Illumination Basic & Scheme
Building Utility & Services
2. Nature of light
3. Terms used in illumination
4. Law of illumination
5. Electrical methods of producing light
6. Sources of light
7. Arc lamps
8. Filament lamps
9. Gaseous discharge lamps
10.Sodium vapor lamp
11.Mercury vapor lamp
15.Compact fluorescent lamp
16.Candle power and its measurement
As we know that almost all human activities
depends on light. Sun is a prime natural source
of light but artificial lighting plays almost main
role in our daily life. These artificial lights are
produced by mechanical lamps and electrical
But due to poor performance the mechanical light
are totally replaced by electrical lights. The
electrical lighting are mainly used for
decorative purpose, advertising, traffic control ,
medical field and street lighting etc.
Electrical lighting has following advantages :
2. Easy to control
4. Easy to handle
5. Steady output
6. Better reliability
7. Suitable for almost all purposes etc.
Sensitivity of Human Eye
As we know natural light consists of seven
colors having different wavelengths. The
average human eye is most sensitive to a wave
length of 5500 0A.
The relative sensitivity of eye for a particular
wave length is the visual effect produced by
the light on the average human eye as
compared with the effect of light having wave
length 5500 0A on human eye.
Sensitivity of Human eye
This is also known as Relative
• It is the unit of luminous flux. One
lumen is defined as the luminous
flux emitted per unit solid angle
from a point source of one candle
The angle subtended at a point by two converging lines lying in the same plane is
called plane angle. It is measured in radians and equal to the ratio of the length
of the arc too its radius,
θ = arc/ radius = l/ r radians
The angle subtended by the partial surface area of a sphere at its centre is called
as solid angle. It is measured in steradians and equal to the ratio of area of the
surface to the square of radius of sphere,
ω = area of surface/ square of radius = A/ r2 steradians
• the unit of solid angle. One steradian is
defined as the solid angle that is
subtended at the centre of a sphere by its
surface having area equal to radius
• ω = area of surface/ square of radius
= A/ r2 steradians
• The light radiating capacity of a source is
called its candle power. The number of
lumens given out by a source per unit
solid angle in a given direction is called
its candle power. It is denoted by C.P.
Total flux emitted = CP X solid angle
= 1 X 4π = 4π lumens
= 4π lumens
• Luminous intensity in any particular
direction is the luminous flux emitted by
the source per unit solid angle in that
• It is denoted by I and its unit is candela
or candle power (CP) .
• Luminous intensity of source in a
particular direction, I = φ / ω
reduction factor of a source of light is the
ratio of its mean spherical candle power
to its mean horizontal candle power.
Reduction factor = MSCP/ MHCP
MEAN SPHERICAL CANDLE-POWER
•Mean Spherical Candle Power, a unit of measure that represents the average output of a
light source measured in all directions (360°)
MEAN HORIZONTAL CANDLE-POWER
The average value of the candle-power of a light source in all directions in a horizontal
plane through the source,
When light falls on a surface, it becomes
visible, the phenomenon is called as
It is defined as luminous flux falling on a
surface per unit area. It is denoted by E
and measured in lumen per square
meter or meter- candle.
E = Ф / A lux
One meter candle or lux is defined as the
illumination produced by a uniform
source of one CP on the inner surface of
a sphere of radius one meter.
In the human eye, the opening of pupil is
controlled by its iris which depends upon the
intensity of light received by the eye. If the eye is
exposed to a very bright source of light, the pupil
of the eye contracts automatically in order to
reduce the amount of light admitted and prevent
damage to the retina. This effect is called glare.
Glare is defined as the brightness within the field
of vision of such a character so as to cause
discomfort and interference in vision.
It is defined as the visible radiations emitted
by it in lumens per watt.
Usually, the light sources do not radiate
energy only in the visible spectrum. The
radiant energy is also accompanied with
infrared and ultra violet radiations.
Sun light produces majority of radiations in
the visible spectrum. The tungsten lamp
produces small radiations so its efficiency is
The efficiency of fluorescent lamp is more than
Whole of the light incident on a reflecting
surface is not reflected. Some portion of it is
absorbed by the surface.
The ratio of the reflected light to the incident
light is called reflection factor.
