Illumination - Types of lamps

Lamps

Vijay Balu Raskar (BE Electrical)

LAMP
A lamp is a replaceable component which is
designed to produce light from electricity.
These components usually have a base of ceramic,
metal, glass or plastic etc.
The origins of the very first lamp dates back to
70,000 BC.
Invention of the first practical incandescent lamp
by Thomas Edison and Joseph Swan in the
nineteenth century.
Invention of the incandescent light bulb from
Thomas Edison and so on.
Types :- Many are available in the market eg.
Incandescent , Sodium, Neon etc…

TYPES OF LAMPS
 There have been significant improvements in lamp efficiency as well
as the different types of lamp after invention.
They can be divided into two main categories: incandescent and
luminescent gaseous discharge lamps.
The gaseous discharge type of lamp is either low or high pressure.
Low-pressure gaseous discharge sources are the fluorescent and low-
pressure sodium lamps.
Mercury vapor, metal halide and high-pressure sodium lamps are
considered high-pressure gaseous discharge sources.

Each light source will be described in terms of its three primary
components: -
(1) light-producing element (lamp), (2) enclosure (luminaire), and (3)
electrical connection.


Fluorescent Lamp
Diagram

NOTE:- A and B are electrodes

Construction:
The figure shows the constructional details of Fluorescent Lamp.
It consists of a long glass tube which is internally coated with a
little quantity of argon gas is also filled in the tube.
There are two electrodes A and B made up of coiled tungsten
filament coated with an electron emitting material.
The control circuit of the tube contains glow type starter, choke L
and two capacitors C1 and C2.
There are two electrodes of which one is fixed while other is U
shaped bimetallic strip made of two different metals.
These electrodes are sealed in a glass bulb, which is filled with a
mixture of helium and hydrogen.
The contacts are normally open.
The coating effect used depends upon the colour effect desired
and may consists of Zinc silicate, Cadmium, Silicate, or calcium
tungstate. They are commonly known as “Phosphrous”

Working:
When the supply is switched ON as electric arc is established between
the electrodes of the starter due to flow of current through small air gap
between the electrodes.
Due to this arc, heat is produced which is sufficient to bend the
bimetallic strip which makes contact with fixed electrode.
This closes the circuit and therefore choke carries large current.
Once the electrodes close, arc vanishes and bimetallic strips cool
down again.
Now the electrodes A and B become hot and due to cooling the choke
circuit open.
The current through the choke coil is suddenly reduced to a small
value.
This change in current induces an e.m.f. which is very high of the order
of 1000 V, in the choke coil.
This e.m.f. induced is sufficient for ionizing the gas molecules between
electrodes A and B, which establishes the discharge between the
electrodes A and B through theBalu Raskar (BE Electrical)
Vijay gas.

The potential difference across the tube falls to about 100-110 V
which is sufficient to maintain the discharge but not sufficient to restart
the glow in the circuit.
So even if starter is removed from the circuit, discharge continues as
the current flows from electrode A and B due to ionization of gas.
If the supply voltage is low, there is difficulty in starting the tube, as
the low voltage is insufficient to establish a glow in the starter.
As choke lowers the power factor, the capacitor C1 used in the circuit
improves the power factor of the circuit.
The capacitor C2 suppresses the radio interference developed due to
arcing.
The function of the inductive choke coil is to supply a large voltage
surge for establishing discharge between the electrodes A and B.


The fluorescent lamp requires three elements or components to produce visible light:
(1) Electrodes (Cathodes)
(2) Gases
(3) Phosphor
Electrodes (Cathodes)
Electrodes are the electron-emitting devices.
Two types of cathodes are in current use.
The hot cathode is a coiled coil or a triple-coiled tungsten filament coated with an
alkaline earth oxide that emits electrons when heated.
The electrons are boiled off the cathode at about 900°C.
The cathode of a cold cathode lamp is a pure iron tube that also has an electron-
emitting material applied inside the tube.
The cold cathodes are subjected to higher voltage, releasing electrons at about
150°C.
Cold Cathode lamps are used in special application such as neon signs and can be
bent into different shapes.
The hot cathode lamp is the most common type of electrode used in fluorescent
lamps for most applications.
Therefore, we shall not describe cold-cathode lamps.

Gases
A small quantity of mercury droplets are placed in the fluorescent tube.
During the operation of the lamp, the mercury vaporizes at a very low pressure.
At this low pressure, the current flowing through the vapor causes the vapor to radiate energy
principally at a single wavelength in the ultraviolet region of the spectrum (253.7nm).
The pressure of the mercury is regulated during operation by the temperature of the tube
wall.
The lamp also contains a small amount of a highly purified rare gas. Argon and argon-neon
are the most common, but krypton is sometimes used.
The gas ionises readily when a sufficient voltage is applied to the lamp.
The ionized gas decreases in resistance quickly, allowing current to flow and the mercury to
vaporise.

Phosphor
This is the chemical coating on the inside wall of the tube or enclosure.
When the phosphor is excited by ultraviolet radiation at 253.7nm, the phosphor produces
visible light by fluorescence.
That is, visible light from a fluorescent lamp is produced by the action of ultraviolet energy on
the phosphor coating on the inside surface of the tube or enclosure.
The phosphor mixture can be altered to change the color of the lamp or the lamp's spectral
power distribution.


