The glowing tubes commonly referred to as neon lights operate on the principle of gas-discharge lighting, which involves passing an electrical current through a noble gas sealed within a glass tube to produce light. While the term “neon” is widely used to describe all such illuminated signage and art, it actually refers to a specific chemical element that produces only one color. The diverse palette of colors available in these signs requires the use of other noble gases and specialized techniques.
The True Neon Light
The element that gives its name to this type of illumination is Neon (Ne), a colorless and odorless noble gas. When a high voltage is applied across a tube containing pure neon gas at low pressure, the gas emits a brilliant, unmistakable reddish-orange glow. This reddish-orange light is highly efficient for signage and is the most energy-conservative color produced by this technology.
How Noble Gases Create Light
The mechanism for light production in all gas-discharge lamps relies on manipulating the electrons within the gas atoms. A high-voltage electrical current is applied to electrodes at either end of the sealed glass tube. This voltage provides the necessary energy to strip electrons from the noble gas atoms, a process called ionization, creating a stream of free electrons and positively charged ions.
The liberated electrons are accelerated by the electric field, colliding with other gas atoms. These collisions transfer energy, forcing the atoms’ electrons into higher energy orbits, a state known as excitation. Since this elevated energy state is unstable, the electrons quickly fall back down to their original, lower energy levels. The excess energy released during this return to stability is emitted as a photon, or light energy. The specific color of the resulting light is determined by the electron energy levels unique to each type of noble gas atom.
Achieving Other Colors
The vast spectrum of colors beyond reddish-orange is achieved by utilizing other noble gases, often in combination with different substances. Argon (Ar) naturally emits a faint lavender or bluish light when excited. To create the strong blue color ubiquitous in signs, a small drop of liquid mercury is added to the argon-filled tube. The electrical discharge vaporizes the mercury, and the combined gas mixture produces the desired blue light.
This argon-mercury combination is also the foundation for nearly all other colors because the discharge produces a significant amount of invisible ultraviolet (UV) light. By coating the inside of the clear glass tubing with various phosphor powders, the UV light is absorbed and then re-emitted as visible light of a different color. Different phosphor blends can convert the UV energy into shades like bright green, yellow, or pink. Other noble gases like Helium, Krypton, and Xenon are also used to create specialized shades, sometimes in conjunction with colored glass.