Glossary of Street Lighting and Illumination Terms

Contains interesting discussions alongside the definitions.

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Arc (electric) -- The visible light, or spark, produced when electricity jumps through air or a gas. Heat is produced also. Switches wear out because each time a switch is opened, a brief arc occurs and the switch contacts suffer a small amount of heat damage.

Arc Light -- Generally refers to a carbon arc lamp. Some of the first streetlights were carbon arc lights. The carbon arc lamp had the electricity jumping a gap between a pair of carbon rods since these did not melt as metal electrodes might given the heat produced. (The carbon rod ends did evaporate gradually so arc streetlights required maintenance in the form of frequent rod replacement. Arc lights still used for theater spotlights today have automatic means of moving the rod ends back together as they wore away.) Mercury and sodium lights are technically arc lights since they contain small arcs passing through mercury vapor or sodium vapor, respectively.

Ballast -- A coil of wire and/or related electronic components used to limit the amount or electric current flowing through a lamp. Almost all lamps other than incandescent lamps used in street lighting require ballasts.

Ballast Box -- The "stand" at the bottom of a streetlight pole used to hold ballasts back in the days when the latter were large and heavy. It does not provide added stability but rather must be rigid enough to withstand the stresses and wind forces sustained by the pole and light while it is attached to the (usually concrete) underground footing.

Bracket Arm -- The more or less horizontal bar that holds the streetlight out over the street and that may or may not have a second bar or an attached cable for additional support.

Candlepower -- A now obsolete measurement  for amount or quantity of light that never had a rigorous definition but was based on the total amount of light emitted by a candle. Different sources generally used ten to twelve lumens as equal to one candlepower.

CFL (Compact Fluorescent) -- Fluorescent lamp with a sharply bent or coiled small diameter tube and a built in ballast, having an overall size not much different from the size of an ordinary incandescent lamp. Usually has a screw in base that is the same as for an incandescent lamp. They are not commonly used for street lighting as light output like that of all fluorescents is greatly reduced at low temperatures.

Cutoff -- Refers to the aiming of light down onto the street and reducing of light that can be seen from above. Full cutoff refers to streetlights that direct no light above their mounting level. Non-cutoff refers to streetlights with little or no aiming of light. Semi-cutoff refers to streetlights with a reflector that aims most of the light downward.

Dime -- A temporary insulating device the size and shape of a coin that is placed between a pair of metal strips specifically intended to hold it and which metal strips if they touched would short out or bypass one lamp in a series circuit. When subjected to a certain voltage kick occurring if the lamp burns out, the dime burns through and becomes an alternate current path for the purpose of keeping the rest of the lamps in the circuit operating. Once in awhile the dime (and also the shunt inside a miniature Christmas tree lamp that performs the same function) fails to work and it is then necessary to go from lamp to lamp to find the burned out one.

Envelope -- Another term for the glass or quartz bulb part of a lamp. Mercury and high pressure sodium lamps have a small inner envelope to contain the arc, maintain proper pressure of the mercury or sodium vapor, and maintain the proper temperature using a small enclosed space. The outer envelope absorbs harmful ultraviolet  light or converts it to visible light using a layer of phosphors, limits the cooling down of the inner bulb, and provides safety from the high temperature of the inner bulb.

External Wiring -- Refers to streetlights where the current carrying wires emerge from the luminare and are strung through the air or along insulators mounted on the bracket arm.

Fluorescent -- Emitting light of one color or a range of colors (usually human visible) when light of another color (usually ultraviolet) is shone upon it. The fluorescent lamp would give off no more than a dim bluish glow plus produce a lot of ultraviolet light mostly absorbed by the glass tube were it not for the fluorescent material (called phosphors) coating the inside of the tube. Mercury vapor lamps also produce ultraviolet light and are often coated with phosphors so they give off white light for more natural looking illumination in addition to the bright bluish  (green-purple) glow of the mercury arc.

Foot-candle -- Level of illumination (or amount of brightness) at any given spot on a one square foot area evenly illuminated by one lumen. (A standard candle at the center of a one foot radius (two foot diameter) spherical enclosure illuminates the inside surface of the enclosure to one foot-candle.)

