Street light

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A street light, lamp post, street lamp, light standard or lamp standard, is a raised source of 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 dawn, or activate automatically in dark weather. It is also not uncommon for street lights to be on posts which have wires strung between them, such as on telephone poles or utility poles.

Contents 1 History of street lighting 2 Disadvantages 2.1 Dangers of street lights 2.2 Light pollution 2.3 Safety 3 Purposes of street lights 3.1 Beacon lights 3.2 Roadway lights 3.3 Security lighting 4 The future 5 See also 6 External links

[edit] History of street lighting

Further information: History of street lighting in the United States

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, plus, there are over 1,000 lamp posts (on average) on every street in the United States.

The first electric street lighting employed arc lamps, initially the 'Electric candle', 'Jablochoff candle' or 'Yablochkov candle' developed by the Russian Pavel Yablochkov in 1875. This was a carbon arc lamp employing alternating current, which ensured that the electrodes burnt down at the same rate. Yablochkov candles were first used to light the Grand Magasins de Louvre, Paris where 80 were deployed. Soon after, experimental arrays of arc lamps were used to light Holborn Viaduct and the Thames Embankment in London - the first electric street lighting in Britain. More than 4,000 were in use by 1881, though by then an improved differential arc lamp had been developed by Friederich von Hefner‑Alteneck of Siemens & Halske. The United States was swift in adopting arc lighting, and by 1890 over 130,000 were in operation in the US, commonly installed in exceptionally tall moonlight towers.

Ultimately, however, arc lights suffered from two major disadvantages. Firstly they emitted an extremely intense and harsh light which, although useful at industrial sites like dockyards, was discomforting in the context of ordinary city streets. Second, they required a high level of maintenance, as the carbon electrodes burned away quite swiftly. With the development of cheap, reliable - and bright - incandescent light bulbs in the closing years of the 19th century, they swiftly passed out of use for street lighting, though they were retained in industrial use for rather longer.

Incandescent lamps, continued to be used for street lighting until the advent of high-intensity discharge lamps, 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 (an open circuit), the current through the string became zero, causing the entire voltage of the street lighting circuit (thousands of volts) to be imposed across the insulating film in the cutout, rupturing it (see Ohm's law). In this way, the failed lamp was bypassed and illumination restored to the rest of the street. (This is the same principle used in Christmas tree lights.) 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.

Today, street lighting commonly uses high-intensity discharge lamps, often HPS high pressure sodium lamps. Such lamps provide the greatest amount of Photopic illumination for the least consumption of electricity. However when Scotopic/Photopic light calculations are used, it can been seen how inappropriate HPS lamps are for night lighting. White light sources have been shown to double driver peripheral vision and increase driver brake reaction time at least 25%. When S/P light calucations are used HPS lamp performance needs to reduced by a minimum value of 75%. This is now a standard design criteria for Australian roads.

The first city to have electric street lights was Godalming, UK (1881).

[edit] Disadvantages

The major criticisms of street lighting are that it can actually cause accidents if misused, and can cause light pollution.

[edit] 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 impaired vision. As a person gets 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.

There is also the simple physical danger: street light stanchions (poles) pose a collision danger to motorists and should be designed to "break-away" when hit, be guarded by a guardrail, or both. Also, very high winds or accumulated metal fatigue occasionally topple street lights with the resulting hazard to anything the light and pole falls upon.

[edit] 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 metal halide 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.

[edit] 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.

[edit] Purposes of street lights

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.

[edit] 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. On Interstate highways this purpose is commonly served by simply placing reflectors at the sides of the road to reflect the light coming from people's headlights.

[edit] Roadway lights

Street lights are not normally intended to light up the driving route (rationale being headlights are preferred for that), but to disclose signs and hazards outside of the headlights' normal view. Because of the dangers discussed above, roadway lights are properly used only sparingly and only when a particular situation justifies increasing the risk. This usually involves an intersection with several turning movements and much signage — situations where the driver must take in much information quickly that is not within the beam of his headlights. In situations like that (A freeway junction or exit ramp) the intersection may receive lighting so that the driver can quickly see all the hazards, and in a well-designed plan there will be gradually increasing lighting for a quarter-minute or so both before the intersection and gradually decreasing lighting after it. The main stretches of a highway remain unlighted to preserve the driver's night vision and increase the visibility of oncoming headlights. If there is a sharp curve where the headlights will not illuminate the road ahead, a roadway light on the outside of the curve is often justified.

