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Fluorescent Lamp

 

Kendriya vidyalaya no -2 Darbhanga

WORK EDUCTION

Fluorescent Lamp

Construction. - The lamp consists of a glass tube commonly 60 cm., 120 cm. 150 cm. long and about

35 mm. in diameter with filament type electrodes sealed into each of its ends. The filaments are made

of tungsten wire and are coated with a material which has the ability to emit electrons. The inner

surface of the tube is coated with a thin layer of phosphor material, ie., fluorescent powder (a mixture

of calcium, zinc and cadmium salt). During the manufacturing of the lamp most of the air is removed

from the tube and a small amount of argon gas and drops of mercury are filled inside it.

Lamp Circuit. - Owing to the long length of the tube it is necessary to heat the electrode so that they

emit sufficient electrons at starting. This is achieved by connecting the electrodes in series with a choke

and the supply through a thermal starting switch (starter). When the lamp switch is turned on, current

passes through the circuit and heat filaments. The heat vaporises the mercury causing electrons to be

emitted from the material with which the filaments are coated. The current also heats the starter which

opens the circuit after a few seconds. The sudden stop of current through the choke produces voltage

of several thousand volts across it because of self-inductance. The high voltage 'kicks', then starts the

flow of electrons from one filament to the other.

How Light is produced - As the electrons flow through the tube with the help of argon gas, they collide

with mercury atoms. This collision produces invisible ultra violet rays.These chemicals are exposed to

strong ultra violet rays, they glow brightly. They produce visible light and increase both the light and

colour efficiency of the tube. Since the tube is coated with such fluorescent powder and when the ultra

violet rays created by the arc inside the tube fall on them, they glow brightly and produce visible rays,

the result is the light which we see.

Lamp Efficiency. One reason for their popularity is that they waste less electric power in the form of

heat. They remain rather cool while in operation they produce aproximately three times as much light 5

incandescent lamp with the same amount of electric power. Another important point in favour of this

type of lamp is the low surface brightness of the lamp, it is not bright at any one spot, the total light

delivered by it is dependent not on a small area of extreme brightness. This results in better lighting in

general, less shadows, less eye-strain and better feeling

Advantages. 1. It can be operated on A.C. and D.C.

2. It takes less time to start than other gas discharge type.

3. Its light is 3 times more than an incandescent lamp.

4. Its light is cool, shadowless and natural daylight.