More than a century after Nikola Tesla became one of the inventors of the fluorescent light bulb, our low-energy and fluorescent lamps essentially still rely on the same ingenious technique:
In a tube coated on the inside with a fluorescent substance and filled with a noble gas and mercury vapour under low pressure, a gas discharge takes place between two electrodes at both ends, causing the mercury vapour to emit ultraviolet light; the fluorescent coating then converts the UV rays into visible light.
Apart from the mercury content, discarded fluorescent lamps are also a source of rare earths such as europium and yttrium which, incidentally, are recovered through techniques developed at the Catholic University of Louvain.
As for the noble gases – in addition to argon, neon is often used for orange-red light and krypton for pale purple light – they have found their way to numerous common and not so common applications:
- argon (39Ar) is used to date ground water
- krypton is used in some types of photographic flashes for high-speed photography
- a diver’s drysuit is sometimes inflated with argon (as an alternative to nitrogen or helium-based breathing gases)
- neon often serves as a heat carrier in cooling installations
- double glazing is often filled with argon: in between the two layers of glass its insulating properties are very efficient.
Like those earth metals, krypton – not to be confused with the fictitious element kryptonite from the Superman films – is rather precious. Conversely, neon and especially argon are not so rare: argon accounts for 0.00015% of the earth’s crust, making it 500 times more common than neon and even twice as abundant as vapour.