Advantages:

i) The light available is much more than the normal incandescent lamp. Fluorescent
lamp gives 2200 to 2400 lumens while normal lamp gives 600 lumens.
ii) The life of fluorescent tube is much more than the incandescent lamp.
iii) The fluorescent tube gives effect of daylight while incandescent lamp gives
yellowish light.
iv) Low power consumption.
v) Higher efficiency.
vi) Instantaneous switching without any warming period.
vii) Using different fluorescent materials various colored light can be obtained.

Disadvantages:
i) Very high initial cost.
ii) Produces radio interference.


Application:-

i. Fluorescent lamps come in many shapes and sizes.
The compact fluorescent light bulb (CFL) is becoming
more popular.
ii. Used for Domestic purpose like School, House, Shops,
malls, temples , etc i.e. for Residential use.
iii. Used for Every Industry like in office, godowns, etc


Sodium Vapour Lamp
Diagram


Construction:
It consists of two glass tube, outer glass tube and
inner glass tube.
The inner glass tube contains two electrodes (see fig.).
 Sodium along with small quantity of neon or argon
gas is filled in the inner tube to make discharge self-
starting.
Sodium vapour is chemically active.
The glass of the tube is made up of suitable material to
resist this action.
To maintain the correct temperature in the discharge,
it is placed in an evacuated outer tube. The outer tube
reduces the heat loss. The transformer includes in the
circuit heats the cathode while choke stabilizes the
discharge.

Working:
When the lamp is not in operation,. The sodium is usually in
the form of solid deposited on the side walls of the tube.
When the lamp is switched on, the discharge is first
established through the neon or argon gas.
This gives out radish colour.
After some time heat is developed due to this discharge
that is vaporizes sodium vapour.
In this way the lamp starts its normal operation giving
yellow colour.
 Capacitor C is connected to have a better power factor.
The operating temperature of this lamp is about 300C.
These lamps are commonly used for illumination of roads,
good yards, and airports.
Efficiency of a Sodium Vapour Lamp under practical
Conditions is about 40-50 LumensElectrical)
Vijay Balu Raskar (BE
/ watt.

Sodium Vapour lamp is only suitable for AC and Therefore,
requires choke control.
Manufactures in 45, 60, 85 and 140Watt ratings.
The average life is about 3000 Hours.
Lamp is not affected by voltage variations.
Light output reduced by 15% due to ageing.
The metallic sodium gradually vaporizes and then ionizes.
Thereby, Producing the characteristics mono-chromatic yello
light, which makes objects appears as gray.
The lamp must be operated horizontally or nearly so, to
take the sodium well spread out along the tube.
Some operated vertically as designed.


Advantages:
i) Its efficiency is higher than that of the filament
lamp.
ii) It has a long life.

Disadvantages:
i) The bright yellow colour obtained is not suitable
for indoor lighting. So it is not useful in houses.
ii) For the necessary output, long tubes are required.
iii) For giving full output, some time (about 10
minutes) is required.


The lamps fails to operate when
i. The filament breaks or burns out.
ii. The cathode stops to emit electrons.
iii. The sodium particles may concentrate on one side of
the tube.
iv. The tube is blackened owing to sodium vapour action on
the glass, in which case the output will be reduced.

 Applications:-
 Used for high way.
 Used for General outdoor lighting where colour
discrimination is not required like Street light, parks, rail
yards, storage yards, etc..


Mercury Vapour Lamp:
Diagram


Construction:
 The lamp consists of two bulbs, inner bulb and outer bulb.
The electric discharge takes place in the inner bulb.
The outer bulb protects the inner bulb and reduces loss of heat.
The inner bulb consists of a small amount of mercury and argon gas.
 The two electrodes E and B are made up of electron emitting
material.
Three electrodes B, E and S are provided in the inner bulb.
The electrode ‘E’ is connected to electrode ‘S’ through a high
resistance.
Choke L and capacitor C forms the control circuits of the lamp.


Working:
When the supply is switched on, the initial discharge is established
between electrode B and S through the argon gas and then between
electrodes B and E.
The heat produced due to this discharge is sufficient to vaporize
mercury and the discharge through the mercury vapour takes place.
 In this normal operation of the lamp, it emits or radiates its
characteristic light.
The electrode ‘S’ is called as starting electrode or auxiliary
electrode.
The choke serves to limit the current drawn by the electrodes to a
safe limit.
The capacitor C improves power factor of the lamp.
These lamps are widely used for outdoor street lighting where a
high illumination necessary, where the colour of light is not
important.

Advantages:

i) Its efficiency is high and output is more.
ii) It has long life.

Disadvantages:

i) The initial time required for warming up is more about 5
minutes.
ii) If lamp goes out while in service, cooling is required for
restarting. This cooling reduces the vapour pressure.
iii) each lamp contains mercury which can be harmful to
both humans and wildlife.

Application:-

Ultra high pressure mercury vapor lamps are used in the
area of photolithography to expose various photoresists.
Phosphor coated or white mercury vapor lamps are
recommended for all applications where color is important.
There are three standard modified mercury vapor lamps:
a. Color Improved: very poor on reds, marginal color, not
recommended.
b. Deluxe White, DX: increased red, good color,
recommended.
c. Warm White Deluxe, WWX: excellent reds, excellent
color, highly recommended, decreased lumens.

Illumination - Types of lamps

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