Gumball -- Collector's term for a streetlight for bracket arm or hung installation, having a more or less spherical shape not including a top mounted compartment for the lamp socket, where the upper hemisphere is the reflector and the lower hemisphere is the (glass) refractor. The "standard" size is about 13 inches in diameter.

Halide Lamp -- A mercury lamp containing chemical compounds involving halogens so that the light produced is whiter compared with the green-purple glow from a pure mercury vapor lamp. (Halide lamps and ordinary mercury vapor lamps may not be arbitrarily interchanged even if the wattages are the same.)

Halogen -- One of the chemical elements chlorine, fluorine, bromine, or iodine. Halogen lamp -- An incandescent lamp whose bulb is filled with one or a mixture of halogens rather than being evacuated in order to cause material naturally evaporating from the filament to tend to redeposit on the filament and thus make the filament last longer.

Head -- Generally, the part of the luminaire that holds the lamp socket and mounting hanger or collar. When the mounting collar is part of or attached directly to the reflector housing, as in a clamshell style, that assembly has been referred to as either the head or the body.

HID (High Intensity Discharge) -- Generally refers to a mercury, mercury & metal halide, or sodium lamp. The electric arc ,or discharge, passing through mercury vapor, sodium vapor, etc. produces an intensely bright glow.

Incandescent -- Being so hot as to give off light (glow). The principle behind the incandescent electric lamp is to maintain a component (the filament) in a white-hot glowing state by the use of electricity flowing through it. What Thomas Edison spent a long time in accomplishing was finding a material for the filament that lasted a reasonably long time without burning out and that could be kept hot enough to give off at least yellowish-white light which requires a higher temperature than giving off reddish light. Still higher temperatures result in a whiter light tending towards blue, which is more desirable for indoor photography. The lamp operating this hot has a much shorter lifetime compared with the same lamp operating at a lower temperature due to accelerated evaporation of the filament..

Internal Wiring -- Refers to streetlights where the current carrying wires pass through a hollow bracket arm and on into the luminaire.

Lamp (electric) -- Correctly, the item commonly referred to as a light bulb or fluorescent tube. (The bulb is the glass portion.)

Lantern -- Another term for "luminaire", used in Europe.

LED (Light Emitting Diode) -- A "lamp" which gives off very little heat for the amount of light produced and also has a low resistance for current flowing in one direction and a very high resistance to current that would flow in the opposite direction. They may be built into panels to provide tiny spots or bars of light, or manufactured as individual units typically cylindrical about 1/8'th to 1/4'th inch in diameter and about 1/4'th to 1/2'th inch long.

     Generally the bulb is shaped so that most of the light goes in one direction and therefore the light appears bright when seen from that direction..A typical illumination usage (as opposed to pilot light usage) has many individual units mounted in an array as each unit gives off relatively little light.

Lumen -- Unit of measure of illumination, equal to the amount of light that comes out of the hole resulting from cutting 1 square foot of surface area from a two foot diameter (one foot radius) opaque non-reflecting sphere at the center of which is a candle of certain defined characteristics burning under certain defined conditions. (A dinner table candle is roughly equivalent but I do not know the standard candle's exact measurements, wax material, wick length, or the other defined conditions.)

     A sphere two feet in diameter has approximately 12 square feet of surface area and the total light emitted by the candle is therefore around 12 lumens. Since the candle itself blocks some of the light, the amount of useful light from the candle is more like ten lumens, which many published references use as the subjective definition of one candlepower.

Lumen-second: -- One lumen for a period of one second, or two lumens for a period of one half second, etc. (and then perhaps the light is turned off). For photography, if the light is dimmer, one can leave the shutter open longer, thus the usefulness of talking about a given light intensity for a given period of time.

     A peanut sized photoflash lamp (now obsolete) which burns fine zirconium wire in a clear glass oxygen filled bulb was rated at 8000 lumen seconds. Since the flash lasts for about 1/50 of a second, the brightness of the light, if it could be sustained, comes out to 8000 x 50, or 400000 lumens which is equal to the light of twenty 400 watt mercury streetlights.

Luminaire -- The "streetlight itself" not counting the pole or bracket arm. It consists of at least the head, lamp socket, reflector if any, refractor or glass cover if any, and lamp. Sometimes it contains a ballast and/or photocontrol.

Lux, or Meter-candle -- Level of illumination at any given spot given one lumen as it evenly illuminates one square meter (10-3/4 square feet) of area.

