I.Operating Principle
A fluorescent lamp is a low-pressure mercury-vapor discharge lamp.

Figure 1. Fluorescent Lamps
Source:https://encryptedtbn0.gstatic.com/imagesq=tbn:ANd9GcQZ24eUEu6hOwnLwvXjWLTZJ1AlXA7WGtjasm6aMuqODp1QJfyeekTpkQXtMWxsjcIoRWSUd3z6YTU8W8oDcmeUvyir7t_vWeNacg7amg&s=10

During operation, an electric discharge excites mercury vapor inside the tube, producing predominantly ultraviolet (UV) radiation. Approximately 90% of the radiated energy is emitted at 253.7 nm, corresponding to the principal resonance line of mercury. The ultraviolet radiation is absorbed by the phosphor coating on the inner surface of the lamp and re-emitted as visible light through photoluminescence.

Compared with incandescent lamps, fluorescent lamps dissipate a smaller proportion of input energy as heat and achieve significantly higher luminous efficacy. Owing to their relatively low power consumption and high luminous output, they have been widely used in indoor lighting applications such as offices, classrooms, and industrial facilities.

II.Structure
Phosphors used in fluorescent lamps consist of a host lattice and activator ions. Upon absorption of ultraviolet radiation, electrons are excited from the ground state ($S_0$) to higher energy levels of the excited state ($S_1$).

After excitation, the electrons undergo non-radiative transition within the excited state, during which part of the absorbed energy is dissipated as lattice vibrations (thermal energy). The electrons then relax to the lowest vibrational level of the excited state.

Subsequently, the electrons return from the excited state ($S_1$) to the ground state ($S_0$) through radiative transition, emitting visible light. This emission process is known as fluorescence.

Figure 2. Jablonski diagram of phosphor fluorescence.
Source:https://en.wikipedia.org/wiki/Fluorescence#/media/File:Jablonski_Diagram_of_Fluorescence_Only-en.svg

III.Spectral Characteristics
The emission spectrum of a fluorescent lamp exhibits a discontinuous, multi-peak distribution consisting of sharp mercury emission lines superimposed on the broad emission bands of the phosphors.
Mercury Emission Spectrum
The mercury emission spectrum is generated directly by the discharge of low-pressure mercury vapor within the lamp tube. It consists of narrow atomic emission lines with characteristic wavelengths including:

• 404.7 nm (violet)
• 435.8 nm (blue)
• 546.1 nm (green)
• 577.0 / 579.1 nm (yellow doublet)

Figure 3. Emission spectrum of low-pressure mercury vapour discharge

Phosphor Emission Spectrum
The phosphor emission spectrum is produced by photoluminescence from the phosphor coating on the inner surface of the lamp, providing a broad spectral background in the visible region.In solid phosphor materials, lattice thermal vibrations (phonon coupling) broaden the energy levels of the activator ions. As a result, discrete atomic transitions are broadened into wide emission bands.
By combining red, green, and blue phosphors or multiple rare-earth phosphors, the spectral gaps between mercury emission lines can be filled. This enables adjustment of the correlated color temperature (CCT) and color rendering index (CRI).