Why are phosphors coated on the inner surface of fluorescent lamp tubes?

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optical ceramics High-pressure sodium-vapour lamp bulb.

Phosphors are coated on the inner surface of fluorescent lamp tubes to convert invisible radiation into visible light.

Inside the lamp, an electric current flows through a gas mixture—typically argon and mercury vapor—which causes the gas to ionize and emit ultraviolet radiation. Because this ultraviolet light is not visible to the human eye, the phosphor coating acts as a transducer: it absorbs the ultraviolet energy and undergoes fluorescence, re-emitting that energy as visible light.21

This process allows fluorescent lamps to produce illumination much more efficiently than incandescent lamps, as they do not rely on the energy-intensive heating of a metallic filament.2