Phosphors and Emitters

When a material is placed in an electric field or irradiated with light, it may itself radiate light either electric or optical energy is absorbed and optical energy is then radiated. A material that exhibits this phenomenon is called a phosphor and the light radiated is called luminescence (see Section 32.10). Luminescence can be fluorescence or phosphorescence. The difference is the time for which emission occurs. 

Light-emitting diodes and III-V lasers are devices that use a direct-band-gap semiconductor to convert electrical energy into visible light. They have long been used for materials with g - 1 eV (the familiar red), but now GaN (with a wider band gap, large v, and smaller A.) gives us a blue emitter. 

Cathode ray tube 1.5-10kV e-beam Zn2Si04 Mn Green 

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