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Fluorescent Lamp Parts
Basically, a fluorescent lamp is made up of five components. See
Figure 5.1.
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GLASS TUBE, coated on the inside
with fluorescent powders called PHOSPHORS.
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Two ELECTRODES (or cathodes) coated
with EMITTER, supported by a glass mount structure, and sealed at the ends of the tube.
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FILLING GAS - usually a low
pressure of Argon or Krypton/Argon mixture.
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Small amount of MERCURY (less than
20 mg), which vaporises during the lamp operation.
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LAMP CAP cemented to each end of
the tube to connect the lamp to the lighting circuit.
Figure 5.1.
Fluorescent Lamp Operation
When the circuit is energised, electricity heats the cathodes. See
Figure 5.2.
The cathodes are coated with
material which, when heated, emits electrons. The electrons establish an electric arc between the
cathodes at opposite ends of the tube. The electrons collide with the mercury atoms, causing
mercury to emit invisible ultra-violet radiation. The ultra-violet is absorbed by the phosphor
coating on the tube and re-radiated as visible light.
Figure 5.2. Principles of
fluorescent lamp operation.
Fluorescent Lamp Shapes
Fluorescent lamps have a limited range of shapes. Most common is the straight tube in
various diameters. See
figure 5.3. The diameter may be quoted in mm, but the universal term is "T" for
tube followed by the diameter in eighths of an inch e.g. 26mm is T8. Most T12 lamps are now being
replaced by more energy efficient T8 lamps of the same length but reduced power.
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