High Intensity Discharge (HID) Lamps
In HID lamps light is generated by a gas discharge inside a confinement called an arc tube or a burner. The gas mixture inside the arc tube is heated, and therefore partially ionized, by maintaining an electrical current between two electrodes. The ionized gas forms the light emitting arc.
To prevent demixing of gas components inside the burner, the lamp is operated at alternating current. As a result of this periodic heating, an acoustic wave with the modulation frequency of the power in the discharge is generated. It propagates towards the walls where it is damped and reflected. Incident and reflected waves interfere which leads to the development of standing acoustic waves. At certain frequencies resonances form. The standing waves interact with the discharge arc and are responsible for its distortion and instability. These take the form of flickering light, reduction in the lamp’s lifetime or even destruction of the lamp. In order to further improve HID lamps it is necessary to understand and control the acoustic resonance phenomenon.