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High and low
• Current is passed through a filament. The temperature of
the wire is raised and light as well as heat energy is emitted.
• Higher the temperature higher is the amount of light energy
• The lamp filament should be of such a material whose
temperature can be raised quickly without causing any
damage to it.
This lamps are used below 40 W
1. Tungsten has a high melting point (3400 degree
2. high resistivity
3. low temperature co-efficient (0.005.1)
4. Low vapor pressure
• The melting point of tungsten is 3400 °C.
• If the tungsten filament is opens at about 2100 °C , it vaporizes
rapidly and life of the lamp is very short.
• Hence the bulb is filled with an inert gas - mostly nitrogen or argon.
• This reduces rate of evaporation of the metal from the filament.
• Temperature can be raised to about 2500 °C without shortening
the life of the lamp.
• These lamps use coiled filaments.
• The gas prevents arcing between two coils of the spiral filament.
Gas filled lamps are used from 40W to 500W and
1.Due to low initial cost
2.low energy consumption convenience
•Under low pressure, mercury discharge lamps emit ultra
•If the pressure is increased we get light having a bluish
•These lamps were widely used earlier but now they have
been replaced by sodium vapour lamps.
•Sodium vapour lamps have replaced mercury lamps due to their
high lumen output for the same power input .
•The efficiency of sodium vapour lamps is between 40 to 50 lumens
per watt with an average life of about 3000 hours.
•The glass tube is made specially chemical resistant to withstand
the chemical reaction of hot sodium.
•The lamp must be operated at horizontal or nearly horizontal
position to keep the sodium uniform throughout
•The lamps must be handled very carefully otherwise if the bulb
breaks, the sodium may spread and catch fire.
•Neon lamps are made up of a glass tube of upto 7-8 m length.
•It has electrodes at the two ends.
•These tubes are operated at a temperature of about 200 °C.
•Hence they are also known as cold cathode discharge lamps.
•They are widely used for decoration and advertisement purposes
Neon tubes can be bent into any desired shape
•Tubes can be made to emit different colours like orange, yellow,
green, blue, red, etc .by mixing suitable chemicals with neon gas.
• In halogen lamps black deposits are formed on the inner
surface of the bulb.
• If a small amount of halogen vapor is added to iodine
gas, a part of the evaporated tungsten is restored to the
filament by means of chemical reaction.
• This reduces blackening caused by deposits of evaporated
The advantages .of a halogen lamps are as follows :
(i) their life is almost double that of conventional
(ii) almost no blackening. Hence a high lumen output
is maintained through out the life of the lamp.
(iii) Better color.
(iv) reduced dimensions.
Halogen lamps are very suitable for outdoor illumination of buildings, play
grounds, car parks, airport runways, etc. They are available upto 5kW.
•The tubes are filled with argon gas at low pressure and one or two
drops of mercury.
•The inner surface of the tube has a thin coating of fluorescent
•Different colours can be obtained depending upon the type of
coating material used.
•It is provided with two electrodes coated with electron emissive
•The mercury vapour emits UV radiation when electric charge is
passed through it.
•The fluorescent coating converts UV radiation into visible light.
CFL or compact fluorescent lamps work on the same principle.
(1). It is compact in size hence saves space compared to
(2). It saves about 70-80% energy as compared to
(3). Its compact size and less weight make it suitable for
(4). Its life is about ten times that of incandescent lamps
(5). Starting and working is flicker free.