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Reading a french article talking about a recycling unit of rare earth from energy saving light bulbs near Lyon - France (here is the official press release of the Salvay company), I was wondering :

  • where is rare earth used in energy saving light bulbs ?
  • what advantages does it provide ?
  • why don't we use them in regular light bulbs ?
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2 Answers

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  • Fluorescent light bulb

    An 'energy saving light bulb' is a fluorescent lamp. Those lamps are filled with a gas containing low pressure mercury vapor and noble gas.

    To generate light, an electrical discharge is sent through an ionized gas. The excited atoms produce ultraviolet radiation.

    The inner surface of its tube is coated of various metallic elements such as rare earth. Those rare earth elements when crossed by UV radiation emits red, blue and green lights, which together are percieved as white.

  • Incandescent light bulb

    Regular bulbs are incandescents light which emit light by heating the filament present inside the bulb. As there is no UV radiation, there is no reason to use rare earth !

References:

Wikipedia - Fluorescent lamp

What's inside a low-energy light bulb? (reference given by Luboš Motl)

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These light bulbs perform electric discharge in the vessel filled with argon which produces ultraviolet radiation and this ultraviolet radiation "activates" lanthanum or other rare earths in the coating which emit colorful visible light – different colors for different elements.

See e.g. this Wired article for some introduction.

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