Bahir Dar University

Bahir Dar Institute of Technology (BiT)

Faculty of Electrical and Computer
Engineering

Electrical Power Engineering Stream

Electrical Installation
Course code: ECEG 4131
Prepared by: Biniyam Zemene (M.Sc.)
12/20/2018 Biniyam.Z 1
 1
CHAPTER

Illumination

12/20/2018 Biniyam.Z 2
 1. Introduction
Radiation from a hot body
• The usual method of producing artificial
light consists in raising a solid body or
vapor to incandescence by applying heat
to it
• It is found that as the body is gradually
heated above room temperature, it begins
to radiate energy in the surrounding
medium in the form of electromagnetic
waves of various wavelengths
12/20/2018 Biniyam.Z 3
 Cont’d
• when the temperature is low, radiated energy is
in the form of heat waves only but when a
certain temperature is reached, light waves are
also radiated out in addition to heat waves and
the body becomes luminous
• Further increase in temperature produce an
increase in the amount of both kind of
radiations but the color of light or visible
radiation changes from bright red to orange,
to yellow and then finally, if the temperature
is high enough, to white
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 Cont’d
• As the temperature is increased, the
wavelength of visible radiation goes
becoming shorter
• It should be noted that heat waves are identical
to light waves except that they are of longer
wave length and hence produce no impression
on the retina
• Obviously, from the point of view of light
emission, heat energy represents so much
wasted energy
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 Cont’d
White light, such as that given by the sun, is
composed of different colours each having different
wave lengths. These are the following.
0.300 To 0.436 µm ----------------------------Violet
0.436 To 0.495 µm -----------------------------Blue
0.495 To 0.566 µm -----------------------------Green
0.566 To 0.589 µm ----------------------------Yellow
0.589 To 0.627 µm ---------------------------Orange
0.627 To 0.780 µm -------------------------------Red
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 Cont’d
• Those colours of white light having
wavelength less than 0.3 µm belongs to the
ultraviolet range and those with wave lengths
greater than 0.8 µm belongs to the infrared
range
• And, the visible spectrum ranges are from 0.4
to 0.7 µm

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 1.1 Terminologies
Plane and Solid angle
• In view of the fact that a source emits the light
in all directions and it is not limited to a
particular plane, the study of plane angle and
solid angle has become essential
Plane angle
• When two straight lines lying in the same
plane meet at a point, there will be an angle
between these converging lines at the meeting
point. This angle is termed as plane angle.
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 Cont’d
• It is measured in radians and equal to the ratio
of the length of the arc to its radius,
θ = arc/ radius = l/ r radians

12/20/2018 Biniyam.Z 9
 Cont’d
Solid angle
• The angle subtended at a point in space by an
area, is termed as a solid angle
• In plane angle, it is the area which is enclosed
by the two lines, but in case of solid angle, it is
the volume which is enclosed by numerous
lines lying on the surface and meeting at a
point

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 Cont’d
• Solid angle is represented

• The angle subtended by the partial surface area

of a sphere at its center is called as solid angle
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 Cont’d
• 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

steradian

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 Cont’d
Luminous Flux(F or ɸ)
• It is the light energy radiated out per second
from the body in the form of luminous light
waves
• Since, it is a rate of flow of energy, it is a sort
of power unit
• Unit of luminous flux is lumen (lm)
• It is defined as the flux contained per unit solid
angle of a source of one candela or standard
candle
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 Cont’d
• Approximate relation between lumen and
electric unit of power
i.e. watt is given as 1 lumen= 0.0016 watt (approx.)
Luminous Intensity (I) or Candle-power of a
point source in any particular direction is given
by the luminous flux radiated out per unit solid
angle in that direction
• In other words, it is solid angular flux density
of a source on specified direction
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 Cont’d

• If dɸ is the luminous flux radiated out by a

source within a solid angle of dω steradian in
any particular direction, then I= dɸ/ dω
• If flux is measured in lumens and solid angle
in steradian , then its unit is lumen/steradiain
(lm/sr) or candela (cd)

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 Cont’d
Illumination or Illuminance (E)
• When the luminous flux falls on a surface, it is
said to be illuminated, the illumination of a
surface is measured by the normal luminous
flux per unit area received by it
• If dɸ is the luminous flux incident normally on
an area d A, then
E = dɸ/d A or E= ɸ/A
• Since flux ɸ is measured in lumens and area in
m2, unit of E is lm/m2 or lux
12/20/2018 Biniyam.Z 16
 Cont’d
Lumen-hour
• It is the quantity of light delivered in one hour
by a flux of one lumen
Luminance (L):
• The luminous intensity (I) per unit of the
apparent area of the source of light (or an
illuminated area).
• The measured brightness of a surface
L= I/A [cd/m2]
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 Cont’d
• Co – efficient of Utilization:- total height flux
radiated out by the source is not utilized on the
working plane
• 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
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 Cont’d
Utilization factor = lumens reaching at the working place
total lumens emitted by the source
Usually it varies from 0.4 to 0.6
• The value of utilization factor depends
upon :
i. The mounting height of lamps
ii. Area to be illuminated
iii. Type of lighting scheme
iv. Color of the surrounding, etc.
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 Cont’d
Depreciation Factor (Maintenance Factor )
• As we are to continue to use the installation,
the illumination produced considerably
decreases due to ageing of the lamps and
accumulation of dusts, dirt and smoke on the
lamps, reflectors, ceiling and walls
• Its value is more if there is much as the ageing
problem increases, etc.
• The value is mostly ranges between 0.8 and 1
12/20/2018 Biniyam.Z 20
 1.1.1. Laws of Illumination
• The illumination on a surface depends upon
the
o luminous intensity
o distance between the source and
o surface and the direction of rays of light
• It is governed by following laws :
1. Inverse square law
2. Lambert’s cosine law
12/20/2018 Biniyam.Z 21
 Cont’d
Inverse square law
i. E is directly proportional to the luminous
intensity I of the source or E α I
ii. The illumination of a surface is inversely
proportional to the square of the distance of
the surface from the source E α 1/r2

12/20/2018 Biniyam.Z 22
 Cont’d

• 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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 Cont’d
• The illumination of a surface is inversely
proportional to the distance between source &
surface
Let, E1 be the illumination on surface A1,
E2 be the illumination on surface A2,
Since exactly the same luminous flux falls on A1 & A2
we can have the relation,
φ = E1A1= E2A2
E2 = E1A1/A2 = E1 (d1/d2)2
i.e., E ~ 1/d2, where d is the perpendicular distance
from the light source
12/20/2018 Biniyam.Z 24
 Cont’d
Lambert’s cosine law
• It states that the illumination on the surface is
proportional to the cosine of the angle b/n the
normal to the surface and the light flux

