Fluorescent Lamp and Working Principle of Fluorescent Lamp

What is Fluorescent Lamp?

A fluorescent lamp or a fluorescent tube is a low weight mercury vapor lamp that uses fluorescence to deliver visible light. An electric current in the gas energizes mercury vapor which delivers ultraviolet radiation through discharge process which causes a phosphor coating of the lamp inner wall to radiate visible light. A fluorescent lamp changes over electrical vitality into useful light a great deal more proficiently than incandescent lamps. The normal luminous viability of fluorescent lighting frameworks is 50-100 lumens for every watt, a few times the adequacy of incandescent lamps with equivalent light yield.

How Fluorescent lamp works?

• When the switch is ON, full voltage will come across the tube light through ballast and fluorescent lamp starter. No discharge happens initially i.e. no lumen output from the lamp.



• At that full voltage first the glow discharge is established in the starter. This is because the electrodes gap in the neon bulb of starter is much lesser than that of inside the fluorescent lamp.
• Then gas inside the starter gets ionized due to this full voltage and heats the bimetallic strip that is caused to be bent to connect to the fixed contact. Current starts flowing through the starter. Although the ionization potential of the neon is little bit more than that of the argon but still due to small electrode gap high voltage gradient is appeared in the neon bulb and hence glow discharge is started first in starter.
• As voltage gets reduced due to the current causes a voltage drop across the inductor, the strip cools and breaks away from the fixed contact. At that moment a large L di/dt voltage surge comes across the inductor at the time of breaking.
• This high valued surge comes across the tube light electrodes and strike penning mixture (mixture argon gas and mercury vapor).
• Gas discharge process continues and current gets path to flow through the tube light gas only due to low resistance as compared to resistance of starter.
• The discharge of mercury atoms produces ultraviolet radiation which in turn excites the phosphor powder coating to radiate visible light.
• Starter gets inactive during operation of tube light.

Basic Physics of Gas Discharge inside the Fluorescent Lamp?

When a sufficiently high voltage is applied across the electrodes, a strong electric field is set up. A small amount of current through the electrodes filaments heats up the filament coil. As the filament is oxide coated, sufficient amount of electrons is produced and they rush from negative electrode or cathode to positive electrode or anode due to this strong electric field. During the movement of free electrons discharge process starts to be established. The basic discharge process always follows three steps:

1. Free electrons are derived from the electrodes and they get accelerated by the electric field applied.
2. Kinetic energy of the free electrons is converted into the excitation energy of the gas atoms.
3. The excitation energy of the gas atoms gets converted into the radiation.

In the discharge process, a single ultra violates spectral line of 253.7 nm is produced at a low pressure of mercury vapor. To generate 253.7 nm ultra violate ray the bulb temperature is kept between 105 to 115°F. The length to diameter ratio of the tube should be such that fixed wattage loss happens at both ends. Where this wattage loss or glow of electrodes takes place is called cathode and anode fall region. This watt loss is very small. Again the cathodes should be oxide coated. Hot cathode provides an abundance of free electrons. Hot cathodes, mean those electrodes which are heated by circulating current and this circulating current is provided by choke or control gear. Few lamps have cold cathode also. Cold cathodes have larger effective area and higher voltage such as 11kv is applied across them to get ions. Gas starts to be discharged due to this high voltage application. But at 100 to 200 V the cathode glow get separated from the cathode, it is called cathode fall. This provides a large supply of ions which are accelerated to the anode to produce secondary electrons on impact which in term produce more ions. But cathode-fall in hot cathode discharge is only at 10 V.