What is Street Lighting? – Objectives, Principles and Types
The main objects of street lighting are :
- To make the traffic and obstructions on the road clearly visible in order to promote safety and convenience.
- To make the street more attractive.
- To increase the community value of the street.
The principle employed for street lighting is different from that of interior lighting. There are no walls and ceiling which reflect or diffuse light, hence only direct lighting scheme can be employed and hard shadows and high contrast cannot be avoided. Because of areas to be illuminated being large, the value of illumination for economic reasons, is very low compared to that for indoor lighting and the question of colour rendering is also of minor importance. High level of illumination is not necessary, because firstly one has seldom to look continuously at nearby objects, secondly it is the impression of whole scene that is required. In fact in case of interior lighting the objects are seen by the light reflected by them but in case of street lighting the objects are seen in relatively bright back ground i.e., silhouetted against a relatively bright back ground. Further more, owing to the low illumination, the eye is in its most sensitive state and, therefore, glare must be avoided.
Two general principles are usually employed in the design of street lighting installations, namely (i) the diffusion principle and (ii) the specular reflection principle.
1. Diffusion Principle:
In this case the lamps fitted with suitable reflectors are used. The reflectors are so designed that they may direct the light downwards and spread as uniformly as possible over the road surface. In order to avoid glare the reflectors are made to have a cut-off of between 30° to 45° so that the filament is not visible except from underneath it. The diffusing nature of the road surface causes the reflection of a certain proportion of the incident light in the direction of the observer, and therefore, the road surface appears bright to the observer. The illumination at any point on the road surface is calculated by applying point to point or inverse-square law method. Over certain proportions of the road the surface is illuminated from two lamps and the resultant illumination is the sum of the illuminations due to each lamp.
2. Specular Reflection Principle:
In this case the reflectors are curved upwards so that the light is thrown on the road at a very large angle of incidence. It is observed that a motorist requires to see objects about 30 metres away. Thus in Fig. 7.51 the observer is shown about 30 metres from the object. Much of the light from the lamp L3 is not reflected towards the observer, where as most of the light from lamps L1 and L2 is reflected towards him. Thus the object will appear silhouetted against the bright road surface due to lamps at a longer distance. The requirement of a pedestrian, who requires to see objects in his immediate neighbourhood, is also fulfilled in this method as some light from the lamps falls directly downwards. This method of street lighting is only suitable for straight sections of the road. This method is more economical also as compared to the diffusion method of lighting but it suffers from the disadvantage that it produces glare for the motorists.
Illumination Level For Street Lighting and Mounting Height of Lamps: The illumination required depends upon the class of street lighting installations. In class A installations, i.e., in important shopping centres and road junctions, illumination level of 30 lumens/m2 is required where as a in poorly lighted suburban streets, illumination level of 4 lumen/m2 is sufficient. An average well-lighted street, will require illumination level between 8 to 15 lumens per square metre. Excellent illumination is considered when the distance apart is not more than 8 times the height of the luminaires ; that is, with the luminaires 8 metres above street level, they should be spaced not more than 64 metres (normal spacing for standard lamps is 50 metres with a mounting height of 8 metres). Lamp posts should always be fixed at the junction of roads and as far as possible lamps near large trees should be avoided.
Types of lamps For Street Lighting:
Mercury vapour and sodium discharge lamps have been found to have certain particular advantages for street lighting purposes ; the most important of these is the lower power consumption for a given amount of light, which, inspite of the higher cost of the lamps, makes the overall cost of an installation with discharge lamps less than that employing filament lamps. The colour and monochromatic nature of the light produced by discharge lamps do not matter much in street lighting installations.