Law of Illumination
The illumination on a surface depends upon the
luminous intensity, distance between the
source and surface and the direction of rays
of light. It is governed by following laws :
1. Inverse square law
2. Lambert’s cosine law
Inverse Square Law
It states that the illumination of a surface is inversely
proportional to the square of the distance of the surface from
the source. E α I/d2
Lambert’s Cosine Law
This law states that the illumination on any surface is
proportional to the cosine of angle between the direction of the
incident flux and perpendicular to the area. E= I Cos α
Electrical method of producing
Following are the methods of producing light :
1. By developing arc between two electrodes
2. By passing a current through a filament
3. By electric discharge through vapors or
The principle of an arc lamp is that when two
electrodes carrying current are separated
through a small distance, an arc is struck
between them. The arc lamps were used in the
past for street lighting purposes but now a
day these are used when extreme brightness
Most commonly use arc lamp is
Carbon Arc Lamp
Carbon Arc Lamp
This is the oldest type of lamp and is still being
employed in cinema projectors and search
lights. It consists of two hard carbon rods
(Electrodes). The diameter of +ve electrode is
double to that of –ve electrode. The –ve
electrode is generally fixed and +ve electrode
is placed in adjustable holder and the process
is manually or automatic. The arc consists of
carbon vapors surrounded by orange red
zone of burning carbon and pale green
Carbon Arc Lamp
When the lamp is OFF, the two electrodes are
touching each other due to spring pressure on
+ve electrode. When the supply is ON a large
current is flow through electrodes. The
temperature of carbon electrode is increased and
thus the +ve electrode is pulled away against its
spring pressure through a small distance by coil
and thus an arc is struck between electrodes. This
arc is maintained by transfer of carbon particles
from one electrode to other electrode.
Carbon Arc Lamp
These particles travel from +ve electrode to –ve
electrode, thus after sometime of operation +ve
electrode become hollow and –ve become
pointed. That’s why +ve electrode is made double
than –ve electrode.
In carbon arc lamp 85 % of light is given by +ve
electrode which produces high intensity light
and only 10 % by –ve electrode and 5 % by air.
The temperature of +ve electrode is 4000 oC and
that of –ve electrode is about 2500 oC. The
luminous efficiency of such lamps is about 9
For maintaining the arc, a minimum voltage required is
given by : V = (39 + 28 L ), where L is length of arc in
Filament or Incandescent Lamp
Working Principle :
As we know when a room heater is switched On
, it gives out red light with heat at the
working temperature of 750 oC and at this
temperature the radiations are mostly in infra
red regions. This working principle is used to
develop the filament lamp.
Filament or Incandescent Lamp
When an electric current is passed through a
fine metallic wire , it raises the temperature of
wire. At low temperature only heat is
produced but at higher temperature light
radiations goes on increasing. As filament
lamp consists of fine wire of high resistive
material placed in an evacuated glass bulb.
This type of lamps are operated at the
temperature of 2500 oC .
A tungsten filament is enclosed in evacuated glass bulb but to
improve its performance some chemical like argon or nitrogen
gas are filled.
Properties of Metal for Filament
1. High melting point : so that it can be operated at
2. High specific resistance : so that it produces
3. Low temperature coefficient : so that filament
resistance may not change at operating
4. Low vapor pressure ; so that it may not vaporize
5. High ductile : so that it may withstand
Sodium Vapor Lamp
This lamp consists of discharge tube made from
special heat resistance glass, containing a small
amount of metallic sodium, neon gas and two
electrodes. Neon gas is added to start the
discharge and to develop enough heat to
vaporised sodium. A long tube is required to get
more light. To reduce overall dimensions of the
lamp, the tube is generally bent into U-shape .
Sodium Vapor Lamp
Working Principle :
An electric discharge lamps require a high voltage
at staring and low voltage during operation. So
at starting a voltage of 450 V is applied across
the lamp to start the discharge. After 10 to 15
minutes, the voltage falls to 150 V because of
low power factor. To improve the power factor a
capacitor is connected across the supply. The
color of light produce is yellowish.
Mercury Vapor Lamp
On the basis of pressure inside the discharge tube,
the mercury vapor lamps are classified as high
pressure Mercury Vapor Lamp and low pressure
Mercury Vapor Lamp. High pressure M.V. Lamps
are of following types :
1. M.A. type : these are operated at 220 -250 V
A.C. main and made in 250 W and 400 W.
2. M.A.T. type : these are made in 300 and 500 W
and operated at 200 -250 V A.C. as well as D.C.
3. M.B. type : This is operated at 200 – 250 V A.C.
and made in 80 W and 125 W.
