Sodium-vapor lamp - a gas discharge lamp that uses sodium
in an excited state to produce light. Two types of such lamps:
low pressure and high pressure.
A low pressure sodium streetlamp
Spectrum of a low-pressure sodium lamp. The
intense yellow band is the atomic sodium D-line
emission, comprising about 90% of the visible light
emission for this lamp type.
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Source: http://en.wikipedia.org/wiki/Sodium-vapor_lamp