Why is Fluorescent Lamp Phosphor Coated?

Fluorescent Lamp is still mostly used- light source in our day to day life. It illuminates the very eye pleasant light. For producing eye pleasant light phosphor is used inside of the fluorescent lamp. It is used to convert ultraviolet ray radiation into the visible radiation. Previously zinc beryllium silicate was used as the basic phosphor. Later contemporary lamps had used the phosphor that was the mixture of calcium halophosphate and magnesium tungstate. This version of phosphor coating produced yellowish white light but later bluish white. But now a day’s Phosphor is actually a semiconductor material that has been mixed with activators. It has peak spectral sensitivity near 253.7 nm. In its band theory, we can see the three zonal bands; they are valence band, forbidden band, and the conduction band. In addition of activators with phosphor material a permitted energy level is created in the forbidden zone. One example of the activator is silver.
The ultraviolet ray to visible ray conversion is shown pictorially below.
Phosphor Model of Zinc Sulfide
A - B : - Electron Jump B - E : - Electron Migration E - D : - Electron Jump D - C : - Electron Jump A - C : - Hole Migration

• A 253.7 nm photon impinges on the phosphor and causes the electron jump from divalent sulfur to divalent zinc. It creates, at this moment, univalent zinc ion in the conduction band. Univalent zinc ion means zinc ion with an excess electron.
• This excess electron migrates from one zinc ion to another through the crystal lattice in the conduction band.
• Just simultaneously, the positive hole migrates from one sulfur atom to another in the valence band.
• While the positive hole gets close vicinity of the activator it captures the electron from the activator. Then a migrated electron in the valance band falls into the positive hole and the photon is released.
• This released photon has longer wavelength thus we get visible rays from the fluorescent lamp.