12/20/2018 Biniyam.Z 25
 Cont’d
• As shown in fig above let Φ be the flux
incident on the surface of area A when in
position1
• When this surface is turned back through an
angle θ,then the flux incident on it is Φ cosθ
• Hence, illumination of the surface when in
position 1 is E1= Φ/A. But in position2

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 Cont’d
Combining all these factors together , we get E= I Cos θ/ r2

12/20/2018 Biniyam.Z 27
 Cont’d
• Thus illumination depends not only on the
perpendicular distance from the light source
but also on the angle that the light falls on the
area to be illuminated
Example: A 250W sodium-vapor street lamp
emits a light of 22,500 cd and is situated 8m
above the road. Calculate the illuminance.
a) Directly below the lamp
b) At a horizontal distance along the road of 6m
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 Cont’d

Soln: Given I=22,500 cd, h = 8m, ox = 6m

d = 10m
a) E= I*cosӨ/h2 but cosӨ=1 because Ө=0
E= 22,500*1/64=351.56 lm/m2
b) Ex=I*cosӨ/d2 and cosӨ=h/d=8/10=0.8
Ex=22,500*0.8/100=180 lm/m2
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 Cont’d
Example: - A standard incandescent lamp
having a luminous intensity of 100 cd in all
directions gives an illuminance of 40 lux at the
surface of a bench vertically below the lamp.
What distance is the lamp above the bench?
Solution
I
E 2
d
I 100
d   1.58m
E 40
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 1.2. Light source and applications
• Light occurs in nature, and sunlight,
moonlight, and starlight are the most important
sources of light to life.
• Natural light sources occur within nature and
are beyond the control of people.
• Man-made light sources can be controlled by
people, these include wood flame, oil flame,
gas flame, electric lamps, photochemical
reactions, and various reactions.
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 Cont’d
• Due to their obvious advantages in terms of
availability, safety, cleanliness, and remote
energy generation, electric lamps have
displaced almost all other man-made sources.
Qualities of Light Sources
• The intent of most light sources is to produce
white light.

12/20/2018 Biniyam.Z 32
 Cont’d
• There are two criteria's to measure:
1.Color temperature which describes whether
the light appears warm (reddish), neutral, or cool
(bluish).
• It is measured in degrees Kelvin (K). For
instance, the color temperature of an
incandescent lamp is about 2700K,
o In more practical sense we generally
consider colors of artificial light sources to
be approximately be in the 2,000 to 10,000
degree K range.
12/20/2018 Biniyam.Z 33
 Cont’d
2. Color rendering index (CRI) which
describes the quality of the light on a scale of 0
(horrible) to 100 (perfect).
• It expresses how a given light source makes
the color of an object appear to human eyes.
• If a light source has a higher CRI, it is
considered to display colors more accurately or
more nearer to Natural light (the sunlight).
• The higher the number of CRI the more natural
an object or color will appear.
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 Cont’d

12/20/2018 Biniyam.Z 35
 Cont’d
• When choosing electric light sources, it is
generally best to select source color temperature
and CRI according to the following table.

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 Cont’d
Generally, electric lamps can be classified in to:
o Incandescent lamps : Electric current pass
through a tungsten wire heats it to
incandescence, and the wire emits light.
o Discharge lamps : Light is produced by
passage of an electric current through a
vapor or gas.
a) Incandescent lamps
o Tungsten Incandescent Lamps
o Tungsten-Halogen lamps
12/20/2018 Biniyam. Z 37
 Cont’d
b) Discharge lamps
o Fluorescent lamps (also known as low
pressure mercury)
o Low Pressure Sodium lamps (LPS )
o High-Intensity Discharge (HID) Lamps
 High Pressure Mercury lamps
 Metal Halide lamps
 High Pressure Sodium lamps (HPS )
o Neon lamp
12/20/2018 Biniyam.Z 38
 Cont’d
Tungsten Incandescent Lamps
• Incandescent lamps emit visible light when
enough electric current passes through the
filament to heat it to incandescence.
• The hotter the filament, the whiter the light.
• All modern Incandescent lamps, also known
as General Lighting Service Lamps (GLS),
consist basically of a filament, enclosed in a
glass bulb generally filled with an inert gas.
• Tungsten is used because of its high melting
point (34000c)
12/20/2018 Biniyam.Z 39
 Cont’d
• Most filaments operate at temperatures in the
region of 2700°C.
• The problem is that as the filament gets hotter,
more evaporation of metal from the filament.
• The rate of evaporation is controlled by the gas
filling, usually a mixture of argon and nitrogen
• The inert gas fill released into the bulb puts
enough pressure on the filament to retard
evaporation
12/20/2018 Biniyam.Z 40
 Cont’d
• Incandescent lamps are capable of producing
from 5 to 20 lumens per watt (lm/W).
• The average lifetime of Tungsten Incandescent
lamps is about 1000h when operating at rated
V.
• The color temperature of incandescent lamps is
about 2700K, generating a warm-toned light.

12/20/2018 Biniyam.Z 41
 Cont’d

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 Cont’d
Advantages of incandescent lamps:
 Inexpensive
 Easy to use, small and does not need
auxiliary equipment
 Easy to dim by changing the voltage
 Excellent color rendering properties
 Directly work at power supplies with fixed
voltage
 Free of toxic components and instant
switching
12/20/2018 Biniyam.Z 43
 Cont’d
Disadvantages of incandescent lamps:
 Short lamp life (1000 h)
 Low luminous efficacy
 Heat generation is high
 Lamp life and other characteristics are
strongly dependent on the supply voltage
 The total costs are high due to high
operation costs.
12/20/2018 Biniyam.Z 44
 Cont’d
Tungsten-Halogen lamps
• A tungsten-halogen lamp is a high-pressure
incandescent lamp containing a halogen gas
such as iodine, bromine or fluorine which
allows the filament to be operated at a higher
temperature.
• Halogen lamps emit intense white light than
incandescent lamps, they use quartz rather than
glass bulbs.
• The use of a small quartz bulb allows the gas
pressure to be substantially increased
12/20/2018 Biniyam.Z 45
 Cont’d
• The halogen gas recycles evaporated particles
of tungsten back onto the filament surface
(halogen cycle)
• Halogen lamps give off whiter light and last
longer than Tungsten Incandescent lamps .
• Lamp life for halogen lamps ranges from
2000h up to 10,000 hours and luminous
efficacy is 15-35 lm/W

