FLUORESCENT LAMPS

Emitter Material Function

Cathode Operation

When the lamp is turned on, the cathode heats up enough for it to emit electrons.  These electrons collide with and Ionize noble gas atoms in the lamp surrounding the filament to form plasma, by a process of impact Ionization.  As a result of avalanche ionization, the conductivity of the ionized gas rapidly rises, allowing higher currents to flow through the lamp.  

Construction

A fluorescent lamp is filled with a gas containing low pressure mercury vapor and argon, xenon, neon, or krypton.  The pressure inside the lamp is around 0.3% of atmospheric pressure.  The inner surface of the lamp is coated with a fluorescent coating made of varying blends of metallic and rare-earth phosphor salts. The lamp cathode is typically made of coiled tungsten which is coated with a mixture of barium, strontium and calcium oxides (chosen to have a relatively low thermoionic emission temperature).

Emitter material

The "emitter material" on the lamp filaments/cathodes is necessary to enable electrons to pass into the gas via thermionic emitter at the lamp operating voltages used.  The material is slowly sputtered off by bombardment with electrons and mercury ions during operation, but a larger amount is sputtered off each time the lamp is started.  The method of starting the lamp has a significant impact on this. Lamps operated for typically less than 3 hours each will normally run out of the emitter material before other parts of the lamp fail.  The sputtered emitter material forms the dark marks at the lamp ends seen in old lamps.  When all the emitter material is gone, the cathode cannot pass sufficient electrons into the gas fill to maintain the discharge at the designed lamp operating voltage.  Ideally, the ballast should shut down the lamp when this happens.  However, some ballasts will provide sufficient increased voltage to continue operating the lamp which will cause overheating of the lamp end and rapid disintegration of the electrodes and their support wires until they are completely gone or the glass cracks, compromising the low pressure gas fill and stopping the gas discharge.