Article Index
Types of Fluorescent Lights
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Discharge lamps
Cold cathode type-only used for decorative lighting
Hot cathode

• a, High pressure fluorescent

(i)Mercury vapour types) Used mainly for street lighting
(ii) Sodium type , ) but sometimes used for deck lighting

• b, Low pressure Hot Cathode Fluorescent type discharge Lamp

Principle- A length of glass tubing contains a small amount of mercury vapour and argon gas, at a very low pressure ( 10-6 atmospheres ). A heater element forming an electrode is situated at each end of the tube. These electrodes may be coated with an oxide to improve thermionic emission. The interior of the tube is coated with fluorescent powder ( a Phosphor coating)

If a suitable voltage is applied between the two electrodes a DISCHARGE strikes between them and the mains voltage is then sufficient to maintain the discharge. This occurs in low pressure so that the lamp will run at a comparatively low temperature and so will not effect the fluorescent coating. The electrons from the electrode collide with the mercury atoms. This dislodges an electron from the atom making the mercury atom a positively charged ION. As the dislodged electron returns to the influence of the ION ( i.e. the electron changes from one energy level to another) a certain amount of electro-magnetic radiation (i.e. a photon) is given off in the form of Ultra-violet light. These rays activate the fluorescent coating and the luminous surface provides a glare free efficient light.