Overhead Streetlights: A History and Spotter’s Guide

October 8, 2016 at 1:20 pm | Posted in Streetlights | Leave a comment
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Now that all the overhead streetlights are going to be converted to LED starting this fall, it’s time to take a look at the history of them.

The NEMA Lights

In the early days, every manufacturer had it’s own design. Here’s a beautiful example from Lexington, MA near Battle Green.

Streetlight near Battle Green, Lexington MA

Streetlight near Battle Green, Lexington MA

The pole is decorative, but the luminaire itself, although beautiful, is simply designed to be functional. The white porcelain protects the metal, and the waves add strength to the reflector and help throw light to the sides. This was known as the “radial wave” design and these are highly desired by lighting collectors.

In 1941 all the manufactures got together and developed the NEMA (The NationalElectrical Manufacturers Association) standard. NEMA luminaires standardized the placement of two latches to mount the optics into the rest of the luminaire. This form had two advantages.

  1. An optical assembly for any manufacturer would work on the luminaire base for any manufacturer. If a globe got smashed, a city could just take a Westinghouse globe out and mount it to a GE luminaire, without needing a different stock for every different manufacturer on the streets.
  2. Optics could be changed, i.e., if a new more efficient reflector was developed, or more light was desired and thus a bigger globe was needed, they could just go out and pop in a new, bigger lamp (since incandescent lamps didn’t need specialized control gear for each different wattage) and new optics on the existing luminaire.

Originally there were a wide variety of NEMA optics– “Radial Wave”, “Half Moon”, “Admiral Hat”, “Gumdrop”, “Teardrop”, “Bucket” to name some. Here’s a NEMA “Gumdrop” I own. It has a 327 watt, 4000 hour streetlight lamp in it.


Line Materials NEMA “Gumdrop”

The “Teardrops” were the same idea, but the refractor extended down farther  to accommodate larger lamps for major streets.  This style is now very popular with those gaudy, atrocious “fake history” lights I’ve seen in Chicago, on Lake Street, the Minneapolis parkways, and numerous other places. I get that as much as I like the old mid-century modern lights the installation is now a maintenance nightmare and need to be replaced, but when the teardrops first went up it was the style of the time; they weren’t putting in fake chimneys to imitate Victorian or Federalist style lighting. There’s plenty of designs that while modern, are still decorative.

And an “Admiral Hat”, using an economical open reflector.


NEMA Lights Today

Some of the previous optics attempted to direct the light in specific directions, but the “open bucket” refractor, which blasts light equally in all directions, is the only one still in common use. However, Xcel has recently introduced a dark sky friendly full cutoff reflector for new installations.

Here’s a NEMA light in St. Paul. Uniquely, Xcel supplies the power and the mast, while St. Paul owns and maintains the luminaire. Since they blast light in all directions, they tend to be on very long poles to have them towards the center of the street. With the newer, more efficient lamps, St. Paul and many small towns still use NEMA heads for streetlighting the much smaller lamp size of smaller and medium sized HID lamps enabled them to still fit even with the required control gear.

St Paul NEMA “Bucket” light,

St Paul NEMA “Bucket” light,

A pair of old and new Xcel NEMA area lights, these are a design by GE called the “Power Bracket”. The much shorter length of HID lamps opposed to the large incandescent lamps for which NEMA luminaires were designed enabled the ballast to be stacked on top in the conventional design, but these moved it into short, integrated arms, which eliminated heat rising and building up around it.  More or less the same model has been in production for many decades.


Old and New GE NEMA Area lights. The one on the left is a 400 watt lamp which is the largest that the NEMA form factor could accommodate, which was enough for just about all purposes.

Also notable are what are pejoratively called “yardblasters” by lighting enthusiasts; cheap fixtures from big box stores that look somewhat like true NEMA heads equipped with a bucket, but the refractor is bolted on, they’re made of stamped metal,  and the ballast is usually sketchy; it’s the buzzing mercury light on your neighbor’s garage that’s shining in your window. Although NEMA luminaires were able to accommodate HID lamps, soon came luminaires designed from the start to take advantage of them: the “clamshell” which evolved into the cobrahead.

