A gas-discharge tube uses electrodes inside a low-pressure gas to create light. When a high potential difference exists between the electrodes, the gas between them ionizes and forms a plasma. The free electrons in this plasma allows current to flow between the electrodes. The plasma either generates light directly, or by causing another material to create light. There are many different types such as the fluorescent, neon, xenon, sodium, metal halide, and switching tubes.

Penning Mixture

A pure gas has a much higher ionization voltage than a mixture of gases. The most efficient mixture is called a Penning mixture, and consists of a noble gas and a small amount of other gases. Two common Penning mixtures are neon with a small amount of argon, and helium with a small amount of xenon.

Fluorescent Tubes

The gas contained in a fluorescent tube is mercury vapor. When excited, the mercury atoms produce ultraviolet light which strike a phosphorous coating on the inside of the tube, and the excited phosphorous creates visible light. Fluorescent tubes use less power and last longer than incandescent globs, but are also larger and more expensive.

Neon Tubes

Neon tubes use a Penning mixture of neon and argon to produce orange light, but other gases can be used to produce other colors. Neon tubes are widely used in large shop signs. A Nixie tube is a type of neon tube that has one anode and many cathodes, shaped to resemble numbers and letters. These were used in displays before LCDs and LEDs became widespread.

Sodium Vapor Lamps

Sodium lamps have an inner tube filled with solid sodium and a Penning mixture. This tube is placed inside a vacuum-sealed glass lamp coated with indium tin oxide. This layer lets visible light out but stops the infrared wavelengths. Startup is a two stage process where the Penning gas heats up the sodium until it forms a vapor. It is why these lamps initially have a different color which turns yellow after a few minutes. Sodium lamps are very efficient and widely used for outdoor lighting.

Metal Halide Lamps

Metal halide lamps are also called high intensity discharge lamps, and are used to light sports fields and large warehouses. Their gas contains a mixture of mercury, argon, and metal halides. The proportions of metal halides in the gas determines the color of their light.

Switching Tubes

Some gas-discharge tubes, called switching tubes, behave like transistors. The Thyratron and Krytron allow large currents to flow when a small current is applied to their control electrode. A Geiger-Muller tube is used to detect radiation and is the main component in a Geiger counter. It allows a short pulse of current to flow through it's electrode when the gas is ionized by radiation.