High Intensity Discharge Lamps

High intensity discharge lamps (HID lamps) are gas discharge lamps which are operated with a high power density. They are generally made as short-arc lamps, i.e., with a small distance between anode and cathode. Further, they are typically arc lamps, i.e., continuously operating lamps rather than flash lamps. The relatively small light-emitting volume allows for collimation or focusing of the generated light, which is essential for some applications such as searchlamps or projection displays.

While all HID lamps share the principle of a high-current electric arc, the specific technology is defined by the gas fill and the pressure:

• Mercury vapor lamps: The oldest type, producing a bluish-green light. They have largely been superseded by more efficient and better-color-rendering alternatives.
• Metal halide lamps: These contain mercury plus metal halides (e.g., iodides of sodium, scandium, or dysprosium). They offer high luminous efficacy and good to excellent color rendering, making them popular for stadiums, retail, and automotive lighting.
• High-pressure sodium (HPS) lamps: Characterized by a strong yellow-orange emission, these lamps are extremely efficient and long-lived, standard for street lighting and horticulture where color rendering is less critical.
• Xenon short-arc lamps: Filled with high-pressure xenon gas, they produce a continuous spectrum closely matching sunlight and offer very high luminance from a tiny arc gap. They are the standard for cinema projection and high-performance searchlights.

Similar to many other gas discharge lamps, HID lamps exhibit negative differential resistance; as the arc current increases, the voltage across the lamp decreases. Therefore, they cannot be connected directly to a constant voltage source (like the mains) but require an electrical ballast to limit the current. The ballast also provides the high ignition voltage required to strike the arc. Modern electronic ballasts allow for more stable operation, dimming capabilities, and sometimes hot-restrike functionality.