“A line spectrum has emission only in certain, sometimes very narrow, wavelength areas, in contrast to a continuous spectrum. Sodium and mercury vapor gas discharge lamps, among others, have this kind of line spectrum. . . .

Neon tubes, as used for illuminated advertising and traffic lights, also have line spectra.

Neon gas is used for red light, and mercury gas for blue light, within transparent glass tubes. Subtractive mixing occurs in the colored transparent tubes, part of the spectrum of the light source is absorbed by the colored glass. Yellow glass absorbs so much of the “blue” mercury lines that mostly “green” can pass through and become visible. “Neon” light adverstising (the name “neon” is used for neon and for mercury tubes) was shown at the World’s Fair in Paris in 1937, when for the first time fluorescent powders were coated on the insides of the tubes. A large assortment of colors was produced by making use of different fluorescent powders. This invention was the start of the development which led to our fluorescent tubes, for lighting purposes.”

—Franz Gerritsen, from his Theory and Practice of Color: A Color Theory Based on Laws of Perception, 1974, translation by Ruth de Vriendt.