The bright, flashing advertisements that illuminate cityscapes are a type of gas-discharge light commonly known as “neon signs.” The element that gives these signs their name and their characteristic red-orange glow is the noble gas Neon. While the term “neon sign” is used universally, only the tubes that produce the fiery red-orange color actually contain pure neon gas. These signs operate by passing a high-voltage electrical current through a sealed glass tube filled with gas at low pressure. The specific color produced depends entirely on the gas used inside the tube.
Neon: The Core Element and Its Unique Glow
Neon is a noble gas, meaning its atoms have a full outer shell of electrons, making it chemically stable and unreactive. To make the gas glow, a high-voltage electrical current, often between 3,000 and 15,000 volts, is applied across the electrodes at either end of the sealed glass tube. This intense electrical energy strips electrons from the neon atoms, a process called ionization, which creates a conductive plasma within the tube.
As the free electrons accelerate through the tube, they collide with other neon atoms, transferring energy to them. This collision causes the electrons in the neon atoms to jump to a higher energy level, putting the atom into an “excited” state. This higher energy state is inherently unstable, and the excited electrons quickly fall back to their original, lower energy level, or ground state.
The excess energy released during this drop is emitted as a photon, or particle of light. The specific distance the electron drops between energy levels is fixed for neon, dictating the exact wavelength of the emitted light. Neon atoms primarily release photons in the red and orange parts of the visible spectrum, resulting in the distinct red-orange light associated with the element.
Expanding the Color Palette Beyond Neon
Although pure neon gas produces only the classic red-orange hue, most advertising signs utilize a wide spectrum of colors. To achieve blues, greens, yellows, and white, manufacturers rely on two main techniques: using other noble gases and applying internal phosphor coatings. The most common alternative gas is Argon, which, when electrified, naturally emits a faint bluish or lavender light.
To create a more vibrant blue, a tiny amount of mercury vapor is added to the argon gas mixture. The electrical discharge causes this mercury to emit a strong ultraviolet (UV) light, which is not visible to the human eye. This UV light is then harnessed by coating the inside of the glass tube with specific phosphors, which are chemical compounds that fluoresce when struck by UV radiation.
The color of the light ultimately produced depends on the chemical composition of the internal phosphor coating. For instance, a green light is achieved using the argon/mercury mixture inside a tube coated with a green-emitting phosphor. White, yellow, and various shades of blue and purple are created by selecting different phosphors, all energized by the UV light generated by the argon and mercury combination. Colored glass tubing can also be used as a filter to modify the final light output; a yellow-coated tube filled with red-glowing neon gas, for example, yields an orange color.
The Manufacturing Process and Sign Durability
The construction of these gas-discharge signs is a highly skilled craft that ensures both the custom shape and long operational life. The process begins with “glass bending,” where artisans use a high-temperature flame to heat soft lead glass tubing, making it pliable enough to be shaped into letters and designs over a pattern. Once the glass is shaped, metal electrodes are sealed onto both ends of the tube to provide the necessary electrical connection.
The evacuation process removes all air and impurities, such as moisture, from the inside of the tube using a vacuum pump. Any remaining contaminants would interfere with the gas’s ability to ionize correctly and shorten the sign’s lifespan. The tube is then filled with the noble gas—either pure neon or the argon/mercury mixture—at a very low pressure before being permanently sealed.
The final sign is connected to a high-voltage transformer, which provides the potential difference necessary to initiate the electrical discharge and maintain the glow. Because noble gases are inert, they do not react with the electrodes or the glass, allowing the light-emitting process to cycle continuously with minimal degradation. This stable, low-current discharge system allows these advertising signs to operate for decades, often lasting over 30,000 hours of continuous use.