If it is desired to truly light a roadway, perhaps because it carries heavy volumes of fast multilane traffic, then to avoid the dangers caused by casual placement of street lights, it should not be lit intermittently, as this requires repeated eye reajustment which implies eyestrain and temporary blindness when entering and leaving light pools. In this case the system is instead designed to eliminate the need for headlights. This is usually achieved with bright lights placed on high poles at close regular intervals so that there is a consistent light level all along the route. The lighting goes from curb to curb.

[edit] Security lighting

Security lighting is similar to high-intensity lighting on a busy major street, with no pools of light and dark, but with the lighted area extending onto people's property, at least to their front door. This requires a different type of fixture and lens. The increased glare experienced by drivers going through the area might be considered a trade-off for the increased security being sought. This is what would normally be used along sidewalks in dense areas of major cities. Often unappreciated is that the light from a full moon is actually brighter than even most security lighting.

[edit] The future

In countries such as France, Germany, Belgium, UK and the northern part of the US, street lamps are burning an average of 4000 hours per year. Considering that the average wattage of a lamp is around 150 watts, considering that a 100,000 inhabitant city contains about 18,000 lamps, such a city spend around 11 giga watt hours (11 billion watt hours). Considering that producing 1 kWh implies the emission of 340 grams of CO₂ (average in Europe), the streetlights of such a city is responsible for the emission of 3700 tons of CO₂ in the atmosphere per year.

Technologies and techniques now exist to:

1. save electricity without impacting the lighting level as perceived by the citizen
2. automatically identify 99% of the lamp and the ballast failures, to save on maintenance cost and increase the security in town
3. leverage the same technologies and the same infrastructure to monitor other environmental data (through temperature, humidity, air pollution, air quality and noise sensors) to build up a real time environmental database and enhance our control for future generations.

Amongst the key technologies and techniques :

1. Electronic communicating ballasts (such as SELC ballasts) are dimming the lamps intelligently when less light level is required (middle of the night in industrial and commercial zones for instance). Moreover they consume 4 to 5 watts while usual magnetic ballast consume up to 20 watts. In 2005, the European Union voted a directive (2000/55/EC) to incitate Cities to deploy such electronic ballast that result in up to 45% electricity savings (financial saving and positive impact on environment). Moreover such electronic ballast can usually identify failures in the lamp, on the electrical network and in the supply cabinet and communicate them through standardized protocols such as EIA709 on powerline. EC directive (2000/55/EC) is related ONLY to fluorescent lighting and fluorescent ballasts. This directive has little to do with the vast amount of street lighting ballasts.
2. Power Regulator can be installed in the cabinet that supplies electricity to the street lights. Power regulators enable electricity regulation and can usually dim the lamps in the darkest time of the night. Unfortunately, power regulators are less efficient than electronic ballast for several technical reasons : length of the supply links, different technologies of lamps that cannot be dimmed at the same levels. Power Regulators can provide actual electricity savings around 20%. However there are other systems to control conventional HPS ballasts that can cut 40% of the power used.

As an alternative, there are solutions to dim existing magnetic ballasts which also identify failure conditions. There are solutions that will pay back the capital cost for adding controls in 2 and not more than 3 years where street lighting costs range from 12 to 10 euro cents per kilowatt hour and are used 11 to 12 hours a day

[edit] See also

• History of street lighting in the United States
• List of streetlight manufacturers and fixtures

[edit] External links

Wikimedia Commons has media related to: Category: Streetlamps

• An enthusiast's guide to street lighting - including many close-up photographs of UK street lighting equipment, as well as information on installations through the ages. (UK)
• A street lighting website with pictures showcasing many street lights.
• A picture website where many members showcase their street light pictures.
• Vicki Sauter's streetlight collection, which includes photos of streetlights from around the world.