12/20/2018 Biniyam.Z 46
 Cont’d
• The color temperature of halogen lamps is
about 3000K, making their light appear
slightly whiter and cooler than incandescent.
• These lamps are used in
o High-intensity desk and reading lamps
o Vehicle headlights
o Spotlights and
o Flashlights
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 Cont’d
• Halogen lamps

12/20/2018 Biniyam.Z 48
 Cont’d
Advantages of tungsten halogen lamps:
 Small size
 Directional light with some models (narrow
beams)
 Low-voltage alternatives
 Easy to dim
 Instant switching and full light output
 Excellent color rendering properties
12/20/2018 Biniyam.Z 49
 Cont’d
Disadvantages of tungsten halogen lamps:
 Low luminous efficacy
 Surface temperature is high
 Lamp life and other characteristics are
strongly dependent on the supply voltage

12/20/2018 Biniyam.Z 50
 Cont’d
Discharge lamps
• This type of lighting relies on the ionization of a
gas to produce light.
• When an arc is struck in a gas or metallic vapor it
radiates energy in characteristic wave bands.
• For example, neon gives red light, sodium gives
yellow and mercury vapor gives four distinct lines
in the visible and two in the ultraviolet region of
the spectrum.
• The color of the light emitted depends upon the
type of gas used
12/20/2018 Biniyam.Z 51
 Cont’d
• All modern discharge lamps operate in a
translucent enclosure containing the
appropriate metals or metal halides; the initial
discharge is usually struck in argon or neon.
• As the metal or metal halide evaporates, it
takes over the discharge from the starter gas
and emits light at its characteristic
wavelengths.
• Because more light and less heat is radiated by
these lamps, they are more efficient in terms of
lumens per watt than filament lamps
12/20/2018 Biniyam.Z 52
 Cont’d
Fluorescent lamps ( low pressure mercury lamps)
• Fluorescent lamps use the principle of
fluorescence, in which minerals exposed to
ultraviolet light are caused to glow.
• Certain materials, such as calcium phosphate,
emit visible light whenever they absorb ultraviolet
light. This phenomenon is known as fluorescence.
• The lamp consists of a glass tube the interior is
coated by phosphorous.
• The tube is filled with mercury vapor at low
pressure and a little argon to assist starting.
12/20/2018 Biniyam.Z 53
 Cont’d
• At each end of the tube is situated an oxide-
coated filament.
• Discharge takes place when a high voltage is
applied across the ends of the tube.
• A fluorescent lamp requires a ballast in order to
work properly.
• A ballast is an electrical component that starts the
lamp and regulates the electric power flow to the
lamp.
• Phosphors are designed to radiate particular
colors of white light, thus enabling the choice of
both the color temperature and CRI of a lamp.
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 Cont’d
Advantages of fluorescent lamps
o Inexpensive
o Good luminous efficacy (40-70 lm/W)
o Long lamp life, 10,000 – 16,000 h

12/20/2018 Biniyam.Z 55
 Cont’d
Disadvantages of fluorescent lamps
• Ambient temperature affects the switch-on and
light output
• Need of auxiliary ballast and starter or electronic
ballast
• Light output depreciates with age
• Contain mercury
• Sensitive for voltage change

12/20/2018 Biniyam.Z 56
 Cont’d
Practical operation
• When the supply is applied, the circuit is
completed via: the choke, first lamp element,
starter switch, second lamp element and the
neutral.
• The elements, which are coated in oxide,
become warm and the oxide coating emits
some electrons and the gas ionizes at the ends
of the tube (this helps the main ionization
process).
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 Cont’d
• The starter contacts (usually of the bimetallic
type) separate, owing to the current passing
through them, and the choke is open-circuited.
• Making a sudden interruption in the current
flowing through the inductor causes it to
produce high voltage.
• When the gas is fully ionized, the choke limits
the current to a predetermined value, and the
light emitted, which is mostly ultraviolet, is
made visible by the fluorescent powder
coating.
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 Cont’d
• The radio interference suppression capacitor is
usually located in the starter.
• The PF correction capacitor is part of the control
circuitry common to all fluorescent lighting
installations.

12/20/2018 Biniyam.Z 59
 Cont’d
Starters
• Three methods are commonly available for
starting the discharge in a fluorescent tube: the
thermal starter, the glow start and the quick
start.
Thermal type start
• The thermal type starter switch has two
contacts mounted on bi-metal strips, a small
heating coil being fitted very close to the bi-
metal strips but no electrical contact between
them.
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 Cont’d
• The contacts are normally closed so that the main
supply is first switched on full heating current
passes through the lamp electrodes as before.
• The current also flows through the starter heater
and so warms the bi-metal strips. After a short
time the bimetal strips warms sufficiently to bend
and open the contacts thus striking the lamp.

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 Cont’d
Glow type start
• The glow type starter switch consists of a small
bulb filled with helium and containing two
contacts, one of which is mounted on a bi-metal
strip.
• The contacts are normally open so that when full
main voltage is applied to the starter contacts.
• This causes a glow discharge, which warms the
bi-metal strip making it bends, so closing the
starter contacts.
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 Cont’d
• The closing of the starter contacts allows full
heating current to pass through the lamp electrodes
and also extinguish the glow discharge. After a
short time the bi-metal strip cools sufficiently to
open the circuit thus striking the lamp.
• If the voltage applied to the starter is insufficient to
initiate a glow discharge and so the starter contacts
remain open until the next starting operation so
lamp will not start.
• A small capacitor is often connected in parallel with
the starter switch contacts to suppress radio
interference.
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 Cont’d

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 Cont’d
Quick start or Instant starter
• In the case of the quick start, starting is
achieved by the use of autotransformer and an
earthed metal strip in close proximity to the
tube.
• When the supply is switched on, mains voltage
appears across the end of the tube, and the
small part of the winding at each end of the
transformer energizes the filaments, which
heat up.
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 Cont’d
• The difference in potential between the
electrodes and the earthed strip causes
ionisation, which spreads along the tube

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 Cont’d
Compact Fluorescent Lamps (CFLs)
• The CFL is a compact variant of the
fluorescent lamp.
• The overall length is shortened and the
tubular discharge tube is often folded into two
to six fingers or a spiral
• For a direct replacement of tungsten filament
lamps, such compact lamps are equipped
with internal ballasts and screw or bayonet
caps.
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 Cont’d
• There are also pin base CFLs, which need an external
ballast and starter for operation
• The luminous efficacy of CFL is about four
times higher than that of incandescent lamps.
• Therefore, it is possible to save energy and costs in
lighting by replacing incandescent lamps with CFLs.

