After the fluorescent tube is permanent, why do both ends turn black?

When the lamp tube works, the filaments at both ends of the lamp tube and the electron emitting substances coated on the surface of the filament will gradually evaporate, and these substances will be deposited on the inner wall of the nearby glass tube after evaporation, making the white fluorescent powder black.

Generally speaking, blackening is directly proportional to the use time, and the longer the use time, the more serious this phenomenon will be, but many factors such as lamp manufacturing technology, usage habits, ballast design and so on have an impact on it.