Maneesh M S Dept. of EEE, SNGIST

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ELECTRIC LAMPS

Here, we will be discussing about the different types lamps used in our day to day life. We

will be covering the following topics

Sodium Vapour Lamp

Metal Halide Lamp

Mercury Vapour Lamp

Fluorescent Tube Light

Compact Fluorescent Lamp

Light Emitting Diode

Incandescent Lamp

Maneesh M S Dept. of EEE, SNGIST

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LAMPS

Lamps are devices which convert electrical energy to light energy. Different types of lamps

are available in market which has different working principles. Here we will learn in details

some of the lamps which we come across in our day to day life. Before we go into detail, we

can familiarize some of the terms related to lamp and light.

Luminous Flux : It is the total amount of light energy emitted per second by a source.

Unit is Lumens.

Luminous Intensity : It is the luminous flux emitted by the source per solid angle.

Unit is Candela.

Illumination : When light falls on a body, it is called illumination. It is measured in

Lumens per square meter or Lux.

Candle Power : It is the light radiating capacity of a source in a given direction.

Denoted by C.P.

1 Sodium Vapour Lamp

Sodium vapour lamps are mainly used for street lighting. They have low luminosity hence

require glass tubes of large lengths, which makes them quiet bulky.

Construction:

The lamp consists of a U shaped inner glass tube filled with neon gas at a pressure of 10mm.

It also contains a small quantity of sodium and argon gas. The initial ionization voltage is

reduced, as the ionization potential of argon is low. Two oxide coated tungsten electrodes are

sealed into the tube at the ends. This tube is enclosed in an outer double walled vacuum

enclosure to maintain the required temperature.

Maneesh M S Dept. of EEE, SNGIST

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Working:

A voltage of the order of 380- 450 volts (depending on the wattage) is necessary to start the

discharge, which is obtained from a high reactance transformer or an autotransformer.

Initially the sodium vapour lamp operates as a low-pressure neon lamp emitting pink colour.

As the lamp gets heated and reaches a temperature of 200° C the sodium deposited on the

sides of the tube walls vaporizes and radiates yellow light. It has a maximum efficiency at

220° C. Proper mounting of the lamp is to be ensured to prevent the sodium blackening the

inner walls of the tube .A capacitor C is used to improve the power factor.

2 Metal Halide Lamp

A metal-halide lamp is an electric lamp that produces light by an electric arc through a

gaseous mixture of vaporized mercury and metal halides (compounds of metals with bromine

or iodine). It is a type of high-intensity discharge (HID) gas discharge lamp. They are similar

to mercury vapour lamps, but contain additional metal halide compounds in the quartz arc

tube, which improve the efficiency and colour rendition of the light. The most common metal

halide compound used is sodium iodide. Metal-halide lamps have high luminous efficiency

and produce an intense white light. Lamp life is 6,000 to 15,000 hours.

The lamps consist of a small fused quartz or ceramic arc tube which contains the gases and

the arc, enclosed inside a larger glass bulb which has a coating to filter out the ultraviolet

light produced.

Metal-halide lamps produce light by making an electric arc in a mixture of gases. In a metal-

halide lamp, the compact arc tube contains a high-pressure mixture of argon or xenon,

mercury, and a variety of metal halides. The argon gas in the lamp is easily ionized when

Maneesh M S Dept. of EEE, SNGIST

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supply is given, and initiates the arc across the two electrodes. The heat generated by the arc

then vaporizes the mercury and metal halides, which produce light as the temperature and

pressure increases.

3 Mercury Vapour Lamp

A mercury-vapour lamp is a gas discharge lamp that uses an electric arc through vaporized

mercury to produce light. The arc discharge is generally confined to a small fused quartz arc

tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated

with a phosphor; in either case, the outer bulb provides thermal insulation, protection from

the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz

arc tube.