Mantle -- A mesh or similar component in a gas or oil lamp that more or less covers the flame. Its purpose is to be the light emitting component by becoming white hot (incandescent), while not cutting off the oxygen supply or significantly detracting from the burning of the flame. A gas or oil flame does not give off much light by itself, and when the fuel-air proportions are set for maximum light output, a lot of smoke and carbon deposits result. Instead the flame is adjusted for maximum heat and the mantle is placed over it.

     Much research went into the design of mantles both long ago for gas streetlights and in recent years for equipment such as "Coleman lanterns". A mantle had to be of a material that would last a reasonable length of time in a white hot condition without disintegrating. (The same properties plus the ability to conduct electricity but not the being in the presence of oxygen were necessary for an incandescent electric lamp filament.) A typical mantle is the burned out remains of a small cloth bag. It is typically supplied and sold unburned for ruggedness during installation. After the bag is burned at the first usage, the ash skeleton remains intact because of the chemicals the bag was soaked in, but it is still very brittle and fragile. When a hole develops in the mantle as it eventually burns away or is mishandled, the light output is of course reduced due to the reduction of the glowiing surface area. Therefore the mantle needs periodic replacement. (See, also, Wick.)

MH  (Metal Halide) -- See "Halide".

Multiple -- Parallel, q.v.

NEMA -- National Electric Manufacturers' Association, a group of industry representatives who set standards (not always adhered to) for various types of electrical equipment including streetlights and traffic signals. Standards included dimensions (streetlight components), weights, current consumption, and behavior (traffic signal controllers). The "NEMA head" is shaped like a can, about 6 inches in diameter and slightly taller, with two external fins opposite each other near the bottom on which a variety of reflectors can be snap-fitted. The lamp is positioned base-up.

Neutral -- The return path or return conductor in an electrical circuit which in modern circuits must be insulated as if it were hot and kept separate from the ground and "ground wires" until the current gets back to the main power switch for the building or street lighting system.

Non-drip candle -- Wax that drips from a candle and accumulates at the base is difficult to re-use as fuel and is therefore wasted. A "non-drip" candle has an outer layer of wax that melts at a higher temperature. As the candle burns, an equilibrium is maintained where a pool of melted wax feeds the wick and  is held in place at the top by the not yet melted outer edge.

     This pool of wax also keeps the outer layer from getting too hot and melting too soon. Then as the melted wax is consumed and the level drops, the top outer edge no longer has enough melted wax lapping around to help dissipate the heat of the flame. It finally melts, commingles with the pool of melted wax, and feeds the flame. If the flame is buffeted by too many air currents, it will get too close to the outer wax edge, melt it, and allow wax to drip. If the top of the candle is thinner, it is more likely to drip because the outer edge is closer to the flame. If a candle is very thick, the flame will burn down inside and the outer walls are too far from the flame to ever melt and be consumed as fuel. One way to avoid wasting the leftover wax is to melt it in a pot and make new candles from it by hand dipping. Walkways are sometimes illuminated in a decorative fashion using candles placed in paper bags. So long as the flame does not get too close to it, the paper bag won't catch fire.

Ohm's Law -- The relationship between voltage, current, and resistance that is always true; voltage equals current times resistance, or current equals voltage divided by resistance, or resistance equals voltage divided by current. Click here for a more involved discussion.

Parallel or Multiple (as opposed to series) -- Refers to an electric circuit where current can flow to and from each lamp or device without having to flow through another device. No device or lamp is dependent on the integrity of any other (as in Christmas tree lights) in order to operate. A schematic diagram of a parallel circuit might be depicted as a ladder with each rung representing a lamp and the rails standing for "hot" and "neutral" respectively.

Photocontrol -- The device, usually cylindrical and the size of a tin can, that contains a light sensitive element and other electromechanical or electronic components to turn lights on at night and off during the day.

Primary -- Refers to wiring (distribution wiring) typically carrying 2400 to 24,000 volts hot to ground, and carrying power from the substation to step down transformers on street poles or in large buildings where the end user voltage (120, 240, etc. volts) is provided. Streetlights connected in series are usually connected to a primary circuit.

Quarter Moon -- Reflector shaped more or less like an upside down (and short) canoe, with no refractor or covering provided for the lamp inside.