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 Cont’d
Low Pressure Sodium lamps (LPS )
• These are the most efficient lamps in terms of
lumens per watt, because the monochromatic (one
color) yellow light they produce is in the area near
the peak of the eye sensitivity curve.
• They are extensively used for highway lighting and
in situations where no color discrimination is
necessary.
• The low-pressure type consists of a U-shaped
double-thickness glass tube, the inner wall of which
is of low-silica glass which can withstand attack by
hot sodium.
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 Cont’d
• Inside the tube is a quantity of solid sodium
and a small amount of neon gas (this helps to
start the discharge process).
• An outer glass envelope stops too much heat
loss from the inner tube.
• The recommended burning position of the
lamp is horizontal, +20°; this ensures that hot
sodium does not collect at one end of the tube
in sufficient quantities to attack and damage it.
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 Cont’d
• The light output is almost pure yellow , which
distorts surrounding colors, and as such is
useful only for street lighting.
• The modern LPS lamp has a high efficacy, a 90
W lamp giving in the region of 140 lm/W.
• LPS lamps are even higher in lumens per watt,
but their color is so poor that their use is
limited to security lighting.

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 Cont’d

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 Cont’d
High-Intensity Discharge (HID) Lamps
• HID includes mercury-vapor, metal halide, and high-
pressure sodium lamps.
• All electric discharge lamps include cylindrical
transparent or translucent arc tubes that enclose the
mercury, various gases, and metal salts and confine
the electric discharge.
• The arc tubes are enclosed in a glass bulb which
performs three functions:
o Excludes air to prevent oxidation of metal
o Increases operating temperature of the lamp
o Reduces ultraviolet radiation emitted by the
12/20/2018
excitation of the vapors
Biniyam.Z 73
 Cont’d
• Like fluorescent lamps, all HID lamps require
ballasts.
• HID lamps operate at high pressures and very high
temperatures. These lamps can get quite hot and
generally should be protected from direct touch.
• HID lamps give full light output over a wide range of
ambient temperatures.
• This makes them suitable for street, stadium, and
parking lot lighting as well as building flood lighting
and interior high bay illumination.
• HID lamps require time to warm up; they get
progressively brighter over several minutes until
reaching full light output.
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 Cont’d
• The lamp’s true light output and color is often
not reached for two to five minutes.
• If power to an operating HID lamp is
interrupted, the lamp must cool before the
ignition circuit can restart it.
• The cool-off period is called the restrike time.
• Some HID lamps must cool more than 10
minutes after being extinguished before they
can restrike and warm back up.
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 Cont’d
High Pressure Mercury lamps
• This type consists of a quartz tube containing
mercury at high pressure and a little argon gas to
assist starting.
• There are three electrodes, two main and one
auxiliary
• The initial discharge takes place in the argon gas
between the auxiliary electrode and the main
electrode close to it.
• This causes the main electrode to heat up and the
main discharge between the two main electrodes
takes place.
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 Cont’d
• Mercury-vapor (MV) lamps produce most of their
light from the excitation of mercury atoms. A
small amount of liquid mercury, which is
vaporized, and argon gas fill is sealed within a
quartz arc tube mounted inside the glass envelope.
• Mercury vapour lamps emit a considerable
amount of energy in two wavebands in the UV
region, the proportion of visible to invisible
radiation being closely related to the gas pressure
in the discharge tube.
• This is used to excite fluorescence in phosphors
coating the inside of the outer bulb.
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 Cont’d
• MV lamps have correlated color temperatures
(CCTs) of 3000 to 7000 K.
• The color rendering indexes (CRIs) of standard
MV lamps range from 15 to 25, while those with
phosphor-coated bulbs are 40 to 55.
• The efficiencies are from 30 to 65 lm/W.
• MV lamps are used in both indoor and outdoor
lighting mainly for industrial and street lighting,
commercial and display lighting.
• They are available with wattage ratings of 40 to
1000 W.
12/20/2018 Biniyam.Z 78
 Cont’d
Typical high-pressure mercury vapor lamps.

12/20/2018 Biniyam.Z 79
 Cont’d
Metal Halide lamps
• Metal halide lamps produce white light of a good
color quality and are available in many sizes, from
compact lamps that can be used in track lighting and
table lamps to huge lamps for lighting stadiums.
• Standard metal halide lamps typically are used where
color is not critical, such as sports arenas, parking
lots, landscape lighting, and building floodlighting.
• Metal-halide (MH) lamps produce light by radiation
from an excited mixture of mercury vapor in their arc
tubes and the products of the dissociation of metal
halides (compounds with iodine or bromine).
12/20/2018 Biniyam.Z 80
 Cont’d
• The construction of an MH lamp is similar to
that of a mercury-vapor lamp.
• In addition to an argon gas fill, the quartz arc
tube contains halide (iodide) additives to
increase the lamp’s luminous efficiency and
provide a more pleasing color balance than can
be obtained from mercury vapor in an argon
gas fill.
• MH lamps have correlated color temperatures
(CCTs) of 2300 to 5400 K and color rendering
indexes (CRIs) of 60 to 93.
12/20/2018 Biniyam.Z 81
 Cont’d
• Efficacies of MH lamps are 75 to 125 lm/W.
• Phosphor coating is not needed on MH lamp
glass envelopes because their emitted light
color is comparable to fluorescent cool white.
• MH lamps emit less UV radiation than MV
lamps, are 50 to 75 percent more efficient than
fluorescent lamps, and have wattage ratings
from 40 to 1000 W.
• The lamp life is typically from 6,000 h to
12,000 h.
12/20/2018 Biniyam.Z 82
 Cont’d

12/20/2018 Biniyam.Z 83
 Cont’d
High Pressure Sodium lamps (HPS)
• HPS lamps produce light by the excitation of
sodium vapor under high pressure.
• HPS lamp is constructed and operates in a way
similar to mercury-vapor and metal-halide
lamps.
• The high-pressure type of sodium-vapor lamp
differs from other discharge lamps in which
the internal arc tube is made of translucent
ceramic polycrystalline alumina (compressed
aluminum oxide).
12/20/2018 Biniyam.Z 84
 Cont’d
• Which is capable of withstanding the intense
chemical activity of the sodium vapour at high
temperature and pressure.
• The arc tube contains a small amounts of
metallic sodium and mercury sealed with a
xenon gas fill.
• compressed aluminum oxide is used due to
their strength at the high operating
temperatures of HPS lamps (about 1300˚C).
• Some HPS lamps use a neon–argon mix in
place of xenon.
12/20/2018 Biniyam.Z 85
 Cont’d
• The efficacy is in the region of 100 lm/W, and
the lamp may be mounted in any position.
• HPS lamps, the most efficient of the HID
lamps, are about twice as efficient as
fluorescent lamps.
• They have wattage ratings from 35 to 1000 W.
On the other hand, low-pressure sodium (LPS)
lamps have wattage ratings of only 18 to 180
W
12/20/2018 Biniyam.Z 86
 Cont’d

