By Keith Dooley, Updated Mar 24, 2022
A high‑pressure sodium lamp uses sodium vapor, along with mercury, to produce bright, energy‑efficient illumination. The lamp’s tube—typically made of aluminium oxide for its high‑pressure resilience—is sealed with xenon gas, which acts as a starter that does not react with the other gases.
Within the tube, the sodium and mercury vapors form an electrical arc when ignited. The temperature—and thus the pressure—of the vapor is governed by the power supplied. Higher power raises the temperature, increasing pressure and light output. An inductive ballast maintains a constant current, preventing voltage spikes and ensuring stable operation.
The ballast contains a coiled wire that generates a magnetic field when current passes through it. This field stores energy, which the ballast releases to deliver the initial surge of voltage needed to start the lamp. Once the lamp is running, the ballast continuously regulates the current, protecting the lamp and maintaining consistent brightness.
