A luminous sign, commonly called a “neon sign,” is a sealed glass tube containing gas that glows when a high voltage electrical current is applied. The term “neon” is often a misnomer, as pure neon gas only produces a characteristic red-orange hue. To achieve the vast spectrum of colors seen in modern signage, manufacturers must use other noble gases, often combined with specialized coatings. The final color depends on the interplay between the gas mixture, the energy applied, and the inner surface of the glass tube.
The Specifics of Yellow-Green Illumination
The yellow-green color is not produced by pure neon gas or any other noble gas alone. Creating this specific shade requires a two-part system utilizing a gas mixture that generates light, which is then converted into the desired visible color. The gas mixture used for nearly all colors outside of the neon red-orange spectrum is primarily Argon gas combined with a small amount of Mercury vapor.
When energized, this combination produces a pale, bluish-lavender light from the Argon, but generates significant invisible ultraviolet (UV) radiation from the Mercury vapor. The UV light acts as an energy catalyst, making a special coating necessary to transform this energy into the bright yellow-green hue.
The interior of the glass tube must be coated with a specialized powdered chemical compound known as a phosphor. This compound absorbs the UV radiation and re-emits it at longer wavelengths that fall within the visible spectrum, specifically the yellow-green range. This Argon and Mercury combination is consistently used to create all colors, including yellows, greens, blues, and whites, that cannot be made with neon alone.
The Physics Behind Gas Excitation
The glow of any noble gas sign is a direct result of physics known as gas discharge or plasma generation. This process begins when a high-voltage electrical current is applied across the electrodes at each end of the sealed glass tube. The electricity accelerates free electrons present in the tube, causing them to collide with the atoms of the noble gas inside.
These collisions transfer energy to the gas atoms, causing their electrons to jump to a higher, less stable energy level, a state referred to as excitation. Because the electrons cannot remain in this high-energy state, they quickly fall back to their original, lower energy level. The excess energy released during this drop is expelled in the form of a photon, which is a particle of light.
The specific color of the light emitted is determined by the distinct energy difference between the higher and lower electron orbits of that element. For example, excited neon atoms produce photons that appear red-orange, while excited Argon atoms release photons that primarily produce a pale blue or ultraviolet light. The gas pressure inside the tube is kept low to allow the accelerated electrons to travel freely and collide efficiently.
How Phosphor Coatings Determine Color
The full palette of colors available in modern signs is achieved by manipulating the interaction between the gas and the internal phosphor coating. The UV light produced by the Argon/Mercury gas mixture is absorbed by the phosphor coating, exciting the coating’s electrons. As these electrons return to their stable state, they re-emit the absorbed energy as visible light.
The chemical composition of the phosphor dictates the exact color that is re-emitted. For yellow-green, the compound is specifically formulated to convert the high-energy UV radiation into lower-energy photons corresponding to the yellow and green wavelengths.
This conversion process allows sign makers to create hundreds of different shades, including colors like turquoise, pink, and white, none of which can be produced by a pure noble gas alone. The vibrant yellow-green color is a result of the coating, with the gas mixture serving only as the internal UV light source.