Mercury Vapor Lamp
It consists of hard glass tube enclosed in outer bulb of
ordinary glass. The space between two bulbs are
completely evacuated to prevent heat loss by convection
from inner bulb. The outer bulb absorbs harmful ultra
violet rays. The inner bulb contains argon gas with
certain quantity of mercury. In addition with two
electrodes on starting electrode having high resistance
in series also provided. The main electrodes are made of
tungsten wire in helical shape. The lamp has screwed
cap and connected to supply with choke. A capacitor is
connected across supply to improve power factor.
Mercury Vapor Lamp
Working Principle :
When the supply is switched ON, full voltage is applied
across main and starting electrodes. This voltage breaks
down the gap and discharge through argon gas takes
place. As the lamp warms up, mercury is vaporized ,
which increase the vapor pressure. This discharge takes
the shape of intense arc. After 5 minutes, the lamp gives
It gives greenish blue color light .
this lamp is always suspended vertically, other wise
inner glass tube may break due to excessive heat.
it is a low pressure mercury vapor lamp. It
consists of a glass tube 25 mm in diameter
and 0.6 m, 1.2 m and 1.5 m in length. The
tube contains argon gas at low pressure about
2.5 mm of mercury. At the two ends, two
electrodes coated with some electron emissive
material are placed.
The colors produce by this tubes are as :
Zinc silicate Green
Calcium tungsten Blue
Cadmium borate Pink
Calcium Holo phosphate White or day light
Magnesium tungsten Bluish white
A choke is connected in series with the tube
which act as a blast and provide a high
voltage at starting glow in the tube. During
running condition the same choke absorbs
some supply voltage and remain a voltage of
110 V across the tube. A capacitor is
connected to improve the power factor.
Advantages of Fluorescent Tube
1. Voltage fluctuation has very small effect on
2. The luminous efficiency is more as length of
rod is more.
3. It gives light close to natural light.
4. Heat radiations are negligible.
Disadvantages of Fluorescent Tube
1. Its brightness is less.
2. Initial cost is more
3. Overall maintenance cost is high.
These lamps are operated at a very low temperature of
about 200 oC that’s why these are called cold cathode
Two electrodes are housed at the two ends of the tube
which contain neon gas. The electrodes are made of iron
or nickel cylinder without any coating and practically
they do not emit electrons. It gives red light whereas
with mixture of mercury and argon it gives bluish green
color. Voltage require for starting and operation is
10000 V. This high voltage is obtained from
When the supply is switched ON at primary side of
transformer, a voltage of 10000 V develops
across secondary side which come across two
electrodes. At this voltage a discharge occurs in
Different colors can be obtained by changing the
constituents of gases and mercury filled in the
Neon lamps are generally used for advertising.
Most of letters having two ends at which
electrodes are placed. In letter having more
than two ends , the tube path is repeated for
Halogen lamp is a special type of tungsten
filament lamp which was developed in 1959,
in this lams, a small amount of halogen vapor
is added to the inert gas of the bulb. Its glass
bulb is small in size and mechanically strong.
It operates at high temperature of 3000 oC .
When the supply is given to the lamp, a filament
glows and produce light. The halogen in
addition to inert gas causes the evaporated
tungsten to resettle back on the filament
during cooling, that’s why lamp can be
operated at high temperature. It provides
high intensity light.
Advantages of Halogen Lamp
1. It is smaller in size.
2. It does not need any blast.
3. Good colors can be obtained.
4. Excellent optical control.
5. Gives same output throughout life
6. It has long life
Disadvantages of Halogen Lamp
1. During maintenance the handling of lamp is
2. Radiant heat is more which heats the
3. Operating temperature is high which effects
Compact fluorescent Lamp( CFL)
The compact fluorescent lamps are becoming
more and more popular now a days because
of their low power consumption, low
running cost, longer life, attractive look,
smooth light and low maintenance. These
lamps are available in different sizes and
designs. They have single rod, double rod,
triple rod or spiral rod. These lamps are
available in different power rating e.g. 5, 7,
9, 11, 18 and 24 watt 220 V
Compact fluorescent Lamp( CFL)
It is basically a low pressure mercury
vapor lamp having two electrodes
coated with electron emissive material
placed in a glass tube. The tube is coated
internally with some fluorescent
material in the form of powder. In the
tube one drop of mercury and argon
gas is filled at low pressure.
