The following points highlight the five main points to be kept in mind while making choice of lighting system for street lighting. The points are: 1. Initial Cost and Cost of Replacements 2. Life of Lamp 3. Operational Costs 4. Quality of Light 5. Efficiency.
1. Initial Cost and Cost of Replacements:
The tungsten filament lamp is the cheapest with the only accessory required a reflector. Next is the fluorescent tube. High pressure mercury vapour lamp and sodium vapour lamp are the costliest. Regarding replacements, the incandescent lamp will again be the cheapest as compared to rod of fluorescent tube or lamp of sodium vapour or mercury vapour lamp.
2. Life of Lamp:
The life span of a tungsten filament lamp is the lowest, about 1000 working hours. The life spans of fluorescent tube are about three times the life of a tungsten filament lamp. The other lamps have also a longer life.
3. Operational Costs:
For the same light output, the sodium vapour lamp, as it has the highest efficiency, will have lowest running costs. The running costs of mercury vapour lamp and fluorescent tube will be, approximately the same. The tungsten filament lamp will have running costs about 3 – 4 times the other as it has lowest efficiency.
4. Quality of Light:
The light emitted by fluorescent tube is most pleasing almost free colour distortion and close to day light. It is free from glare, has low brightness and does not cause hard shadows. Sodium vapour lamp has a characteristic yellow light that does not show the tube appearance of the objects and is annoying.
The light emitted by a mercury vapour lamp is having a bluish tinge and is devoid of red light, producing colour distortion. Tungsten filament lamp produces inferior quality of light that is likely to produce glare unless the system is carefully designed. It also produces hard shadows.
The luminous efficiency of sodium vapour lamp is the highest of the order of about 60 lumens/watt. The mercury vapour lamp and the fluorescent tube have about the same efficiency of about 40 lumens/watts. The tungsten filament lamp with an efficiency of about 13 lumens/watt stands lowest.
So, for a given luminous output required, the number of lamps will be highest (3 to 4 times) for Tungsten filament lamp and lowest for sodium vapour lamp. Mercury vapour lamp and fluorescent tube closely follow the sodium vapour lamp.
Considering all the factors taken together, it appears that the fluorescent tube, with its quality of light close to daylight, almost free from glare and hard shadows along with lowest operational costs is most suitable for this purpose. Nevertheless it is most widely used.
However, if the colour distortion of mercury vapour lamp and sodium vapour lamp could be ignored, these may be used with still lower operational costs. The tungsten filament lamp with all the disadvantages, except the lower initial cost, it is most unfavoured one. However, still it is much use in our country especially in suburban areas.