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

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 A2

Source

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 1/d2
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 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 = 1/d2 cos θ
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 Light Sources

Following are the methods of producing light :

• Incandescent lamps produce light by heating
a filament until it glows.
• Discharge lamps produce light by ionizing a
gas through electric discharge inside the lamp
• Solid-state lamps use a phenomenon called
electroluminescence to convert electrical
energy directly to light.
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 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 .
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 1. Outline of Glass bulb
2. Low pressure inert gas (argon, nitrogen, krypton,
xenon)
3. Tungsten filament
4. Contact wire (goes out of stem)
5. Contact wire (goes into stem)
6. Support wires (one end embedded in stem;
conduct no current)
7. Stem (glass mount)
8. Contact wire (goes out of stem)
9. Cap (sleeve)
10. Insulation (vitrite)
11. Electrical contact

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 Properties of Metal for Filament
1. High melting point: so that it can be operated at
high temperature.
2. High specific resistance: so that it produces
more heat.
3. Low temperature coefficient : so that filament
resistance may not change at operating
temperature.
4. Low vapor pressure : so that it may not vaporize
5. High ductile: so that it may withstand
mechanical vibrations
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 Arc Lamp
The principle of an arc lamp is that when two
electrodes carrying current are separated
through a small distance of 2-3mm, 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 is required.
Most commonly use arc lamp is
Carbon Arc Lamp

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 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.

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 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.
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 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 lumen/watt.
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 Fluorescent Tube
It is a low pressure mercury vapor lamp.
Dimensions: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.

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• The cathode filament emits electrons after
getting heated due to supply of current.
• These electrons while accelerating, collide
with argon and mercury vapor atoms. Excited
atoms of mercury give radiation but they are
not in visual spectrum, because wavelength is
ultraviolet.
• Inner surface of the tube is coated with
phosphor material. Hence the reemission of
radiations gives different colours.

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 Fluorescent Tube
The colors produce by this tubes are as :
Material Color
Zinc silicate Green
Calcium tungsten Blue
Cadmium borate Pink
Calcium Holo phosphate White or day light
Magnesium tungsten Bluish white

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 Fluorescent Tube

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 Fluorescent Tube
Working :
A choke is connected in series with the tube
which act as a balast 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.

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 Advantages of Fluorescent Tube
1. Voltage fluctuation has very small effect on
light output.
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.

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 Disadvantages of Fluorescent Tube

1. Its brightness is less.

2. Initial cost is more
3. Overall maintenance cost is high.

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 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
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 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.

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 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
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 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
required.
It is used in offices, shops, hotels,
hospitals, cinema halls, residential
buildings etc.
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 Neon Lamp
These lamps are operated at a very low temperature of
about 200oC that’s why these are called cold cathode
discharge lamps.
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 transformer.
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 Neon Lamp
Working :
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 neon gas.
Different colors can be obtained by changing the
constituents of gases and mercury filled in the tubes.

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 Neon Lamp

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 Neon Lamp
Applications :
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
some portion.

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 Halogen Lamp

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 .

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 Halogen Lamp
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.

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 Halogen Lamp

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 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

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 Disadvantages of Halogen Lamp
1. Radiant heat is more which heats the
surroundings.
2. Operating temperature is high which effects
its life.

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 Lighting Schemes
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
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 Direct 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 shadows.
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 Indirect Lighting
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 shadowless illumination which is
useful for drawing offices and composing
rooms. It is also used for decoration
purposes in cinema halls, hotels etc.
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 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.

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 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
interior decoration.

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 General Lighting

This system employs such type of luminaries,

shades and reflectors which give equal
illumination in all the directions.

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 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
possible.
4. It should provide light of suitable colors.

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 Methods of lighting
• Decorative lighting:
✓Provides a visual or decorative impact within a
space. fairy lights, lanterns, battery operated
light chains, small pebble lamps etc.
✓Can create focal points and add some sparkle
to a space. Fairy lights draped across a fireplace
or a cluster of small pebble lights on the floor
can provide decoration and ambience for a
social event or celebration, or simply to
provide a soft glow for an evening in.
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• Ambient lighting:
✓Provides an area with overall illumination.
Also known as general lighting, it radiates a
comfortable level of brightness without glare,
enabling you to see and move around the
space safely.
✓In some small spaces such as a bathroom or
utility room, the ambient lighting also acts as
the primary source of task lighting.

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• It can be accomplished with chandeliers,
ceiling or wall-mounted fixtures, recessed or
track lights and with lanterns mounted
outside the building. Having a central source
of ambient light in all rooms is fundamental to
a good lighting plan.

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• Task lighting
✓Enables you to perform specific tasks, such as
reading, writing, preparing and cooking food,
doing homework, working on hobbies and so
on. It can be provided by recessed and track
lighting, pendant lights, under-cabinet
lighting, and also through the use of portable
floor, table and desk lamps.
✓Task lighting should be free of distracting glare
and shadows, however it must be bright
enough to prevent eye strain.
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• Accent lighting
✓Adds drama to a room by creating visual
interest.
✓As part of an interior design scheme, it is used to
draw the eye to architectural features,
houseplants, paintings/photographs, sculptures
and other prized possessions.
✓It can also be used to highlight the texture of
materials such as a brick or stone wall, window
treatments, furniture or outdoor landscaping.
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• To be effective, accent lighting requires as
least three times as much light on the focal
point as the general lighting surrounding it.
• Accent lighting is usually provided by recessed
and track lighting or wall-mounted picture
lights.

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 Factors required for Light Scheme
The following factors are required to be
considered while designing the lighting
scheme :
1. Illumination level
2. Quality of light
3. Co efficient of utilization
4. Depreciation factor
5. Space height ratio

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 Illumination Level
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.
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 Illumination Level
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

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 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.
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 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 place
total lumens emitted by the source
Usually it varies from 0.5 to 0.8.

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 Depreciation Factor
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
factor.
Usually it varies from 1.3 to 1.6.

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 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.

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 Thank You

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