 “Clamshell” Street Lights

Because of their large circular filaments, incandescent street light bulbs needed to be mounted vertically. But no such limitation applied to mercury  lamps, with their very small arc-tubes. About 1950, manufacturers started designing fixtures specifically for these lamps which were smaller and had an oval shape. Equipped with 400 watt mercury lamps they produced a relatively massive amount of light compared to what came before. Although new mercury installations are now banned due to inefficiency they were much more efficient than incandescents and were good at spreading light out to either side down the street, not into neighbors windows in front of and behind the light. Line Materials was first with their Ovalite, and in what has become the eternal problem for companies putting out a new product with a distinctive name,  all similar fixtures that the competition came out were informally called that. Today “clamshell” is collector slang for them.

The first “clamshells” had odd proportions, as they were marketed as being able to hold an economical incandescent lamp vertically, and then be updated to the expensive but efficient and high-tech mercury lamps later.  The original Line Materials Ovalite and GE Form 109 were two examples of these. Later they revised the design for mercury lamps only, as in the Westinghouse OV-20 and it’s smaller and larger siblings, the OV-10 and OV-35. The medium-sized ones with 400 watt lamps were by far the most common, since expensive mercury lamps weren’t used for local streets and these were the ideal wattage for business districts.

GE Form 109, Bar in Hudson, WI

GE Form 109, Bar in Hudson, WI

Here’s my Westinghouse OV-20. To keep the light somewhat sane in my garage, this uses a 250 watt coated mercury lamp (the original clear tubular lamps it was designed for, now long out of production, produced light somewhat between that and today’s 400 watt mercury lamps).

Westinghouse “Clamshell”

Westinghouse “Clamshell”


OV-20 Mercury Clamshell. The blue box is the external ballast. To keep it somewhat sane in my garage this is only a 250 watt lamp.

Some of these still survive in the field, particularly in small towns and private installations but they are rapidly disappearing with the LED onslaught. Here is one in Lake City, MN in 2014:

Westinghouse OV20 “Clamshell” Streetlight. Not the “paint can” ballast on the top of the pole.

Westinghouse OV20 “Clamshell” Streetlight. Not the “paint can” ballast on the top of the pole.

With advancing technology shrinking the size of ballasts, it became possible to integrate the smallest of the lamp and ballast combinations into the fixture, But this form didn’t last. Instead it morphed into what we all know but may not love, the cobrahead.

The Cobraheads

The first Cobraheads were the Westinghouse Silverliners.

OV25 Silverliner

OV 25 Silverliner, the first Cobrahead model, Great Bear Shopping Center, Bloomington, MN Parking Lot

Like “Clamshells”, Cobraheads generally came in three sizes, small (generally equipped with 100-250 watt lamps but a few are as small as 50), medium (250 and 400 watt lamps), and large (700 and 1000 watt lamps). The large sizes are essentially extinct; I don’t know of any in the area and the fixtures have been out of production for decades. A 400 watt sodium lamp produces as much light as a 700 watt mercury lamp and can be accommodated in a medium fixtures, and generally you want a different form-factor, like a high-mast tower, for 1000 watt sodium lights. This area tended to be conservative and use medium-sized fixtures for 250 watt lamps, the largest size found on local streets and the smallest on the freeways.

Here’s my 1960s GE M250R Cobrahead, showing what they look like on the inside:

1960s GE M250R Cobrahead Streetlight

1960s GE M250R Cobrahead

Although the idea of a cobrahead was to integrate the control gear, they were available without. Perhaps this was for direct replacement of “clamshells”, or utilities had surplus ballasts lying around or they just wanted to be able to procure them separately. Here’s a remote ballasted OV-25 on Stinson Blvd., Minneapolis, where the ballasts are in the bases. Such an arrangement isn’t possible with high pressure sodium and pulse start metal halide lights, because they require a high voltage pulse to start so the control gear must be near the lamp.

Westinghouse Silverliner OV25 Streetlight

Remote Ballasted OV-25. Probably the oldest Cobraheads still on the streets of the area.

A Spotter’s Guide to Cobraheads

Recognizing cobraheads is challenging even for experts since the differences are subtle and the same model (especially true for GE) might have different molds.  This is not an exhaustive list, but includes some of the most common models in the area.  A few city owned lights on metal poles are older, but none of the wood pole lights are older than the mid-1980s Northern States Power Company conversion from mercury to sodium.  In all cases, looking at the lens is not useful for identification; most models could be ordered with several kinds . Although small fixtures could be equipped with 250 watt lamps, our area was very conservative and used medium luminaires for that wattage, they are almost gone from the freeways but can still be found at larger intersections.

