Gas-discharge lighting was first discovered and commercialized in the early 1900s. When inventors ran high-voltage electric current through different gases, they discovered that some corroded the wire inside the glass tube. Noble gases, known for being chemically unreactive, were tried and found to produce vivid colors. Neon, in particular, gives off a bright glow. The other noble gases, argon, helium, xenon, and krypton, are also used to create bright, colorful signs and displays. Radon, the other noble gas, is radioactive and not used in signs.
Neon makes up a fraction of the air you breathe; purifying it is simple and inexpensive. It’s is the most common gas used for signs, giving off a strong red glow. Only small amounts of the gas are needed to make a neon sign. While the sign fixtures use high voltages, their power consumption is very low, in the milliwatts, making them energy-efficient.
Abundant in air, argon is inexpensive to produce. Its light is fainter than neon. A tiny amount of mercury is usually added to produce stronger light. These lamps have a light-blue color, though you can produce other colors by coating the inside of the glass tube with ultraviolet-sensitive phosphors. The mercury gives off ultraviolet light and makes the phosphors glow.
In cold climates, helium can be added to the argon to heat the lamp faster, making its operation more efficient.
In addition to its use with argon, helium can be used alone to produce a pinkish-red glow. Pure helium is more of a specialty item for gas-discharge lamps than neon or argon. This gas is more rare; most helium is produced naturally by radioactive decay and found in natural gas deposits.
Xenon gas can be used to produce a bright lavender light. As with helium, it’s not often used by itself for sign lighting, though it’s long been used for strobe lights and flash photography. Xenon can be mixed in various proportions with other noble gases to make signs of different colors.
Krypton emits a characteristic yellow-white light. This makes it useful for other colors; if the lamp’s glass is colored, the light from the krypton will take on that new color. As with xenon, krypton is also used for lighting applications other than signs, such as airport approach lights.