Radial Wave -- A now obsolete reflector style consisting of a circular plate typically about 20 inches in diameter with and ruffles from center to perimeter, suggestive of a ballerina's tutu dress. The lamp protruded a few inches below and was exposed. The underside was almost always white as opposed to polished "silver".

Reflector -- Any polished or light colored object intended to aim (by "bouncing") light in a desired direction as opposed to just block or absorb it.

Refractor -- A transparent panel or dish which also serves as a lamp cover and which has molded ridges to aim by bending (refract) light in desired directions. For street lighting, the ideal is a choice of refractors to produce reasonably uniformly lit areas of specific shapes and sizes. Were it not for the desire to redirect the light, streetlights would have been equipped with simple flat or dished plain glass lamp covers.

Relamp -- Change a light bulb, as in the joke, "how many (xxx) does it take to ... ?"

Remote Ballast -- Refers to streetlight where the ballast is mounted on the pole or in the pole base. Early ballasts for high wattage mercury lamps were heavy, sometimes over thirty pounds, so mounting them in the luminaire was not practical at first.

Secondary -- Power company wiring and circuits, typically carrying 120 to 480 volts, directly feeding the streetlights or buildings, or the "output" winding of a any transformer (used to provide the power at the voltage for actual usage).

Series -- Refers to an electrical circuit where current has to flow through each lamp or other device in turn. This is the way most Christmas tree lights work, where when one lamp is removed, the entire string goes dark. In reality, both with modern Christmas lights and in series streetlight circuits, there are devices in or at each lamp that bypass a burned out lamp to keep the rest of the lights on, and which work "automatically" most of the time but are not perfect.

Slipfitter -- The sleeve or collar on the luminaire that slides over the bracket arm. There is often a U shaped rod inside that is tightened around the bracket arm much like car muffler clamps. Alternatively, bolts tightened from the outside dig into the bracket arm. Usually at least one bolt is provided as a fine adjustment for leveling the luminaire. Fixtures mounted on vertical pipes almost always use screw on pipe fitings and nipples rather than slip fitters to avoid dropping off if the connection should loosen although this writer has seen fixtures that came loose from horizontal bracket arms and hung precariously from their wires.

Steradian -- Unit of measure for a "solid angle" such as the apex of a cone or pyramid. The solid angle is equal to one steradian when the apex of the "cone" or "pyramid" is at the center of a sphere and the base is the intersection with the surface of the sphere and (taken as the portion of the sphere's surface as opposed to flat) the area of the base is equal to the square of the sphere's radius. Example: One lumen is the amount of light emitted from a candle (of the required material and burn rate) in one general direction, specifically within a solid angle equal to one steradian.

Teardrop -- Collector's term for a "gumball" like streetlight where the refractor is somewhat elongated i.e. like a deep bowl. (The reflector is more or less hemispherical.)

Upswept -- Refers to a bracket arm that more or less curves upward out into the street. It generally permits use of a pole that is not as tall or a mounting point on a pole below other items such as electrical wires on that pole.

Wick -- A long, porous, usually fibrous, component in an oil lamp or candle through which the fuel soaks and rises via capillary action to feed the flame at a controlled rate. If the wick goes dry, it itself burns away if it is not made of a fireproof material such as fiber glass. Materials through which the fuel or melted wax cannot soak fast enough therefore don't make good wicks.

     In the case of a candle, as the wax is consumed and the candle shortens, the flame gets taller as the exposed wick lengthens and more smoke is given off. Eventually the wick gets so long that the melted wax soaking it is used up before it gets to the top of the wick and the end of the wick then burns away. To reduce smoke, modern candles have wicks that tend to curl as the wax around them is melted by the flame. When this wick has reached the length where the end goes dry, it points sideways and keeps the flame from getting too tall. Also because the outer sides of the flame are hotter, the excess wick end burns away faster. (See, also, Mantle.)

Xenon Lamp -- There are two kinds,.(1) an incandescent lamp whose bulb is filled with xenon gas permitting longer lasting operation at higher temperature and more lumens per watt. (2) A discharge arc lamp similar to a mercury or sodium lamp but using xenon gas to provide a high intensity white light that is not too yellowish and not too bluish.

Some of the information in this section courtesy of fellow streetlight collector Joe Maurath.

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