General Electric Luminaires usually had their logo on the bottom and sometimes have a two piece door. The control gear is mounted to the smaller door facing the pole. This “Powr/Door” was a marketing tool, advertised that if you wanted to buy cheap mercury lights you could upgrade to the (at the time) more expensive sodium lights (or even some wonderful technology that hadn’t been invented yet), you would just have to replace the door rather than the whole fixture. In practice, this was rarely done since the cost and work with replacing the entire fixture wasn’t much more.  Small models were known as the M250 series, medium models were the M400 (designating the largest wattage lamp they could use); “A” denoted a two piece door; no suffix and later “R” denoted a one piece door.

There are a few pre-1986 M250As in city owned installations like this one:

M250A GE Streetlight

pre-1986 M250As

M250A2s, produced from 1986 on, have a very boxy look:

GE M250A2 Streetlight


The single door version, the M250R2 was produced from 1985 on:

GE M250R2 Streetlight

GE M250R2

Another M250R2 with a flat lens, these are the ones I see Xcel putting in now if a new fixture is needed:

GE M250R2 Streetlight


Here’s a old  M400A, produced 1967-1985:

GE M400 Streetlight

GE M400

In 1986 the M400 models were replaced by the M400A2 and M400R2. Here’s an R2; simplified but similar looking:


GE M400R2

In  1997 by the M400R2 was replaced by the very different looking M400R3, also known as the MSRL.  There is also the two door A3 version but it there are any around they are rare. In 2008 production of the M400R2 started again and then was discontinued very recently:


GE M400R2

Cooper Industries luminaires are very “fat” for lack of a better term. They all are to some degree, but the model OVX was chunky to an extreme.

Cooper OVS Streetlight

Cooper OVS

Cooper OVZ Streetlight

Cooper OVZ

The model OVZ, which looks virtually identical, replaced the OVS. In 1992 the model OVX was introduced, which this area seems to have switched to rather than the OVZ.  The Cooper OVX was somewhat notorious because it was a small fixture that could be equipped with medium-sized 400 watt lamps, which tended to overheat and prematurely fail.

Also of note was the L-250, as used by a few non-NSP/Xcel utilities in the area.  This was designed as a Westinghouse model, When Cooper bought Westinghouse’s streetlight business in 1982 they marketed it for a while under the Crouse-Hinds (another company they owned) name:



American Electric luminaires, are very generic looking. The model 13 was generally a very light grey, appearing almost white in the sun, and had a blunt end:

Amercian Electric Model 13 Streetlight

Model 13

In 1988 the model 113 replaced it. These were much more rounded and as ordered in this area had polycarbonate refractors that tended to yellow. One identifying feature of both is the metal extends slightly down encircling the refractor. In 2003 the very similar model 115 replaced the 113. I’ve not noticed any 115s in the area since Xcel tended to use GE fixtures by that time, the difference is the 115 had a bulge where the “10” sticker is in this photo.

American Electric Model 113 Streetlight

Model 113

The medium-sized American Electric lights were the model 25 and it’s replacement the 125. A quick way to distinguish an AE 125 from a very similar looking GE M400R3 is the door of a 125 is tapered to meet the back of the housing:

American Electric Model 25 Streetlight

Model 25 Streetlight

American Electric Model 125 Streetlight

Model 125

Also by American Electric is the small DuraStar 2000 and its medium-sized sibling, the Durastar 3000, made of polycarbonate rather than aluminum. These saw quite a bit of use in the southern part of the state as well as northern Iowa. Although anything but “Dura”ble after baking in the hot sun for decades, these were extremely light and easy to mount, the neck would mount to the pole and then the rest of the luminaire would twist on. Also odd is the external ignitor, the black box on the near side. This was a sold as a feature so you could replace it without even opening the door, but in practice it took all of  10 seconds to open the main door which you would do to try a new lamp first, and if a new lamp and photocell doesn’t fix it, industry practice is just to junk the fixture at that point anyway.

American Electric Durastar 2000 Streetlight

Durastar 2000

Near as I can tell 90% of the cobraheads on surface streets in the area are GE M250R2s, American Electric 13s, or Cooper OVSs, these being apparently what NSP used for the mass conversion; the American Electric 113s and OVXs not existing at the time but used for later replacements.