12/20/2018 Biniyam.Z 87
 Cont’d
Neon lamp (Neon tube)
• Neon lamps are closely related to fluorescent
lamps in operating principles.
• While their primary applications are signs and
specialty lighting, neon lamps can be used for
architectural lighting applications.
• Neon lamps last 20,000 to 40,000 hours, are
reasonably energy efficient, with an efficacy of
up to 50 lm/W
12/20/2018 Biniyam.Z 88
 Cont’d
• When thinking of neon lamps, imagine tubular
lighting that can be formed into just about any
shape and be made to create just about any color
of light and neon tubing usually terminates in
base wire connectors.
• There is no starter or choke and a high voltage
transformer (10kV) is necessary to initiate the gas
discharge.
• Neon tube are gas-filled tube.
• There are in fact several different gases used to
give different colors, including helium, nitrogen
and carbon dioxide.
12/20/2018 Biniyam.Z 89
 Cont’d

12/20/2018 Biniyam.Z 90
 Cont’d
Other Light sources
Induction Lamps
• An induction lamp is somewhat like a fluorescent.
• It contains a low pressure of mercury which, when
excited, radiates UV, which in turn is absorbed by a
phosphor coating and reradiated as visible light.
• The induction lamp has no electrodes and the
discharge is created by a magnetic field generated
externally to the bulb.
• An induction coil is located in a glass bulb having a
phosphor powder coating on the inside and
containing a low pressure of mercury.
12/20/2018 Biniyam.Z 91
 Cont’d
• A radio frequency (2.65MHz) current in the
coil causes a U.V. emission in the gas.
• The phosphor powder converts the U.V.
radiation into light in the visible spectrum.
• There are no electrodes required in the lamp,
however, special luminaires are required to
prevent electromagnetic interference (EMI).
• The lamps operate at high frequency so there is
no flicker. 55W and 85W ratings are available.
• Colors are the same as fluorescent tubes.
12/20/2018 Biniyam.Z 92
 Cont’d
• Lamp life can be 60,000 to 100,000 hours and an
induction lamp used every day for 12 hours will
last more than 20 years.
• Typical applications include street lighting and
lighting in hard-to-maintain locations.

12/20/2018 Biniyam.Z 93
 Cont’d
Light-Emitting Diodes (LEDs)
• LEDs are semiconductor devices that produces
light when an electrical current applied to
them.
• LED lights are widely used as replacement of
bulbs in general lighting.
• Extremely energy efficient (130 lumens/watt)
and extremely long lasting light bulbs.
• LED light bulb can reduce energy
consumption by 80 -90% and last around
100,000 hours.
12/20/2018 Biniyam.Z 94
 Cont’d
• They even light up faster than regular bulbs.
• They are more expensive presently.

12/20/2018 Biniyam.Z 95
 1.3. Lighting Schemes
• Lighting or illumination is the deliberate use of light
to achieve a practical or aesthetic effect.
• Lighting includes the use of both artificial light
sources like lamps and light fixtures, as well as
natural illumination by capturing daylight.
• This can save energy in place of using artificial
lighting
• Proper lighting can enhance task performance,
improve the appearance of an area.
• Depending largely on the distribution of the light
produced by the fixture, lighting is classified as:
general, accent, task lighting,
12/20/2018 Biniyam.Z
or decorative 96
 Cont’d
General lighting
• General Lighting provides an area with overall
illumination.
• Also known as “ambient” lighting,
• General lighting radiates a comfortable level of
brightness, enabling one to see and walk.
• General lighting is typically seen as the starting point
for lighting a space or a room

12/20/2018 Biniyam.Z 97
 Cont’d
Task Lighting
• Task lighting, or directional lighting, is aimed at a
specific task.
• It is a way to provide more light on a specific area to
perform a task that requires more light than the
ambient fixtures can give.
• Desk lamps, ceiling pendant fixtures, and appliance
lights are all good examples

12/20/2018 Biniyam.Z 98
 Cont’d
Accent lighting
• Accent lighting enhances decorative or architectural
features by selectively illuminating such features as
fireplaces, paintings, sculpture, plants, or aquariums.
• Accent lighting is also a sort of a directional lighting
that adds drama to a room by creating visual interest.
• Accent lighting requires at least three times as much
light on the focal point as the general lighting.

12/20/2018 Biniyam.Z 99
 Cont’d
Decorative lighting
• Light strips, pendants, chandeliers, and sconces are
all examples.
• Light fixtures that draw attention to themselves and
add character to the room being lighted.

12/20/2018 Biniyam.Z 100

 Cont’d
Luminaires and Lighting Systems
• A luminaire is a device forming a complete
lighting unit
• It comprises of a light source and electric
operating devices (transformer, ballast, ignitor,
etc.).
• It also includes the parts for positioning and
protecting the lamp/s (casing, holder, wiring)
• The function of luminaire is
 To direct light to desired locations,
 Creating the required visual environment without
12/20/2018
causing glare or discomfort.
Biniyam.Z 101
 Cont’d
• Choosing luminaires that efficiently provide
appropriate luminance patterns for the application
is an important part of energy efficient lighting
design.
• Luminaries are characterized by the manner in
which light is distributed

12/20/2018 Biniyam.Z 102

 Cont’d
1. Direct light: - the most commonly used type of
lighting scheme.
• In this scheme more than 90% of total light flux is
made to fall directly on the working plane.
• Though it is more efficient but causes hard shadows
and glare.
• It is mainly used for industrial and general out-door
lighting.

12/20/2018 Biniyam.Z 103

 Cont’d
2. Semi-direct lighting: - in this lighting scheme 60-
90% of the total light flux is made to fall down wards
directly with the help of semi-direct reflectors.
• The remaining light is used to illuminate the ceiling
and walls.
• Such a lighting scheme is best suited to rooms with
high ceiling where there is a high level of uniformity
plane.

12/20/2018 Biniyam.Z 104

 Cont’d
3. General diffuse lighting: - in this scheme lamps
made of diffusing glass are used, which gives nearly
equal illumination in all directions.
• When light fittings are completely enclosed or
concealed, as with some globes and ceiling panels,
the light is diffused since it passes through the glass
or plastic.
• Diffused lighting is also glare- free and produces a
flat appearance.

12/20/2018 Biniyam.Z 105

 Cont’d
4. Semi indirect lighting: -
• In this scheme 60-90% of total light flux is thrown up
wards to the ceiling for reflection and the rest reaches
the working plane
• This lighting scheme has soft shadows and is glare
free.
• It is mainly used for indoor decoration purpose.

