Which of the following is a HID lamp type?

• A. Low pressure sodium
• B. Solid-state lamp
• C. Electron beam lamp
• D. Electroluminescent lamp

Answer: (A)

High-intensity discharge lamps generate light from an electric arc through a gas or vapor and require a ballast to regulate current. Low-pressure sodium fits this category because it produces light by ionizing sodium vapor at low pressure, forming an arc that emits primarily a single yellow-orange line. This gives extremely high efficiency and long life, which are hallmarks of HID lighting. However, its color rendering is very poor, since it’s essentially monochromatic.

The other options don’t fit HID because they rely on different principles: solid-state lamps are LEDs, not gas-discharge; electron beam lamps use a focused electron beam in a vacuum tube, not a typical HID arc in a gas; electroluminescent lamps emit light through electroluminescence in solid materials, not via a gas discharge.