 A Couple of Non-Cobrahead luminaires

The first High Pressure Sodium Lamps on metro freeways used an open cone shape refractor to accommodate the larger 250 watt lamps. These were found on I-35W south of downtown and I-35E north of downtown. The other mercury lights on the freeways were not converted until around 1990.


Cone Streetlights


Cone Streetlights

Holophane, now owned by the parent company of American Electric has made glass for streetlights forever, and they also started making streetlights in order to sell more glass.:


Holophane Streetlight

St Paul used lights with the same style refractor but a much large head:


St. Paul Streetlight

Here’s an unusual, unknown light from Rochester, MN:


Rochester Streetlight

Holophane also makes “fake history” “teardrop” models; the Minneapolis Parkway Models are Holophane MPU “Memphis” luminaires combined with some of the optional attachments:


Memphis Streetlight

And the rarest of the rare, a “Turtle”. I saw this on a side street in Aurora, MN and posted a photo to a lighting message board, asking “what the heck is this?” Turns out it’s a UK model, GEC Z8422 , and everyone was amazed at one turning up in the United States. A small number of low pressure sodium vapor models were exported to Canada, but this was the first mercury vapor model seen in North America, and the first of any model in the US. One can only wonder how on earth it ended up there.

GEC Z8422


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Streetlight Poles, Lamps, and Control Gear

October 7, 2016 at 1:52 am | Posted in Streetlights | Leave a comment

Here is a guide to street lighting pole types and the lamps that they use.

Types of Poles:

Davit Poles: These are the Mn/DOT standard, and although not used by most other states, a few do, like Rhode Island and North Dakota. They are actually two pieces, the top piece slips over the bottom and curves between the pole and luminaire, giving a modern and seamless appearance.

Some poles are mounted on the concrete in the median, and some are on the sides of the road.  Lights in the median are more economical to install, make use of wasted light emitted towards the back of the fixture, are less vulnerable to knockdowns. However they must be maintained on the left side of the road, and lane closures are required if there is not a full shoulder. Current practice is to mount them on a concrete median if one exists and there is a full shoulder.


Relamping a luminaire on a davit pole, Bloomington.

Tenon Poles (“Turnpike” Style): These use poles farther back (less likely to be hit by texting drivers), with floodlights aimed forward at about a 45 degree angle. For a while Mn/DOT liked these due to reduced maintenance, but they caused a lot of light spill into neighboring residential neighborhoods, and equipping them with shields reduced their effectiveness. They’ve also been problematic to convert to LED, but now they figured out that a standard LED fixture pointed down will work for roads up to four lanes, and have found special fixtures for wider applications.

“Turnpike” luminaire on I-494, Plymouth

“Turnpike” luminaire on I-494, Plymouth

Bent Straw Poles: In some areas a more decorative look is desired. Mn/DOT uses bronze “bent straw” pole poles with square “shoebox” luminaires to provide something somewhat better looking than standard davit poles while still being efficient and easy to maintain. Notable locations are the I-35E “Parkway”, The Wakota Bridge, and MN 36 and Stillwater Blvd.

Bent Straw Installation on MN 36, Stillwater

“Bent Straw Installation” on MN 36, Stillwater

Truss Arm Poles are the oldest poles that can still be seen occasionally in Minnesota.  Mn/DOT has not installed new ones since the 1970s and almost all have been replaced, but there’s a few stragglers around. These came in both the straight and curved varieties. Also, the Hudson Bridge, built by Wisconsin, has straight truss lights on both sides of the river (While the St. Croix Crossing, built by Mn/DOT, will have davit poles). Generally truss arm poles are used in the eastern half of the country while a different style, called the “upsweep” style is used in the west.


Curved Truss Arm, Bloomington.

Straight Truss Arm Pole, Shakopee

Straight Truss Arm Pole, Shakopee

High Mast Luminaires are the ones on those super tall 100-120 foot poles. Mn/DOT has traditionally liked them due to ease of maintenance (the fixtures are mechanically lowered on a pulley for relamping) and they give a smooth, even light enabling drivers to easily discern complicated interchange. After complaints about light pollution, Mn/DOT now avoids installing them in residential areas, and in fact has replaced one (at MN 62, US 169, and US 212) with davit poles.


High Mast Lighting

High Mast light in front of Best Buy.

High Mast light in front of Best Buy.

Types of Lamps

Incandescent. Although we all know about incandescent lamps, there were some differences in the lamps used in early streetlights. They cared more about life than efficiency, so they were under-driven to the point of having a decidedly yellow cast. And they cared about lumen output rather than watts, so they have even lumen ratings, like 2000 and 4000, and weird watt ratings, like 327.


