Unit I : Fundamentals of Illumination

What is Illumination?
In general illumination means light. But technically light and
illumination are two different things.

Light is just a cause of light i.e. source of light. While

Illumination is the effect of light at a surface.

Need of Illumination?
1. Human beings are depend on light for all activities. 2.
Natural source of light that is sun is available only during
daytime.
3. We can not restrict our activities only for daytime.
4. So the concept of Illumination comes in the picture.

Light:
" Light is the medium of Illumination that makes sight possible."

OR
" Light is defined as radiant energy which produces
visual sensation upon human eye."

★ Unit of light is lumen hour.

Luminous Flux:-
" It is defined as total light energy radiated per second from a
light source. "

It is denoted by ' φ '

 Illumination
" When the light falls upon any surface the phenomenon is called
as illumination. "

Illumination is denoted by ' E '

E=φ
Α

Where,
φ = Luminous Flux
A = Area

Unit of illumination is lumen / m2 or 'Lux '

Solid angle

" When the lines lines are drawn from the boundary of a portion
such that lines met at the centre and form a cone, then angle formed
by conical surface is called as solid angle. "

★ Solid angle is denoted by ω.

★ Unit of solid angle is steracian.
Brightness or Luminance :-
" It is defined as the ratio of luminous intensity per unit area. "
Luminance = luminous intensity
Area
Unit of Luminance is Lux / m2

Lumen :-
" It is defined as luminous flux emitted per unit solid angle. "
 Lumen = luminous flux
Solid angle

Candle Power :-
" It is the light radiating capacity of the source. "

" It is defined as the number of luminous flux given out by the

source in a unit solid angle. "

Candle Power = lumens

Solid angle

Luminous Efficiency / Lamp Efficiency

" It is defined as the ratio of luminous flux to the wattage of
light source. "

Luminous Efficiency = luminous flux emitted by source

Wattage of light source
● Unit of luminous efficiency is Lumen / watt

Space height ratio :-

" Space height ratio is defined as the ratio of space between lamp
to the height od lamp above the working plane. "

Space height ratio = space height ratio .

Height of lamp above working plane
 Utilization Factor

" It is defined as the ratio of total lumen reaching at working

plane to the total lumen given out by lamp. "

★ The direct lightening utilization factor is 0.25 to 0.5.

★ For indirect lightning utilization factor is 0.1 to 0.3.

Maintainance Factor
" Maintainance factor is defined as ratio of illumination under normal
working condition to the illumination when everything is clean. "

Maintainance factor = Illumination under normal

condition Illumination under
perfect condition

● The ratio of maintainance factor should be less than 1

(MF<1) Depreciation Factor

" It is reciprocal of maintenance factor. "

" It is defined as illumination when everything is clean to

illumination under normal working condition. "

Depreciation Factor = Illumination when everything is clean

Illumination under normal working
condition

Laws of Illumination

There are two laws of illumination

1. Inverse square law.
2. Lamberts cousine law.
 Inverse square law

Where,
S = Point Source of light
A1, A2, A3 = Area of parallel spaces
D1, D2, D3 = distance of parallel surface from point source

Inverse square law states that,“ The illumination of a surface is

inversely proportional to the square of distance between parallel
surface and point source of light."

E=1
d2

Lamberts cosine law

Lamberts cosine law states that,“ Illumination of surface is directly

proportional to the cosine of angle between normal and incident flux."
 EB = EA × cosθ1
EC = EA × cosθ2
ED = EA × cosθ3

Polar Curves
“Polar curve is a graphical representation of light intensity with
respect to angular position in horizontal and vertical plane."

Types of Polar Curve

1. Horizontal Polar Curve
2. Vertical Polar curve

Horizontal Polar Curve

If a candle power is measured at angular position in a
horizontal plane, a horizontal polar Curve is obtained.

Vertical Polar Curve

If a candle power is measured at angular position in vertical
plane, a vertical polar curve is obtained.

Applications of Polar curve

1. To know the intensity of light emitted in diffrent direction. 2.

 To determine the mean horizontal candle power (MHCP) and
mean hemispherical candle power (MHSCP)
3. Polar curves are required to calculate number of lamp in
illumination design.
4. It indicates the coverage of light which helps in determining the
lightning scheme.

Mean Horizontal Candle Power (MHCP)

Mean horizontal candle power (MHCP) is the average (mean)

of candle power in all direction in a horizontal plane (x- axis ) which
passes through the centre of the source.

Mean Spherical Candle Power (MSCP)

Mean Spherical candle power (MSCP) is defined as the average

of candle power in all planes (X- plane, Y-plane, Z-plane ) which
passes through the light.

Total MSCP of light = Total lumen given out by lamp.

4π
Mean Hemispherical Candle Power (MHSCP)

Mean hemispherical candle power (MHSCP) is defined as the

average (mean) of candle power in all direction within the hemisphere
either above the horizontal plane or below the horizontal plane.

