16 Arc Tube

All gas discharge lamps have a chamber which is filled with a gas, and where two electrodes of opposite polarity at either end of the arc tube pass an electric current between them. The space where this happens is called the Arc Chamber, or Arc Tube.

For Fluorescent lamps, the entire length of the lamp is the arc tube, while for the HID lamps the arc tube is much smaller.

The electrodes at either end of the tube are called cathodes, and they are directly connected to the external circuitry of the lamp.

All visible and invisible waves of EM radiation are produced in the arc tube by moving electrons crashing into the gas that fills the arc chamber. As the electrons impart energy to the atoms of the gas, they emit energy in the form of visible light. All light is produced by collisions between electrons and the atoms of the gas that fills the lamp.