12/20/2018 Biniyam.Z 106

 Cont’d
5. Indirect lighting scheme: - in this scheme more than
90% of total light flux is thrown upwards the ceiling for
diffuse reflection by using inverted or bowl reflector.
• In this scheme the glare is reduce to minimum.

12/20/2018 Biniyam.Z 107

 Cont’d
• Asymmetric luminaires are usually designed
for special applications.
• Asymmetric up lights, for instance, are indirect
luminaires with a stronger distribution in one
direction
• Adjustable luminaires are generally direct
luminaires that can be adjusted to throw light
in directions other than down.
• These include track lights, floodlights, and
accent lights.
12/20/2018 Biniyam.Z 108
 Cont’d

12/20/2018 Biniyam.Z 109

 Cont’d
STYLES OF LUMINAIRE
• Some common luminaires styles are the
following.
Downlights
• Downlights are often called cans or top hats.
• A type of direct luminaire, they are usually
round and recessed in the ceiling.
• Their principal use is general illumination in a
wide range of residential and commercial
applications
12/20/2018 Biniyam.Z 110
 Cont’d
• A luminaire consisting of a lamp set in metal
cylinder, recessed into or mounted on a ceiling
to direct a beam of light downward.

12/20/2018 Biniyam.Z 111

 Cont’d
Troffers
• Troffers are widely used in offices, stores,
schools, and other commercial and institutional
facilities for general lighting in work and sales
areas.
• Troffers are the most common type of
fluorescent luminaire.
o Lensed troffers use a plastic lens to refract
light and distribute it within the desired area
below. The lens serves to cut off light
distribution to minimize glare.
12/20/2018 Biniyam.Z 112
 Cont’d
o Parabolic troffers use parabolically shaped
aluminum or plastic louvers to shield the lamp
for improved visual comfort.
o Recessed indirect troffers are open, and light
from fluorescent lamps is bounced off the
interior box of the troffer. Like other troffers,
these are actually direct luminaires.

12/20/2018 Biniyam.Z 113

 Cont’d

12/20/2018 Biniyam.Z 114

 Cont’d
Architectural Lighting Fixtures
• Architectural lighting fixtures are fixtures that are
not decorative but rather functional and not
readily noticeable. They are used to illuminate
architectural shapes and forms.
Wall washers

12/20/2018 Biniyam.Z 115

 Cont’d
Accent Fixtures
• Accent fixtures allow light to focus on art and
building surfaces.
o Recessed accent lights appear as down lights but
internally permit rotation and elevation of the
light beam.
o Eyeballs and pull-down accents resemble down
lights, but their appearance belies the ability to be
adjusted.

12/20/2018 Biniyam.Z 116

 Cont’d
• Track lighting systems are designed to accent art and
retail displays; for ease and flexibility of use, lamp
holders can be relocated to any point on the track.

12/20/2018 Biniyam.Z 117

 Cont’d
Cove Lights
• This consists of placing continuous series of
fluorescent tubes in a groove along one or more walls
of a room, about 12 inches from the ceiling.
• The light reflects off the ceiling and bathes the room
in indirect light.

12/20/2018 Biniyam.Z 118

 Cont’d
Decorative Lighting
• Chandeliers
• Pendants
• Sconces
• Torchiers
• Lanterns

12/20/2018 Biniyam.Z 119

 Cont’d
Soffit Lighting
• This refers to built in light source under a panel.
• It may be fixed to a ceiling or under a cabinet.
• Soffit lighting is often used over a sink or other work
areas.

12/20/2018 Biniyam.Z 120

 Cont’d
Task Lights
Ceiling Drums
• Ceiling drums are round or square luminaires
that mount to the ceiling surface.
• They typically are used as corridor lights, rest
room lights

12/20/2018 Biniyam.Z 121

 Cont’d
Vanity Lights
• The vanity light is a task light for bath vanity
and mirror areas.
• It can mount horizontally above the mirror or
vertically to the sides.

12/20/2018 Biniyam.Z 122

 1.4. Design of lighting schemes
• The lighting scheme should be Provide:
i. Adequate illumination
ii. Light distribution all over the working
plane as uniform as possible
iii. Suitable color of light
iv. Avoid glare and hard shadows as far as
possible

12/20/2018 Biniyam.Z 123

 Cont’d
1. Illumination level: - In order to see the details
of the things that surround us the source has to
illuminate them very well.
• For various tasks or activities and the size and
characteristics of the room in which they will
be carried out, there is a range of illumination
levels established in terms of quality and
quantity.
• Codes for Interior Lighting Design gives
recommended maintained illuminances for a
wide variety of installations.
12/20/2018 Biniyam.Z 124
 Cont’d
• The level of illuminance required depends on the
following factors:
1. The importance of the visual task and the
consequences of errors.
2. The nature of the task/activity and its dependence
on high illumination
3. Requirements for precision or accuracy in the
task activity (The difficulty of the visual task. )
4. The duration for which the task is undertaken.
5. The age of the person(s) performing the
12/20/2018
task/activity (The eyesight
Biniyam.Z
of the user ) 125
 Cont’d
• This recommended illuminance must be
maintained throughout the life of the
installation and must take account of the
reduction of light reaching the working plane
because of lamp ageing, dust collection and
deterioration of the decor.
• The design illuminance (maintained
illuminance) is taken as the illuminance at the
end of the maintenance period (typically 2
years).
12/20/2018 Biniyam.Z 126
 Cont’d
2. Uniformity of Illumination: It has been
found that visual performance is best if the range
of brightness within the field of vision is not
greater than 3:1, which can be achieved by
employing general lighting in addition to
localized lighting.
• Otherwise due to the frequent accommodation
of pupil or iris of the eye, fatigue is caused and
may cause discomfort and affect visual
performance.
12/20/2018 Biniyam.Z 127
 Cont’d
3. Quality (Color) of light: The appearance of
the body color entirely depends upon the color of
the incident light.
• In general the composition of the light should
be such that the color appears natural.
4. Shadows: In lighting installation, formation
of long and hard shadows causes fatigue and are
undesirable.
• However a certain amount of shadow is
desired as it helps to give shapes to solid
objects and make them easily recognized.
12/20/2018 Biniyam.Z 128
 Cont’d
5. Glare
• Glare is defined as a condition in which vision is
affected by an excessive luminance and/or
excessive differences in luminance in the visual
field (i.e; parts of this field are excessively
bright).
• Glare occurs whenever one part of an interior in
the field of vision is much brighter than the
general interior.
• Care must be taken by lighting designers to
ensure that bright light sources are not placed in
the field of vision.
12/20/2018 Biniyam.Z 129
 Cont’d
• The source can be direct as in the case of a
luminaire or window or indirect, reflected
from a working surface such as a display
screen and highly polished surfaces. Glare can
also be caused by excessive contrast such as a
dark background coupled with a bright
foreground.
• Glare from luminaires is reduced by
decreasing the luminance of the source. With
opal and prismatic diffusers the source
luminance is reduced by increasing the area of
the source.
12/20/2018 Biniyam.Z 130
 Cont’d
• Special low glare luminaires utilize louvres to
shield the lamp from direct view.
• Glare can be subdivided into disability and
discomfort glare.
• Disability Glare, defined as glare causing an
actual physiological reduction in visual capability
(impaired, perception of contrasts and forms),
• Disability Glare occurs when vision is actually
impaired. It can be the cause of accidents and a
serious reduction of visual performance. This is a
more extreme form of glare and is most likely to
occur when there is an area close to the line of
sight which has a very high luminance.
12/20/2018 Biniyam.Z 131
 Cont’d
• The most common causes of glare indoors are
windows and electric light sources which are
seen either directly (direct glare) or indirectly
by reflection (reflected glare).
• Discomfort Glare, defined as glare considered
solely in terms of its psychological effect on
the occupant.
• Discomfort Glare often occurs, after
continuous exposure to high background
contrast or high source luminance either
directly or indirectly.
12/20/2018 Biniyam.Z 132
 Cont’d
• Discomfort glare experienced by the occupant
of a room increases with time and leads to
premature fatigue and a reduction in
performance, performance attitude, and well-
being. It can cause eye fatigue and headaches.
• Glare Index of discomfort : Codes for interior
lighting uses a glare index system and provides
limiting glare indices for most building
interiors.
• This is a numerical index which may be
calculated for lighting schemes.
12/20/2018 Biniyam.Z 133
 Cont’d
• In fulfilling the above requirements, in designing
a good lighting scheme, we have to consider
i. The intensity of illumination required
ii. The selection of the required lamps and
fittings
iii. The size of the room
iv. The conditions under which the
illumination is used etc.
• The recommended illumination level for various
occupancies can be found from the Ethiopian
Building Code Standard, Electrical Installation of
Buildings (EBCS-10).
12/20/2018 Biniyam.Z 134
 Cont’d

