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A street light or street lamp, also known as a light standard or lamp standard, is a raised light on the edge of a road, turned on or lit at a certain time every night. Modern lamps may also have light-sensitive photocells to turn them on at dusk and off at sunrise, or activate automatically in dark weather. It is also not uncommon for street lights to be on posts which have wires strung between them telephone poles or electrical poles.

History of street lighting
Today, street lighting is most commonly achieved using high-intensity discharge lamps, often sodium vapor lamps. Such lamps provide the greatest amount of illumination for the least consumption of electricity.
Before the advent of high-intensity discharge lamps, street light circuits used incandescent light bulbs. These were often operated as high-voltage series circuits. To avoid the problem of the entire street going dark if a single lamp burned out, each individual street-lamp was equipped with a film cutout; a small disk of insulating film that separated two contacts connected to the two wires leading to the lamp. If the lamp failed, the entire voltage of the street lighting circuit (thousands of volts) was imposed across the insulating film in the cutout, causing it to rupture. In this way, the failed lamp was bypassed and illumination restored to the rest of the street. The circuit usually contained an automatic device to regulate the electrical current flowing in the circuit, preventing the current from rising as additional lamps burned out and thus preserving the life of the remaining lamps. When the failed lamp was finally changed, a new piece of film was also installed, once again separating the electrical contacts in the cutout. This style of street lighting was recognizable by the large porcelain insulator that separated the lamp and reflector from the light's mounting arm; the insulator was necessary because the two contacts in the lamp's base may have routinely operated at a potential of several thousands of volts above ground/earth.
And before incandescent lamps, gas lighting was employed. The earliest lamps required that a lamplighter tour the town at dusk, lighting each of the lamps, but later designs employed ignition devices that would automatically strike the flame when the gas supply was activated.
Desirability of Street Lights
Many residents favor street lights in their neighborhood and around town because they believe it makes it easier to navigate at night, and their proponents claim they also help reduce crime. A variety of studies have supported this view but at least one indicated that it had little or no effect.
Disadvantages
The major criticisms of street lighting are that it can actually cause accidents if misused, and can cause light pollution.
Dangers of Street Lights
There are two optical phenomena that need to be recognized when considering installing a street light.
The loss of night vision because of the accommodation reflex of the driver's eyes is the greatest danger. As he emerges from an unlighted area into a pool of light from a street light, his pupil quickly constricts to adjust to the brighter light, but as he leaves the pool of light, the pupil's dilation to adjust to the dimmer light is much slower, so the driver is speeding down the road with impared vision. As a person get older, the eye's recovery speed gets slower so his driving time and distance under impaired vision also increases.
Oncoming headlights are more visible to a driver if they are against a black background than a grey one. The contrast creates a greater awareness of the oncoming vehicle.

Light Pollution
In urban areas light pollution can hide the stars and interfere with astronomy. In settings near astronomical telescopes and observatories, low pressure sodium lamps may be used. These lamps are advantageous over other lamps such as mercury and halogen lamps because low pressure sodium lamps emit lower intensity, monochromatic light. Observatories can filter the sodium wavelength out of their observations and virtually eliminate the interference from nearby urban lighting.
The light pollution also disrupts the natural growing cycle of plants.

Safety
A common misconception is that merely installing street lights will automatically make streets safer and lower crime, so political pressure can be a major factor in installation of street lights. Untrained officials often innocently assume that if some is good, more must be better, and so install the brightest lights they can. But misuse of street lights can cause accidents, and crime lighting is an entirely different type of lighting than used for automobile navigation.
Purposes of Street Lights
Decorative omnidirectional beacon lights on a standard. Beacon lights can also be on an arm with a downward reflector.There are three distinct main uses of street lights, each requiring different types of lights and placement. Incorrect misuse of the different types of lights can make the situation worse by compromising visibility or safety.

Beacon Lights
A modest steady light at the intersection of two roads is an aid to navigation because it helps a driver see the location of a side road as he comes closer to it and he can adjust his braking and know exactly where to turn if he intends to leave the main road or see if someone is at the intersection. A beacon light's function is to say "here I am" and even a dim light provides enough contrast against the dark night to serve the purpose. To prevent the dangers caused by a car driving through a pool of light, a beacon light must never shine onto the main road, and not brightly onto the side road. In residential areas, this is usually the only appropriate lighting, and it has the bonus side effect of providing spill lighting onto any sidewalk there for the benefit of pedestrians.