Al chem street-lamps [20]

I presume this is on low pressure sodium lamps that are also known as sodium oxide lamps. They consist of a glass outer layer with a coating that reflects infrared light as well as another layer that will only let visible light out while keeping infrared light (which is heat) in. Then there are two smaller glass pipes inside that contain the sodium metal as well as asmall mixture of neon and argon gasses, which are used to light the Sodium. When the electrical current goes through the gas mixture, it causes it to emit a dim pinkish light, which then heats up the sodium.Then, when the sodium vapourizes, it changes and emits a bright orange light.

In short summary, the neon and argon mixture acts as a starter after the inital current pushing their electrons to jump up to a higher energy level that is unstable. When they return to the original state, light and heat are both released. The light energy emitted here is seen as weak pinkish (or red colour) with heat energy generated to pump up the Sodium electrons into vapour that in turn settle down to emit the characteristic orange (Sodium) glow. This creates repetitive glowing cycles until the lamp is turned off.