Street lighting is done to make the traffic and obstruction on the road clearly visible for safety and convenience. Also, it increases the community value of the street by making the street more pleasant and attractive.

Street lighting makes use of only direct lighting schemes as there are no walls or ceilings which reflect or diffuse light. In street lighting the illumination level is quite low in comparison to indoor lighting as the provision of street lighting is only for visibility purposes.

Design of Street Lighting

Street lighting makes use of following general principles.

1.Diffusion Principle

The diffusing nature of street or road surface causes the reflection of a certain portion of incident light in the direction of the observer which makes the surface of the road appear brighter to the observer.

In the case of the diffusion principle, the lamps are filled with suitable reflectors. Consequently, this helps to direct the light downwards and spread as uniformly as possible over the road surface.

To avoid the glare, the reflectors are build to have a cutoff of between 30o to 40o so that the filament or the luminaries is visible underneath.

We make use of point to point or law of inverse square method (Lambard cosine law) to calculate the illumination at any point on the road surface.

Over a certain portion of the road, where we illuminate the surface using two or more lamps then the resultant illumination is the sum of the illumination between each lamp.

2.Specular Reflection Principle

According to the specular reflection principle, the reflectors of the lamps are turned upwards so that the light is thrown on the road at a very large angle of incident.

Through this arrangement, the pedestrian can also see their objects in their immediate neighborhood.

Relative to the diffusion method of lighting this method is more economical but produces unnecessary glares which can cause accidents during driving.

Illumination Level of Street Lighting

The illumination level of any streets or roads depends on nature and type.

Types of Road Road Characteristics Recommended Illumination Recommended Height

 

A Important Traffic Routes Carrying Fast Traffic 20 to 30 lux 7.5 m to 10 m
B Main road carrying mixed traffic like city, main roads including cycles. 15 to 20 lux 7.5 m to 10 m
C Secondary roads with considerable traffic like slow-moving vehicles, shopping streets 10 to 12 lux 7 m to 8 m
D Residential and unclassified roads 6 to 9 lux Less than 7.5 m

Types of Lamps for Street Lighting

Generally, mercury vapor and sodium discharge lamps are used because these are low-power consuming lamps for the given amount of light although their initial cost is high, their overall performance is more economical than the filament lamps.

Nowadays, efficient lightings are carried on by using various LED lights. These lights are effective from both economic and electrical power points of view. It must also be noted that the color and monochromatic nature of light does not have a significant effect on street lighting installation.

Mounting Height

The recommended mounting height for light fittings is 7.5 meters with a tolerance of +1.5 meters ( can also be +2.5 meters) and -0.5 meters between the light centers and the roadway immediately below the luminaries.

Lamppost should always be fixed at the junction of the road and as far as possible. Lamps near the trees should be avoided.

Spacing of the Light

The spacing between lights should be uniform and about 29 meters to 31 meters in case of “cut-off” type fittings.

In the case of the “non-cut-off” type fittings, the spacing between the light should be 31 meters to 40 meters.

However, the normal spacing for the standard lamp is 50 meters with a mounting height of 8 meters.

Arrangement of Lighting Source

We can mount the lighting source on one or both sides, in parallel or equal zigzag arrangements or the middle depending upon the width of the road and required intensity of the light.

The zigzag arrangement gives more uniform lighting than other arrangements.

Great consideration should be taken while lighting road junctions and crossings so that the obstructions are clearly visible.

Street Lighting for Curves

Light along the curves should be located on the outer sides of the curves.

Two lamps opposite each other should be located where a row of lights crosses from one side of the road to the other.

Solar Street Lighting

Following are the ways of solar street light installation.

1.Stand Alone

The necessary equipment such as the battery, PV modules, and control circuits are installed in a single post, and lighting is installed.

Here, the maintenance and operation of the system are quite difficult and it yields less efficient lighting.

2.Centralized System

In this case, the PV module is only installed on the post, and the control circuit long with the battery operating system (BOS) is centralized in a particular place.

Hence, the operation and maintenance of this type of system will be quite easier relative to the stand-alone system.


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