Advantage of CFL
1. Low energy consumption.
2. Low maintenance cost
3. It stars instantly
4. It does not heat the surroundings
5. Excellent color properties
6. Low operating cost
7. More life
Applications of CFL
The compact size, longer life, low running
and maintenance cost, instant glow
makes these lamps suitable for all
places where uniform illumination is
It is used in offices, shops, hotels,
hospitals, cinema halls, residential
Measurement of Candle Power
The candle power of a source in any given
direction is measured by comparing it
with a standard or substandard source
with the help of an optical instrument
Photometers are mostly based on inverse square
law of illumination and may be classified as
stationary and portable photometers.
The stationary photometers are usually installed
in a dark room with dead black walls and
ceiling in order to eliminate error due to
The portable photometers are direct reading
instruments used to measured illumination in
houses, offices , commercial and industrial
Lighting schemes are classified according to
the location, requirement and purpose etc.
are as under :
1. Direct lighting
2. Indirect lighting
3. Semi direct lighting
4. Semi indirect lighting
5. General lighting
As is clear from the name, in this system
almost 90 to 95 % light falls directly on the
object or the surface. The light is made to
fall upon the surface with the help of deep
reflectors. Such type of lighting scheme is
most used in industries and commercial
lighting. Although this scheme is most
efficient but it is liable to cause glare and
In this system, the light does not fall directly on
the surface but more than 90 % of light is
directed upwards by using diffusing
reflectors. Here the ceiling acts as a source
of light and this light is uniformly
distributed over the surface and glare is
reduced to minimum. It provides shadow
less illumination which is useful for
drawing offices and composing rooms. It is
also used for decoration purposes in cinema
halls, hotels etc.
Semi direct Lighting
This is also an efficient system of lighting and
chances of glare are also reduced. Here
transparent type shades are used through
which about 60 % light is directed downward
and 40 % is directed upward. This also
provides a uniform distribution of light and is
best suited for room with high ceilings.
Semi indirect Lighting
In this system about 60 to 90 % of total light is
thrown upward to the ceiling for diffused
reflection and the rest reaches the working
plane directly. A very small amount of light is
absorbed by the bowl. It is mainly used for
This system employs such type of luminaries,
shades and reflectors which give equal
illumination in all the directions.
Design of Indoor Light Scheme
While designing a good lighting schemes, the
following points must be kept in mind :
1. It should provide adequate illumination.
2. It should provides uniformly distributed light
all over working plane.
3. It should avoid glare and shadows as far as
4. It should provide light of suitable colors.
Factors required for Light Scheme
The following factors are required to be
considered while designing the lighting
1. Illumination level
2. Quality of light
3. Co efficient of utilization
4. Depreciation factor
5. Space height ratio
This is the most vital factor in deciding the
number and wattage of luminaries so that we
are able to see and recognize the object
properly. Colors of the body have the
property of reflecting the light in different
proportions, degree of illumination, its
distance from the viewer, contrast between
the object to be seen and its surroundings.
Type of work recommended illumination level
Offices 100-400 lumens/ meter square
Schools 250-400 lumens/ meter square
Industry 1000 lumens/ meter square
Shops 250-500 lumens/ meter square
Hotels 80-100 lumens/ meter square
Hospitals 250-3500 lumens/ meter square
Quality of Light
This means that the illumination should not be
harmful to the viewers. It should be glare free,
shadow less and contrast free. Direct glare
from the source of light is most common
factor. Presence of polished and glassy surface
will cause indirect glare unless diffused light
is used. Hard and long shadows can be
avoided by using a large number of lamps
and adjusting the mounting height.
Co – efficient of Utilization
A surface to be illuminated receive light either
directly from the lamps or reflected from the
ceiling and walls or both. In this case, the
total flux reaching the surface will never be
equal to the flux emitted by the lamp, due to
absorption by reflectors, ceiling and walls.
Utilization factor = lumens reaching at the working
total lumens emitted by the source
Usually it varies from 0.5 to 0.8.
The total flux emitted by the source and its
fitting may be reduced due to deposition of
dust upon the surfaces. Similarly quantity of
light reflected from the ceiling and walls also
decreases with the passage of time. This is
called as depreciation facto.
Usually it varies from 1.3 to 1.6.
Space Height Ratio
The ratio of space (horizontal distance ) between
the two adjacent lamps to the vertical height
of the lamps above the working plane is
called space height ratio.
So the distance between the lamps is not too
much. An ideal scheme could be when there
is large number of small size lamps are used
also it increases the cost of installation. So the
space height ratio is 1 to 1.5.
Building Services: S.M. Patil