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Roadway lighting product information

Driving at night with just headlights can be a frightening experience, and it is made worse when the street signs are barely visible. Roadway lighting has increased road safety and saved countless lives since it was invented two centuries ago. The history of roadway lighting is long and varied, with many different lamp types used over the years. Today, the overwhelming majority of roadway lights are either sodium vapor lamps or fluorescent tubes. These two types of gas-discharge lighting are very energy efficient, but the growing field of solid-state lighting may provide lamps that are even better.

Roadway lighting originated in England at the start of the nineteenth century with the invention of the gas lamp, and quickly spread to the rest of the world. Every night, lamplighters would go around town lighting the gas lamps using a burning taper on the end of a stick. While a few towns still use gas lighting today, it was largely replaced by electric lighting by the start of the twentieth century. The first electric roadway lighting used the incandescent globe invented by Thomas Edison, but the most widely used type today is the gas-discharge globe. Large sodium vapor lights usually have their own support poles, while the smaller fluorescent lights are often attached to utility poles that also carry power lines.

Large gas-discharge lamps are used to illuminate large areas of roadway, such as freeways, carparks, and roundabouts. They have a distinctive two-stage starting process where they glow dimly as the metal vapor is ionized, followed by a dramatic increase in brightness. Gas-discharge lamps require a ballast to limit the flow of current in this final stage, which would keep increasing until the lamp was destroyed. Mercury vapor lamps were first used in 1940's but were replaced by sodium vapor lamps in the 1970's, and are now banned in some countries in an effort to reduce mercury usage and light pollution. Sodium vapor lamps have a distinctive orange glow, and are brighter and more efficient than mercury vapor lamps. Metal halide lamps are less efficient but have a true white color, and are starting to be used for roadway lighting.

Fluorescent lighting is commonly used for residential streets and footpaths. It is a better choice for small areas because it is cheaper to install and it produces white light. The most common fluorescent lamp used for roadway lighting has a long case with two fluorescent tubes mounted side by side. Two tubes are used so that the lamp will continue working in the event that one tube fails. Fluorescent lights work by passing current through mercury vapor which produces ultraviolet light in its excited state. The phosphorous coating on the tube absorbs the ultraviolet light and releases the energy as visible light. The glass tube prevents any unabsorbed ultraviolet light from escaping, ensuring that only harmless visible light is produced.

Roadway lighting has saved many lives and provides many other benefits for motorists and pedestrians, but it also the cause of a few problems. The light levels just inside tunnels entrances need to be adjusted to create a gradual transition between the outside lighting and tunnel lighting, to avoid a rapid change that would disorientate motorists. Older sodium vapor lamps have a tendency to suddenly turn off for a short time because of overheating, a condition known as cycling, and the lamps need to be replaced before it becomes too frequent. Roadway lighting is also a major source of light pollution, with mercury vapor lamps causing problems for astronomers, but the switch to sodium vapor lights has made some improvement in this area.

The future of roadway lighting will probably involve a move away from current lamp types towards solid-state lighting. These produce light without using filaments or gases, and are more efficient and last longer. This is already happening with traffic lights, which are being changed over to lamps that have many light emitting diodes (LEDs). Although they are not cheap or bright enough to be used for other roadway lighting just yet, they will be in a few years.

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