What are the four principal types of electric discharge lamps?

The four principal types of electric discharge lamps include sodium, mercury vapor, halide, and fluorescent. Each of these lamps operates using a different method of generating light, specifically through gas discharge.

Sodium lamps, including low-pressure and high-pressure types, are particularly known for their efficiency and are commonly used in street lighting due to their high lumen output and long lifespan. Mercury vapor lamps are some of the earliest types of electric discharge lighting, producing a bluish light. While they have seen reduced usage with the rise of more efficient options, they are still recognized for their effectiveness in certain applications.

Metal halide lamps are an advanced version of mercury vapor lamps that include halide compounds, leading to better color rendering and efficiency. Finally, fluorescent lamps utilize a different gas mixture to produce light through a phosphor coating inside the tube, making them widely used in commercial and residential lighting applications.

In contrast, other options either mix different types of lamp technologies, such as combining incandescent or LED—which do not fall under electric discharge lamps—or focus on older or less efficient lamp types. Thus, the choice that accurately categorizes the four principal electric discharge lamps is indeed sodium, mercury vapor, halide, and fluorescent.