STREET LIGHTING

Objectives of street lighting:

The objectives of street lighting are

1) Ensure that fast traffic moves safely,
2) Facilitates pedestrian movements,
3) Ensure that pedestrians and vehicle drivers recognize obstacles; and enhance security,
4) The first is only important for through routes. In residential areas, where traffic moves slowly, it will not be an important concern.

Definitions and description:

The lamp is the source of light, which converts electrical energy into radiation. The main types of lamps in use are:

1) Tungsten lamps (designated GLS): in which a small wire contained in a glass bulb is heated to a high temperature in a vacuum.

2) Fluorescent lamps (designated MCF and TL): in which ultra-violet radiation causes phosphor powder to glow inside a tube.

3) Discharge lamps: in which an electric current is passed through a mixture of gasses in a sealed tube. Examples are low-pressure sodium, high pressure sodium and high pressure mercury (designates SOX, SON and MBF or HPL-N respectively).

The lantern refers to the complete street lighting fixture, including the lamp and the pole or other means of raising it above the street

The luminous flux represents the quantity of radiation emitted by a lamp, measured in units of lumens.

The intensity of light by a lamp is measured in units of candelas.

The Luminance of a point on the road surface that is receiving light from a lantern equals the intensity of the light emitted by the lamp divided by the square of the distance of the point from the lamp.

It relates to the incident light falling on a surface and so is higher close to the lamp. It is measured in terms of lux (Lumens per square metre).

Another measure of the intensity of light is Luminance, which is a measure of the light reflected from the road surface. It is measured in candelas per square metre.

It is commonly used to assess lighting requirements for heavily trafficked roads and has limited relevance to the needs of low-income areas.