The mercury in the tube is a liquid at normal temperatures. It needs to be vaporized and

ionized before the tube will conduct electricity and the arc can start. So, like fluorescent

tubes, mercury vapour lamps require a starter, which is usually contained within the mercury

vapour lamp itself. A third electrode is mounted near one of the main electrodes and

connected through a resistor to the other main electrode. In addition to the mercury, the tube

is filled with argon gas at low pressure. When power is applied, there is sufficient voltage to

ionize the argon and strike a small arc between the starting electrode and the adjacent main

electrode. This starting arc discharge heats the mercury and eventually provides enough

ionized mercury to strike an arc between the main electrodes. This process takes from 4 to 7

minutes, so mercury lamps are slow starting.

Maneesh M S Dept. of EEE, SNGIST

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The mercury vapour lamp is a negative resistance device. This means its resistance decreases

as the current through the tube increases. So if the lamp is connected directly to a constant-

voltage source like the power lines, the current through it will increase until it destroys itself.

Therefore, it requires a ballast to limit the current through it

4 Fluorescent Tube Light

A fluorescent lamp tube is filled with a gas containing low pressure mercury vapour and

argon, xenon, neon, or krypton. The inner surface of the lamp is coated with a phosphor

coating. The lamp's electrodes are typically made of coiled tungsten and usually referred to as

cathodes. They are coated with a mixture of barium, strontium and calcium oxides.

When power is first applied to the circuit the starter contacts connects the two filaments of the

fluorescent lamp and the ballast in series to the supply voltage. The current through the

filaments causes the argon gas to heat up. This heat vaporizes the mercury in the tube. When

the starter contacts opens the current through the filaments and the inductive ballast is

abruptly interrupted. This will create a very high voltage between the filaments. This high

voltage will initialize the discharge through the vaporized mercury gas. The mercury will

emit UV radiations which is absorbed by the phosphor coating and emits visible light.

5 Compact Fluorescent Lamp

The principle of operation in a CFL is the same as in fluorescent tube light except that it is

compact in size. The tube of the CFL consist low pressure mercury vapour and argon gas.

When supply is turned ON, the argon gas heats the mercury vapour and discharge occurs. The

UV rays emitted by the mercury vapour is absorbed by the phosphor coating and converts it

to visible light.

Maneesh M S Dept. of EEE, SNGIST

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6 Light Emitting Diode

It is a P-N junction diode which emits light when energy is applied on it. This phenomenon is

generally called electroluminescence, which can be defined as the emission of light from a

semi-conductor under the influence of an electric field. When a PN junction is reverse biased,

the charge carriers recombines as the electrons cross from the N-region and recombine with

the holes existing in the P-region. The energy dissipated during recombination of the

electrons and the holes is in the form of heat and light.

The electrons dissipate energy in the form of heat for silicon and germanium diodes but in

Gallium arsenide phosphide (GaAsP) and Gallium phosphide (GaP) semiconductors, the

electrons dissipate energy by emitting photons. If the semiconductor is translucent, the

junction becomes the source of light as it is emitted, thus becoming a light emitting diode

(LED) but when the junction is reverse biased no light will be produced by the LED and, on

the contrary, the device may also get damaged.

The figure below shows the structure of an LED.

Maneesh M S Dept. of EEE, SNGIST

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It consist of two electrodes – Anode (+) and cathode (-).

On the top of the cathode, a the PN semiconductor die is placed. The N region is in contact

with the cathode. The P region of the semiconductor die is connected to anode via a wire

bond. The semiconductor die is placed on a reflective cavity which reflects all the lights

emitted from the LED. The transluceent covering protects the LED from physical damage and

allows the light to pass through. The epoxy lense on the top is used to focus the light beam.

7 Incandescent Lamp

Incandescent light bulbs consist of an air-tight glass enclosure or bulb with a filament of

tungsten wire inside it. The bulb is filled with an inert gas, to reduce evaporation of the

filament and prevent its oxidation. Contact leads and the base provide electrical connections

to the filament. Incandescent light bulbs usually contain a glass stem anchored to the bulb's

base that allows the electrical contacts to run through the envelope without air or gas leaks.

Small molybdenum wires inside the stem supports the filament. When an electric current

passes through the filament, it heats the filament to typically 2,000 to 3,300 K. The heated

filament emits light. The useful part of the emitted energy is visible light, but most energy is

given off as heat in the near-infrared wavelengths.