Nature of light
Light is a form of electromagnetic energy radiated from a body and human eye is
capable of receiving it. Light is a prime factor in the human life as all activities of human
being ultimately depend upon the light.
TERMS USED IN ILLUMINATION
The following terms are generally used in illumination.
Color
The energy radiation of the heated body is monochromatic, i.e. the radiation of
only one wavelength emits specific color. The wavelength of visible light lies between
UNIT 1
4,000 and 7,500 Å. The color of the radiation corresponding to the wavelength is shown
in Fig. 6.1.
Light:
It is defined as the radiant energy from a hot body that produces the visual
sensation upon the human eye. It is expressed in lumen-hours and it analogous to watthours, which denoted by the symbol ‘Q’.
Luminous flux:
It is defined as the energy in the form of light waves radiated per
second from a luminous body. It is represented by the symbol ‘φ’ and measured in
lumens.
Ex: Suppose the luminous body is an incandescent lamp.
The total electrical power input to the lamp is not converted to luminous flux, some of
the power lost through conduction, convection, and radiation, etc. Afraction of the
remaining radiant flux is in the form of light waves lies in between the visual range of
wavelength, i.e. between 4,000 and 7,000 Å, as shown in Fig
Solid angle
Solid angle is the angle subtended at a point in space by an area, i.e., the angle enclosed
in the volume formed by numerous lines lying on the surface and meeting at the point
(Fig. 6.5). It is usually denoted by symbol ‘ω’ and is measured in steradian.
Luminous intensity
Luminous intensity in a given direction is defined as the luminous flux emitted by the
source per unit solid angle
Luminous flux emitting from the source
It is denoted by the symbol ‘I’ and is usually measured in ‘candela’.
Let ‘F’ be the luminous flux crossing a spherical segment of solid angle ‘ω’. Then
luminous intensity I =d@/dt lumen/steradian or candela.
Lumen:
It is the unit of luminous flux.
It is defined as the luminous flux emitted by a source of one candle power per unit solid
angle in all directions.
Lumen = candle power of source × solid angle.
Lumen = CP × ω
Total flux emitted by a source of one candle power is 4π lumens.
Candle power (CP)
The CP of a source is defined as the total luminous flux lines emitted by that source in a
unit solid angle.
Illumination
Illumination is defined as the luminous flux received by the surface per unit area.
It is usually denoted by the symbol ‘E’ and is measured in lux or lumen/m2 or meter
candle or foot candle.
Lux or meter candle
It is defined as the illumination of the inside of a sphere of radius 1 m and a source of 1
CP is fitted at the center of sphere.
Brightness
Brightness of any surface is defined as the luminous intensity pen unit surface area of
the projected surface in the given direction. It is usually denoted by symbol ‘L’.
If the luminous intensity of source be ‘I’ candela on an area A, then the projected area
is Acos θ.
Mean horizontal candle power (MHCP)
MHCP is defined as the mean of the candle power of source in all directions in
horizontal plane.
Mean spherical candle power (MSCP)
MSCP is defined as the mean of the candle power of source in all directions in all planes.
Mean hemispherical candle power (MHSCP)
MHSCP is defined as the mean of the candle power of source in all directions above or
below the horizontal plane.
Reduction factor
Reduction factor of the source of light is defined as the ratio of its mean spherical candle
power to its mean horizontal candle power.
Lamp efficiency
It is defined as the ratio of the total luminous flux emitting from the source to its
electrical power input in watts.
It is expressed in lumen/W
Specific consumption
It is defined as the ratio of electric power input to its average candle power.
Space to height ratio
It is defined as ratio of horizontal distance between adjacent lamps to the height of their
mountings.
Coefficient of utilization or utilization factor
It is defined as the ratio of total number of lumens reaching the working plane to the
total number of lumens emitting from source.
Maintenance factor
It is defined as the ratio of illumination under normal working conditions to the
illumination when everything is clean.
Depreciation factor
It is defined as the ratio of initial illumination to the ultimate maintained illumination
on the working plane.
Waste light factor
When a surface is illuminated by several numbers of the sources of light, there is certain
amount of wastage due to overlapping of light waves; the wastage of light is taken into
account depending upon the type of area to be illuminated. Its value for rectangular area
is 1.2 and for irregular area is 1.5 and objects such as statues, monuments, etc.
Absorption factor
Normally, when the atmosphere is full of smoke and fumes, there is a possibility of
absorption of light. Hence, the total lumens available after absorption to the total
lumens emitted by the lamp are known as absorption factor.
Beam factor
It is defined as the ratio of ‘lumens in the beam of a projector to the lumens given out by
lamps’. Its value is usually varies from 0.3 to 0.6. This factor is taken into account for
the absorption of light by reflector and front glass of the projector lamp.
LAWS OF ILLUMINATION
Mainly there are two laws of illumination.
1. Inverse square law.
2. Lambert's cosine law.
Inverse square law
This law states that ‘the illumination of a surface is inversely proportional to the square
of distance between the surface and a point source’.
Proof:
Let, ‘S’ be a point source of luminous intensity ‘I’ candela, the luminous flux emitting
from source crossing the three parallel plates having areas A1 A2, and A3 square meters,
which are separated by a distances of d, 2d, and 3d from the point source respectively as
shown in Fig.
Inverse square law
Luminous flux reaching the area A1 = luminous intensity × solid angle
∴ Illumination 'E1' on the surface area 'A1' is:
Similarly, illumination 'E2' on the surface area A2 is:
and illumination ‘E3’ on the surface area A3 is:
2. Lambert’s Cosine Law:
Very often the illuminated surface is not normal to the direction of light as AC in Fig. but is inclined as AB. The area over which the light is spread is then increased in the ratio-
According to this law the illumination at any point on a surface is proportional to the cosine of the angle between the normal at that point and the direction of luminous flux.
