Editing Phosphor (section)

{{Short description|Luminescent substance}}
{{For|the chemical element|phosphorus}}
[[File:Luc Viatour phosphore poudre.jpg|thumb|Example of phosphorescence]]
[[File:IBM PC 5150.jpg|thumb|[[Monochrome monitor]]]]
[[File:CRT Phosphors.jpg|thumb|[[Aperture grille]] CRT phosphors]]

A '''phosphor''' is a substance that exhibits the [[optical phenomenon|phenomenon]] of [[luminescence]]; it emits light when exposed to some type of [[radiant energy]].  The term is used both for [[fluorescence|fluorescent]] or [[phosphorescence|phosphorescent]] substances which glow on exposure to [[ultraviolet]] or visible light, and [[cathodoluminescence|cathodoluminescent]] substances which glow when struck by an [[electron beam]] ([[cathode ray]]s) in a [[cathode-ray tube]].

When a phosphor is exposed to radiation, the orbital [[electron]]s in its [[molecule]]s are excited to a higher [[energy level]]; when they return to their former level they emit the energy as light of a certain color.  Phosphors can be classified into two categories: [[fluorescent]] substances which emit the energy immediately and stop glowing when the exciting radiation is turned off, and [[Phosphorescence|phosphorescent]] substances which  emit the energy after a delay, so they keep glowing after the radiation is turned off, decaying in brightness over a period of milliseconds to days.

Fluorescent materials are used in applications in which the phosphor is excited continuously: [[cathode-ray tube]]s (CRT) and plasma video display screens, [[fluoroscopy|fluoroscope screens]],  [[fluorescent light]]s, [[Scintillation counter|scintillation sensors]], most white [[Light-emitting diode|LED]]s, and [[luminous paint]]s for [[black light]] art.  Phosphorescent materials are used where a persistent light is needed, such as glow-in-the-dark watch faces and aircraft instruments, and in [[radar|radar screen]]s to allow the target 'blips' to remain visible as the radar beam rotates. CRT phosphors were standardized beginning around [[World War II]] and designated by the letter "P" followed by a number.

[[Phosphorus]], the light-emitting chemical element for which phosphors are named, emits light due to [[chemiluminescence]], not phosphorescence.<ref>{{cite book|author =Emsley, John|year = 2000|title = The Shocking History of Phosphorus |location= London|publisher =Macmillan|isbn=978-0-330-39005-7}}</ref>