dark trenches listening to the roar of the canons , unable to guess the passage of time, the quest to find a glow-in-the dark substance intensified. Watchmakers suddenly remembered Radium, a discovery made at the end of the 19th century. Radium was radioactive and had the property of luminescence. Radium was used in the watch industry for many years, but abandoned in favor of tritium as a result of its high levels of radioactivity; however, too late for hundreds of "Radium Girls in the 1920's". These young girls were employed by US watch companies to paint the Radium and zinc sulphide onto the dials using a small camel haired brush which they pointed with their lips in between paint applications. They had no inkling that this greenish white glow-in-the-dark paint was lethal. Each time they ingested a small amount of radium - an alpha-particle emitter, chemically similar to calcium and thus a bone-seeker, the Radium was accumulating in the bone-marrow and causing bone cancer and other genetic damages.

Tritium, on the other hand is radioactive but in much lower levels than radium, was used until the 1990's. Then came a breakthrough Super-luminova, all the photo luminescence qualities of tritium, but not radioactive and thus less hazardous and more environmental friendly.

There is a distinct difference though. Tritium is permanently stimulated by electrons (betarays) which it emits when it spontaneously transforms into helium. Thus tritium can glow for dozens of years even in total darkness. (see inset) On the other hand Superluminova requires to be stimulated by either violet or ultraviolet light which at first is very bright, but then gradually fades after a few hours in the dark.

As with Super-Luminova the energy emitted by the electrons is stored at quite a stable level, resulting in light being emitted over a several hour period.

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