12/20/2018 Biniyam.Z 135

 Cont’d
• The following are some of the conditions that
should be considered when the illuminations
are used:
1. Maintenance Factor (MF)
• Maintenance Factor (MF) is the term used to
take account of the reduction in illuminance
over the maintenance period due to:
i. Reduced reflectance's due to the accumulation
of dirt and dust on room surfaces. Room
Surface Maintenance Factor (RSMF).
12/20/2018 Biniyam.Z 136
 Cont’d
ii. Reduced light output from the luminaire due to
the accumulation of dirt and dust on the
luminaire. Luminaire Maintenance Factor (LMF )
iii. Reduced light output due to lamp ageing. Lamp
Lumen Maintenance Factor.(LLMF)
iv. Reduced light output due to lamps failing.
Manufacturer data will give the percentage lamp
failures for a specific number of hours operation.
The Lamp Survival Factor (LSF) will be 1 if spot
lamp replacement is carried out
MF = RSMF x LMF x LLMF x LSF
12/20/2018 Biniyam.Z 137
 Cont’d
Example. Calculate the maintenance
factor for an installation where the
LLMF, LMF and RSMF are as shown
in the figure. The luminaires are cleaned
after 3000 hours, the lamps are replaced
after 6000 hours and room surfaces are
cleaned after 6000 hours. Spot
replacement of failed lamps is also
carried out.
MF = RSMF x LMF x LLMF x LSF
Maintenance factor at 6000 hrs = 0.9 x
0.75 x 0.8 x 1 = 0.54
12/20/2018 Biniyam.Z 138
 Cont’d
2. Utilization Factor/Coefficient of Utilization
• The whole light radiated by the lamps doesn’t
reach the working plane. The ratio of lumens
reaching the working plane to the total light
given out by the lamp or lamps, when the
installation is new, is known as utilization
factor or coefficient of utilization.
• Utilization factor takes account of the loss of
light due to absorption on room surfaces.
• It depends on three factors:
12/20/2018 Biniyam.Z 139
 Cont’d
1. Type of Luminaire A
luminaire with a concentrated
light output directed on the
working plane will have a higher
UF than a luminaire with a
dispersed light output.
2. Room index. This takes
account of the length (L) and
width (W) of the room and the
height of the luminaires above
the working plane (Hm).
12/20/2018 Biniyam.Z 140
 Cont’d
3. Reflectance's of Room Surfaces. Bright
colors with high reflectance's result in a higher
UF.

• A high utilization factor will mean fewer

lamps are needed resulting in a more efficient
energy usage and a lower capital cost.

12/20/2018 Biniyam.Z 141

 Cont’d
• To determine the
Utilization Factor:
1. Obtain reflectance
factors for room surfaces
from the architect or
interior designer.
2. Acquire manufacturer's
data for luminaire selected.
3. Calculate room index.
4. Evaluate utilization
factor from manufacturer's
data.
12/20/2018 Biniyam.Z 142
 Cont’d
• The formula for obtaining the room index is:

• Typical Manufacturer's data for a typical twin

tube fluorescent luminaire used to calculate
Utilization Factors.
12/20/2018 Biniyam.Z 143
 Cont’d

12/20/2018 Biniyam.Z 144

 Cont’d
Example
• Calculate the Utilization Factor for a room
with the following dimensions: Length 8m;
Width 6m; Height 3m; height of working plane
0.8m. The room reflectance’s are Ceiling 0.5;
Walls 0.3 and Floor 0.2.

