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.

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.

Arc lights had two major disadvantages. First, they emit an intense and harsh light which, although useful at industrial sites like dockyards, was discomforting in ordinary city streets. Second, they are maintenance intensive, as carbon electrodes burn away swiftly. With the development of cheap, reliable and bright incandescent light bulbs at the end of the 19th century, they passed out of use for street lighting, but remained in industrial use longer.

Incandescent lamps were used for street lighting until the advent of high-intensity discharge lamps, were often operated as high-voltage series circuits. To avoid having the entire street go dark if a single lamp burned out, each 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 circuit (thousands of volts) to be imposed across the insulating film, penetrating 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 voltage in the circuit, preventing the current from increasing as additional lamps burned out , preserving the life of the remaining lamps. When the failed lamp was replaced, a new piece of film was installed, once again separating the 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 operated at 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 calculations 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 in street light installations.

• The loss of night vision because of the accommodation reflex of the driver's eyes is the greatest danger. As he/she emerges from an unlighted area into a pool of light from a street light, his/her pupil quickly constricts to adjust to the brighter light, but as he/she leaves the pool of light, the pupil's dilation to adjust to the dimmer light is much slower, so the driver is speeding along with impaired vision. As a person gets older, the eye's recovery speed gets slower, so driving time and distance under impaired vision also increases.

• Oncoming headlights are more visible against a black background than a grey one. The contrast creates greater awareness of the oncoming vehicle.

There is also the physical danger. Street light stanchions (poles) pose a collision danger to motorists and should be designed to "break-away" when hit, be protected by guardrails, or both. High winds or accumulated metal fatigue occasionally topple street lights.

[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 misconception is that installing street lights will automatically make streets safer and reduce crime, so political pressure can be a major factor in installation of street lights. Untrained officials often assume that if some is good, more must be better, and install the brightest lights possible. 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 illuminate the driving route (headlights are preferred), but to reveal signs and hazards outside of the headlights' beam. Because of the dangers discussed above, roadway lights are properly used 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 drivers must take in much information quickly that is not in the headlights' beam. In these situations (A freeway junction or exit ramp) the intersection may be lit so that drivers can quickly see all hazards, and a well designed plan will have gradually increasing lighting for a approximately a quarter minute before the intersection and gradually decreasing lighting after it. The main stretches of highways remain unlighted to preserve the driver's night vision and increase the visibility of oncoming headlights. If there is a sharp curve where headlights will not illuminate the road, a light on the outside of the curve is often justified.

If it is desired to light a roadway, perhaps due to heavy and fast multilane traffic, to avoid the dangers of 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 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 consistent light 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 increased security. This is what would normally be used along sidewalks in dense areas of cities. Often unappreciated is that the light from a full moon is brighter than 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 are 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 perceived by citizens
2. automatically identify 99% of lamp and ballast failures, saving on maintenance cost and increase security
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 a real time environmental database and enhance our control.

Amongst the key technologies and techniques :

1. Electronic communicating ballasts (such as SELC ballasts) dim lamps intelligently when less light is required (such as middle of the night in industrial and commercial zones). They consume 4 to 5 watts while magnetic ballasts consume up to 20 watts. In 2005 the European Union voted a directive (2000/55/EC) to incitate Cities to deploy electronic ballast that result in up to 45% electricity savings. Such electronic ballasts can usually identify failures of the lamp and the electrical network and communicate them through protocols such as EIA709 on powerline. EC directive (2000/55/EC) relates ONLY to fluorescent lighting and fluorescent ballasts. This directive has little to do with the vast majority 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 reasons. The length of the supply links, different technologies of lamps that cannot be dimmed at the same levels. Power Regulators can provide electricity savings around 20%. There are other systems to control conventional HPS ballasts that can cut power use by 40%.

There are methods 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
• Street light interference

[edit] External links

• 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)
• Frangioso's Street Light Gallery - Shows a lot of pictures of streetlights in Boston area.
• Lighting-Gallery.net - Website that allows streetlight or lightbulb collectors join and show pictures of collections, streetlights scenes, and their lightbulb collections or unusal bulbs. This site also allows movies of light-related. This also has a forum to talk about light general related.
• Vicki Sauter's streetlight collection, which includes photos of streetlights from around the world.