Edmund Germer’s invented Fluorescent lamp and it is a hot cathode low-pressure mercury vapor lamp.
It consists of a long horizontal tube, due to low pressure maintained inside of the bulb; it is made in the form of a long tube. The tube consists of two spiral tungsten electrode coated with electron emissive material and are placed at the two edges of long tube. The tube contains small quantity of argon gas and certain amount of mercury, at a pressure of 2.5 mm of mercury. The construction of fluorescent lamp is shown in Fig.
Normally, low-pressure mercury vapor lamps suffer from low efficiency and they produce an objectionable colored light. Such drawback is overcome by coating the inside of the tube with fluorescent powders. They are in the form of solids, which are usually knows as phosphors.
A glow starter switch contains small quantity of argon gas, having a small cathode glow lamp with bimetallic strip is connected in series with the electrodes, which puts the electrodes directly across the supply at the time of starting. A choke is connected in series that acts as ballast when the lamp is running, and it provides a voltage impulse for starting. A capacitor of 4μF is connected across the starter in order to improve the power factor.
Working of Fluorescent Lamp
At the time of starting, when both the lamp and the glow starters are cold, the mercury is in the form of globules. When supply is switched on, the glow starter terminals are open circuited and full supply voltage appeared across these terminals, due to low resistance of electrodes and choke coil.
The small quantity of argon gas gets ionized, which establishes an arc with a Starting glow. This glow warms up the bimetallic strip thus glow starts gets short circuited. Hence, the two electrodes come in series and are connected across the supply voltage.
Now, the two electrodes get heated and start emitting electrons due to the flow of current through them. These electrons collide with the argon atoms present in the long tube discharge that takes place through the argon gas. So, in the beginning, the lamp starts conduction with argon gas as the temperature increases, the mercury changes into vapor form and takes over the conduction of current. In the mean time, the starter potential reaches to zero and the bimetallic strip gets cooling down. As a result, the starter terminals will open. This results breaking of the series circuit.
A very high voltage around 1,000 V is induced, because of the sudden opening of starter terminals in the series circuit. But in the long tube, electrons are already present; this induced voltage is quite sufficient to break down the long gap. Thus, more number of electrons collide with argon and mercury vapor atoms.
The excited atom of mercury gives UV radiation, which will not fall in the visible region. Meanwhile, these UV rays are made to strike phosphor material; it causes the re-emission of light of different wavelengths producing illumination. The phenomenon of the emission is called as luminescence.
This luminescence is classified into two ways. They are:
Fluorescence: In this case, the excitation presents for the excited periods only.
Phosphorescence: In this case, even after the exciting source is removed, the excitation will present. In a lamp, the re-emission of light causes fluorescence, then such lamp is known as fluorescent lamp.
Advantages of fluorescent lamp
Compared to filament lamps, the efficiency is 3 to 4 times more in fluorescent lamps means these lamps has High efficiency.
The life of the lamp is three times of the ordinary filament lamp.
The quality of the light obtained is much superior. Gives diffused, glare free, shadow less and cool white light.
These lamps can be mounted on low ceiling, where other light sources would be unsatisfactory.
Lower power consumption.
No warming up period is required as in the case of other discharge lamps.
Low heat radiation
Different colour lights can be obtained using different types of fluorescent powders
Disadvantages of fluorescent lamp
The initial cost is high because of choke and starter.
The starting time as well as the light output of the lamp will increases because of low ambient temperature.
Because of the presence of choke, these lamps suffer from magnetic humming and may cause disturbance.
The stroboscopic effect of this lamp is objectionable