• From the Table the Utilization factor can be

read as 0.63
12/20/2018 Biniyam.Z 145
 Cont’d
Spacing Luminaries:
• Correct spacing is of great importance to
provide uniform illumination over the whole
area.
• Too wide a spacing will result in a serious
falling off in illuminance between the
luminaires.
• The illuminance between the luminaires must
not be allowed to fall below 70% of the value
directly below the fitting.
12/20/2018 Biniyam.Z 146
 Cont’d
• The ratio of the horizontal spacing between
rows to the height of the luminaries above the
working plane, called space to height ratio.
This is S/Hm where S is the distance between
luminaires (fittings) and Hm their mounting
height above the working plane.
• Manufacturers will specify a recommended
SHR for each of their luminaires.
• Ensuring that luminaires are spaced within the
recommended value will mean an acceptable
variation in illuminance across the working
plane.
12/20/2018 Biniyam.Z 147
 Cont’d
• The recommended spacing to mounting-height
ratio (SHRMAX) must not be exceeded.
• For most installations a spacing to mounting
height ratio of 1 : 1 to 2 : 1 above the working
surface is usually considered adequate and the
working surface is normally taken as 0.85 m
above the floor level.
• To maintain an even distribution of
illuminance from the luminaires, those
adjacent to the walls of the room should be
fixed at half the spacing distance
12/20/2018 Biniyam.Z 148
 Cont’d

12/20/2018 Biniyam.Z 149

 Cont’d
• Mounting height is largely governed by the
type of the building and type of lighting
scheme employed.
• The correct mounting height of luminaires is
important since glare may result if fittings are
placed in the line of vision.
• In practice, the number and spacing of the
luminaires are often determined by the ceiling
height, the beam structure, or modular size of a
suspended ceiling
12/20/2018 Biniyam.Z 150
 Cont’d
Example 1. A factory area is 40m long, 20m
wide and is 8m high. Point source luminaires are
suspended 1.5 meters below ceiling level. The
working plane is 1 meter high. Calculate the
minimum number of luminaires which must be
installed to conform with a recommended SHR
of 1.5 : 1.

12/20/2018 Biniyam.Z 151

 Cont’d

• This means that the minimum number to

conform with SHR requirement is 3 rows with
5 luminaires per row.
12/20/2018 Biniyam.Z 152
 Cont’d
• More than this number can be used if desired
for reasons such as balance, effect, control or
ease of installation.
• Assuming that three rows of five luminaires is
suitable, the actual spacing is determined as
follows:

12/20/2018 Biniyam.Z 153

 Cont’d
• Note: If work is to be carried out at the
perimeter of the room, a spacing of 0.33 S to
the wall may be used.
For Linear Luminaires
• The relevant spacing maximum transverse and
axial spacing data will be supplied by the
manufacturer.
• The spacing is usually taken between centers.
• (Note: the maximum recommended transverse
SHR is usually different from the axial SHR
where linear luminaires are used).
12/20/2018 Biniyam.Z 154
 Cont’d
• Where high levels of illuminance are required,
it is common practice to use continuous rows
of luminaires with the transverse spacing at the
maximum permissible.
• In this way, installation costs will be kept to a
minimum, particularly where luminaires are
suspended below the ceiling.

12/20/2018 Biniyam.Z 155

 1.5. Method of lighting calculation
• In order to estimate the number and the type of
light fittings for a given area. It is necessary to
know
 What level of luminance is required,
 The area to be illuminated,
 The maintenance factor and the coefficient of
utilization, and
 The efficiency of the lamps to be used.
• The most common methods are the following:
a. Watts per square foot method
b. Lumen method
12/20/2018 Biniyam.Z 156
 Cont’d
A) Watts-per-Square-Foot Method
• Applicable for rough calculations.
• It consists in making an allowance of watts per
square foot of area to be illuminated.
• According to NEC 220- standard illumination
is about 3 watt per ft2.

12/20/2018 Biniyam.Z 157

 Cont’d
Example: - A house has an external dimension of
30ft by 50ft. If an 80w fluorescent lamps, and 220V
supply is used, determine the size of the service
wire and the number of lamps required.
Solution:
A = 30ft X 50ft = 1500ft2
Total wattage required = 1500ft2 X 3w/ft2
= 4500w = 4.5kw
No of lamps required = 4.5Kw/80w = 56.1
= 56 lamps – each 80w
12/20/2018 Biniyam.Z 158
 Cont’d
B) Lumen Method
• This method is most suitable for interior
lighting design, where a high proportion of
light on the working plane is reflected by
internal surfaces.
• The lumen method, sometimes called the
luminous flux method of calculation.
• It is used to calculate the average illuminance
on working planes, or to calculate the number
of luminaires required to provide a specified
average illuminance in rooms.
12/20/2018 Biniyam.Z 159
 Cont’d
The formula used is expressed as:

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Example 1.
• An office area measures 16m x 8m and is 2.7
meters high. It is to be illuminated to an average
value of 500 lux. 600mm x 600mm recessed
luminaires, each containing 4 lamps are used.
Each lamp has an output of 1400 lumens.
Utilization factor is 0.5 and maintenance factor is
0.75. Assume that Desk height is 0.7m and
SH ratio = 1.5 : 1
i. Calculate the number of luminaires required.
ii. Sketch a layout of the scheme indicating the
spacing between luminaires.
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Solution:

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• Desk height 0.7m therefore Hm = 2 m
• SH ratio = 1.5 : 1, Therefore max spacing s =
3 meters
• Min. no of rows = 8/3=2.7 (i.e. 3) i.e 30/3 =
10
• 3 rows of 10 would give a spacing of 1.6m
between centers.
• An alternative layout would be 4 rows of 8
luminaires.
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• 4 rows of 8 luminaires would be preferable as

they would give a square layout with identical
spacing.
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Example 2. An office area measures 30m x 15m.
The ceiling to desk height is 2m. The area is to be
illuminated to a general level of 500 lux using twin
lamp 32 watt luminaires with a SHR of 1.25. Each
lamp has an initial output of 85 lumens per watt.
From the manufacturer data, lamp lumen
maintenance factor (LLMF) = 0.87, luminaire
maintenance factor (LMF) = 0.81 , room surface
maintenance factor (RSMF) = 0.95 and lamp
survival factor (LSF) = 0.95. Assuming that a
bright interior with room reflectance's 70% ceiling,
50% walls and 20% floor.
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a. Find the utilization factor

b. Find the maintenance factor
c. Calculate the number of luminaires required
and design a suitable lighting scheme
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Solution:
a. For room reflectance’s 70% ceiling, 50% walls
and 20% floor. The top row of the table applies.

from the table, U F = 0.69

b. Maintenance Factor (M.F.) = LLMF x LMF x
RSMF x LSF
= 0.87 x 0.81 x 0.95 x 0.95 = 0.636
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Exercises
1. A factory measures 50m x 30m x 6m high. A
general lighting scheme is to illuminate the
whole area to 500 lux maintained illuminance
using 1000 watt metal halide lamps with an
initial efficacy of 90 lumens per watt.
Maintenance factor is 0.6 and utilization factor is
0.5. A space height ration of 1.5 : 1 is
recommended for the luminaire chosen and a
mounting height of 5m over working plane is
assumed. Design a suitable lighting scheme.
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2. Two light sources each having a uniform
intensity of 64,000 cd is mounted 8m high. If
the illumination midway between the lamps on
ground level is same as the illumination level
produced by one of the lamp vertically below it,
calculate the distance between the poles.

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 Question?

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