Total MHSCP = Luminous flux emitted in hemisphere

2π

Reduction Factor
The ratio of mean spherical candle power (MSCP) to mean
horizontal candle power (MHCP) is called as reduction factor.

Reduction Factor = MSCP

MHCP

Formulae
Total MSCP of lamp = total lumen given out by lamp
4π
E = Total MSCP
Wattage of lamp

Lamp efficiency = Total lumen

Wattage of lamp
Numericals

1. A 230 volt lamp has a total flux of 2500 lumens and takes a
current of 0.7A , calculate,
a) Lumen per watt
b) MSCP per watt

Given : Total Voltage= 230v

Total flux = 2500 lumen
Current = 0.7 Ampere

Power = V × I
= 230 × 0.7
Power = 168 watt

Lumen per watt = lumen

Watt
 = 2500
161
Lumen per watt = 15.327

Total MSCP = total lumen given out by lamp on working plane

4π
= 2500
4π
Total MSCP = 198.943

MSCP per watt = Total MSCP

Wattage
= 198.943
168
MSCP per watt = 1.235

2. A lamp of 500 watt having MSCP of 1000 is suspended 2.7

meter above the working plane. Calculate
1) Illumination directly under the lamp at working plane.
2) Lamp efficiency.

Givrn:- = P = 500 watt,

MSCP = 1000
Distance = 2.7m.

Illumination = total MSCP

Distance2
= 1000
(2.7)2
Illumination = 137.17 lux

Lamp efficiency = total lumen

Watt
Total MSCP of lamp = Total lumen
 4π
Total lumen = total MSCP × 4π
= 1000 × 4π
Total lumen = 12566.370

Lamp efficiency = total lumen

Watts
= 12566.370
500
Lamp efficiency = 25.132

Calculation Method
Need of calculation method
● Lightening calculation helps us to identify most appropriate
lightening design.

● So to get proper lightening design and proper illumination we

need lightning calculation method.

1. Watt / m2 method
● It consists in making an allowance of watt per square metre of
area to be illuminated desired on the assumption of an average
figure of overall efficiency of the system.

2. Lumen or light flux method

● This method os applicable to those causes where the resources
of light are such as to produce an approximate uniform
illumination over the working plane or where an average value is
required.
Steps :-
1. From the size of lamp and from their efficiency total lumen output
are determined.

2. Multiplying the total lumen output from the source of coefficient of

utilization, the lumens received on the working plane are
determined.
 3. If the lamps and surroundings are not perfectly clean then the
determination of lumens received on working plane , the
depreciation factor or maintainance factor should be included.

total lumens received on working plane =

No. Of lamps×wattage of lamp×efficiency of each lamp ×coefficient
of
Utilization
Depreciation Factor

3. Point to point method

This method is applicable where the illumination at a point due to
one or more sources of light are required.
Point to point method is also known as inverse square method.
Steps :-
1. If the polar curve of lamp and its reflector giving candle power of
the lamp in diffrent direction is known.

2. The illumination at any point within the range of the lamp can be
calculated from Inverse square law.

3. If two and more than two lamps are illuminating the same
working plane, the illumination due to each can be calculated
and added.

Features Of Illumination
1. Economy
The cost of design of illumination should be low.

2. Life
The life of illumination scheme should be more.

3. Comfort
The illumination scheme should be comfortable to everyone.

4. Less Flicker
Flicker should be always for any type of illumination scheme.

5. Apperance
 The appearance of illumination scheme should be good.

6. Maintainance
For any type of illumination scheme maintenance and repairing
should be less.

7. Less Glare
The illumination scheme should not produce any type of Glare.

Advantages of Good Illumination

1. Good illumination scheme encourage person for better work.
2. Good illumination scheme increases overall efficiency. 3.
Correct and proper illumination scheme avoid accident. 4. Good
illumination scheme provide pleasant atmosphere. 5. By proper
illumination scheme, energy saving will be efficient.
Photometry
Meaning of word Photometry

Photo - ligjt
Metry - measurement

“ the science of measurement of light is known as photometry."

It also measures the human visual response to light.

● The human eye reacts differently to the diffrent wavelength of

visible light.

● The photometry depends on phenomenon of how sensitive

human eye is for a wavelength.

● A standard measure of a response of the human eye to light of

diffrent wavelength is known as luminosity function.

Main physical quantities of photometry are

1. The luminous intensity of light.
2. Luminous flux.
3. Illumination of surface.

Uses of photometry
1. Photometry is often used in the study of liquids and solution in
chemistry.

2. Photometer can help to measure masses of organic or inorganic

materials in a solution or liquid.

3. In astronomy, photometry is utilized by applying filters to restrict

certain wavelengths and allowing other desired wavelength
through to measure.

4. Photometer plays an important role in allowing scientists to

observe and capture images of celestial bodies.