Principle of Lighting

1. Vijay Balu Raskar (BE Electrical)

2. Vijay Balu Raskar (BE Electrical)

3. MAIN OBJECTS of Street Lighting
Vijay Balu Raskar (BE Electrical)

4. There are no walls and ceiling which reflect or diffuse light.
Hence, Direct lighting scheme can be employed.
Hard Shadows and high contrast cannot be avoided.
Illumination to be very large than indoor lighting.
High level of illumination is not necessary.
For low illumination, the eye is in its most sensitive state, so glare
must be avoided.
Principle of street lighting is different from indoor lighting.
High pressure sodium (HPS) lighting is being used in most of the
City. It is five times as efficient as incandescent lighting, and twice as
efficient as mercury vapor lighting.
HPS lights have orange-pink lamp color, and allow you to see all
colors of objects.
They do not attract flying insects the way other lamps do.
HPS lighting is installed to replace incandescent and mercury vapor
lighting to save
energy and reduce operating cost.(BE Electrical)
Vijay Balu Raskar

5. “When light falls on a surface, depending upon the nature of the
surface, some portion of light energy is reflected, some portion is
transmitted through medium of the surface and the rest is absorbed.”
PRINICIPLE OF STREET LIGHTING:-
There are two general principle:-
1) Diffusion Principle and
2) The specular reflection principle.
Types of Lamps For street Lighting:-
Mercury vapour and Sodium Discharge Lamps are used.
They having particular advantages for street Lighting.
They are lower power consumption.
Discharge lamps are colour and monochromatic nature. But,
no matter for street lighting.
Vijay Balu Raskar (BE Electrical)

6. 1. Diffusion Principle of Street Lighting
Example:-
Fig a Fig b
What Is Diffused Light?
ANS:-Diffused light is a soft light with neither the intensity nor the glare of direct light. It is
scattered and comes from all directions. Thus, it seems to wrap around objects. It is softer
and does not cast harsh shadows.
Vijay Balu Raskar (BE Electrical)

7. Explanation:-
Reflectors are used in this principle.
They are designed to spread light uniformly and downwards.
Reflectors made to have cut-off of between 30° to 45°.
Filament is not visible except from underneath.
The illumination at point is calculated by Inverse Square Law
method.
 Due to diffusion nature, the road surface appears bright to
the observer.
Reflected light scattered in all directions.
The viewed see the illuminated surface not the light Source.
The diffusion nature of the road surface causes the reflection
of a certain proportion of the incident light in the direction of
observer. Vijay Balu Raskar (BE Electrical)

8. Sometime, surface is illuminated from two lamps and resultant
illumination is sum of two illuminated lamps.
If surface that is uniformly illuminated by a beam of light appears
to be equally bright when viewed from all possible angle the
reflection is said to be perfectly diffused.
Vijay Balu Raskar (BE Electrical)

9. Fun Fact
The human eye cannot see all the rays on the light
spectrum. Infrared rays, for example, are too long for
the human eye to perceive, and ultraviolet rays are too
short. So, direct (spectral) light seems stronger than
diffuse light. However, the total light transmission is the
same. Our perception can play tricks on us.
Vijay Balu Raskar (BE Electrical)

10. What Causes Light Diffusion?
ANS:-When a light beam strikes a smooth surface, most of it reflects back in the
same concentration. This is specular reflection, which gives us direct, bright light. A
mirror is a common example of a smooth surface that causes specular reflection.
What happens on a rough surface?
ANS:Even microscopic irregularities create roughness. The law of reflection is not
broken. Each ray reflects back at the same angle at which it struck the object but in a
different direction. So diffuse light is scattered light. This scattering is what causes the
diffusion and softness of the light beam.
______________________________________________________________
Application of Diffuse Light
ANS:-Photographers use the principle of diffuse light to create pictures with vivid
details because there are no sharp shadows to distract attention. On a sunny
day, they use light diffusers to create soft shadows. Horticulturists are now
discovering that diffused light creates a better growing environment in greenhouses.
It allows for a greater horizontal spread of light and exposes the middle leaf layers to
light. Drivers find that wet roads have a greater glare than dry roads. That is
because the cracks and crevices on the road's surface fill up with water, creating a
smooth surface. This results in specular reflection that creates the annoying glare.
Fog lamps try to make use of the principle of diffused light to provide a safer beam.
Vijay Balu Raskar (BE Electrical)

11. 2. Specular Reflection Principle of Street Lighting
Example:-
Fig a Fig b
This method is more economical than diffusion method.
This method produces “Glare for the motorists”
Vijay Balu Raskar (BE Electrical)

12. Vijay Balu Raskar (BE Electrical)

13. Explanation:-
Reflectors are used in this principle but curved upwards.
They are designed to spread light very large angle of incident
Reflectors made to thrown light to large angle.
Filament is visible.
The illumination at point is calculated by Lambert’s Cosine
Law method.
 Due to Reflection nature, the road surface or object shown
30 meter away to the observer.
A beam of light reflected but not scattered.
This method is suitable for straight sections of the road.
The viewed see the illuminated surface and the light Source.
 This method produces “Glare for the motorists”
The illumination required depends upon the class of steet
lighting installation. Vijay Balu Raskar (BE Electrical)

14. Vijay Balu Raskar (BE Electrical)

15. Vijay Balu Raskar (BE Electrical)

16. COMPARISION
Vijay Balu Raskar (BE Electrical)