Sodium vapour lamp viewed through didymium glasses
These glasses are used by glassblowers since they effectively filter out the characteristic sodium yellow glow emitted when heating glass containing sodium making their work easier and unobscured in the flame. Didymium was once thought to be one element but later discovered to be two: Praseodymium and Neodymium. Together they strongly absorb the two sodium yellow emission lines with wavelengths close to 590 nm.
In this case the glasses are used on a 70w High pressure Sodium vapour lamp similar to the ones used as street lights in the UK. The lamp is filled with a mixture of inert gases, usually neon and argon or xenon and a sodium-mercury amalgam. When the lamp is turned on, the inert gases (called starter gas) are excited first and they emit a dim blue/pink light depending on the gas. After a few minutes the temperature is high enough to vapourise the amalgam and the bright yellow light of the excited sodium vapour can be observed.
