There are several light sources which have been used in electric street lights. The main types used since the 1940s in the United States have been Incandescent, Fluorescent, Low Pressure Sodium and High Intensity Discharge.
Incandescent
Incandescent lamps are among the first and least efficient light sources used in street lighting. Incandescent lighting was a popular option for street lighting until the 1950s, when other lamps proved to be more efficient and lower maintenance.
Fluorescent
Fluorescent lamps gained popularity for street lighting applications in the 1950s. The lamps were more efficient than their Incandescent counterparts, and required less maintenace. The lamps were primarily used in downtown areas and parking lots. They were good for any place requiring a lot of light over a large area. The popularity of these lamps was relatively short-lived, as more efficient, compact and lower-maintenance High Intensity Discharge lamp technology was advanced.
Low Pressure Sodium
Low Pressure Sodium (LPS) lamp is by far the most efficient light source used in street lighting. LPS lamps are used in various parts of the country, including San Diego, CA. The lamps produce a monochromatic orange-yellow light, from lamps which are long and skinny. LPS is also a good way to reduce sky glow for stargazers. The light is easily filtered out for viewing the stars through telescopes. Drawbacks of using LPS lamps include the color rendering. When the lamp is on, everything around it looks either orange-yellow, black or shades in between. Also, as the lamp ages, it uses more wattage, which lighting designers need to account for. That increase in wattage does result in little-no lumen depreciation, meaning it the light output from the lamp stays fairly constant over it's life.
High Intensity Discharge
There are three main types of High Intensity Discharge (HID) lamps. HID lamps in general requre an external ballast to operate. These lamps usually take between 1 and 5 minutes to reach full brightness, and if there is a dip in electricity, these lamps will shut off. The lamps must cool sufficiently to restrike, which usually takes from 1 minute to 10 minutes. Street lights using these lamps have been labelled on the bottom of the fixture since the late 1970s. A number on the bottom multiplied by 10 yields the wattage rating of the fixture (e. G. "10" means 100 watts, "25" means 250 watts. The only exception is 1000 watt fixtures, which are labelled as "X1". ) Mercury Vapor has a blue label, Metal Halide has a red label and both types of sodium lamps have yellow or gold labels.
Mercury Vapor
Mercury Vapor was the first widely accepted HID lighting source. Clear mercury vapor lamps cast a blue-green light, which some say makes people look like "walking cadavers. " Advances in technology have lead to color corrected mercury vapor lamps, which cast a relatively clean white light. This is done by coating the outer glass globe of the lamp with phosphors.
By the late 1950s, mercury lamps were very widely used around the US. The lamps were about as efficient as fluorescents, the fixtures were smaller, and lasted much longer. The lamps could also operate in extreme cold. One main difficulty with mercury lamps was "lumen depreciation. " Lumen depreciation is a drop in light output of the lamp over time. In a lot of cases, a mercury lamp will burn for years past it's rated life, but it will burn much dimmer while using the same amount of wattage.
Metal Halide
Metal Halide Lamps are a distant cousin of mercury lights. The basic lamp is the same as a mercury lamp, but with other metallic elements added. The result is a good quality white light. Metal hallide has not gained wide acceptance as a source of street light. It is mostly found in parking lots and inside commercial and industrial buildings. The light is more efficient than mercury vapor, but the lamp life is shorter. Another problem incurred with metal hallide is "color shift. " The color of the light produced by each lamp varies slightly, which leads to a cluttered effect. There are now lamps on the market that keep color shift to a minimum, helping to alleviate that problem. Of course, since the Metal Halide lamps are related to mercury, they too suffer from lumen depreciation, but not as extreme as MV.
High Pressure Sodium
High Pressure Sodium (HPS) lamps are now commonly used around the US in street lights. The lamps were developed in the early 1970s and are more energy efficent than mercury and metal hallide lamps. The lamps give off an amber color, have virtually no problem with color shift, and last for long periods of time. The lamps begin to incur problems when they near the end of their life. Lumen depreciation is a problem with HPS, though still not as severe as the depreciation seen with Mercury. The lamps begin to "cycle, " which means they turn themselves off and come back on a minute later. This problem has been addressed with the recent introduction of non-cycling HPS lamps. Advances in photocontols can also stop cycling: Lighting Systems Technologies, Inc has information on these. This page is in no way shape or form part of any corporation or government agency. It is the creation of Jim Terry. Please, feel free to link to this page!
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