How Do Sodium Street Lamps Work?

Function

Sodium street lamps are a type of gas-discharge lamp. Gas discharge lamps work by running an electrical current through an ionized gas which can then be used to vaporize another material, such as sodium. For gas-discharge lamps, the electrical current passing through the gas causes some of the electrons to collide with gas and metal ions. This causes these ions to absorb energy and go up to a "higher energy" state, which is essentially a state at which the ion has stored energy. Then, when the ion goes back to its previous, lower energy level, this energy is released in the form of light.

Low Pressure Sodium Lamps

Low pressure sodium lamps 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 a small 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 vaporizes, it changes and emits a bright orange light.

High Pressure Sodium Lamps

High pressure sodium lamps are smaller and have additional materials within them, such as the metal mercury. The gas mixture heats up the mercury, which then heats up the sodium and causing it to vaporize. Because these lamps are made at high pressure, the light that the sodium emits is broadened. This makes it easier to distinguish the colors of objects under this light, which is why they are often used when good visualization of color is important. While not as efficient as low pressure sodium lamps, the higher pressure variety is still relatively efficient.