How Do Xenon / HID bulbs work?

High voltage passes current through two opposing electrodes that ionizes the xenon gas in the glass capsule thereby creating an arc.

The temperature within the capsule rises quickly ionizing the gas mixture lowering the resistance between the two electrodes.

 The ballast then constantly provides the bulb with constant high power ensuring that the bulb is up to operating temperature – this can take a few seconds and is noticeable when the lights are turned on and get brighter gradually.