Line Materials NEMA “Gumdrop”

Rather than line voltage, many incandescent streetlights, in a blocks-long version of Christmas lights, were run in series, the supply voltage was around 2000 volts with each lamp running at 50 volts. This controlled voltage drop for the enormously power hungry incandescent lamps. Early models had porcelain insulators but improved wire insulation later enabled them to be wired through the luminaire neck like paralleled lights. Obviously you didn’t want the street to go dark if a lamp burned out or was removed, so these had “Jones Sockets”, so 1) If the bulb was removed, a spring in the mogul socket shorted the contacts together, 2) the mogul socket itself fit into another special socket. The receptacle in the base would short if the socket were pulled out, and in between the prongs was a clay disc to short out if the lamp burned out.


Jones Socket Patent Drawing

To relamp, you would pull the socket out, replace the shunt, screw in a new lamp, and push the socket back in, all without affecting the rest of the street. My light was originally a series string so I pulled out the shunt and replaced it with a piece of credit card wrapped several times with electrical tape

Since the 1950s, arc discharge lighting, and then High Intensity Discharge (HID) lighting has been predominate for lighting streets, due to their compact size, efficiency, long life, and lack of concern about color rendering. Light is produced when electrons are bumped outwards away from the nucleus due to the input of energy, when they fall back into place they emit the extra energy as light. In fire and incandescent lamps the energy comes from heat; HID lamps are much more efficient using an electrical arc directly.

Low Pressure Sodium Vapor: Commercialized in the 1930s. these arc discharge lights use sodium metal (combined with neon and argon to get it started). They are bulky and produce a pure yellow light. Astronomers like them because if they see something pure yellow in their telescope they know what it is. These were used years ago around Met Stadium, and on major north-south streets in Richfield. Mn/DOT never liked them because it was difficult to control the light. A surviving installation is at the Thunderbird Hotel parking lot in Bloomington. Another notable use is bridges over the Mississippi, for example the “Big Blue” bridge in La Crosse, where the yellow light doesn’t attract mayflies.

Original Low Pressure Sodium Light from the Golden Gate Bridge, at the Visitor’s Center

Original Low Pressure Sodium Light from the Golden Gate Bridge, at the Visitor’s Center


Low Pressure Sodium Light, San Jose, CA

High Pressure Sodium Vapor: In the 1960s High Pressure Sodium was  (HPS) was developed. These have a golden hue and are now the de-facto standard for street lighting. The gasses are at higher pressure and mercury is added to decrease the size needed and add other colors than pure yellow to the spectrum. Usually 100 and 150 watts are used for residential streets, 200, 250, and 400 for wider streets and freeways, and 1000 watts for the 100 foot towers on the freeways. The 1000 watt lamps are roughly 8 times as efficient as household incandescent. The initial HPS installations in the Twin Cities that I recall, from at least the 1970s, were I-35W in south Minneapolis. I-35E north of dowtown St. Paul, and Lyndale Ave in Bloomington.  New installations from around 1980 on were HPS, with mass conversion in the early 1990s.

As a side note, Europe developed “White SON” for indoor use, which drives high pressure sodium hard enough that it produces a pleasant incandescent like white color common in shops, but it was never common in the US and even in Europe was less common than ceramic metal halide. White SON is more expensive, less efficient, and shorter lived then regular high pressure sodium so it was never used for street lighting.


White “SON” lamp, note European style lamp with small, high strength glass instead of the huge bulging shapes of US lamps.

18 Watt Low Pressure and 35 watt High Pressure Sodium Vapor lamps. These are smaller than are used for street lights, more typical for small wall mounted outdoor lights. They each produce about as much light as a 100 watt incandescent.

18 Watt Low Pressure and 35 watt High Pressure Sodium Vapor lamps. These are smaller than are used for street lights, more typical for small wall mounted outdoor lights. They each produce about as much light as a 100 watt incandescent.

Closeup of a new Low Pressure Sodium lamp. The silver drops are sodium metal. Since sodium is violently reactive and will attack stadard glass, water, air, and just anything else the inner tube is made of the same material as laboratory beakers.

Closeup of a new Low Pressure Sodium lamp. The silver drops are sodium metal. Since sodium is violently reactive and will attack stadard glass, water, air, and just anything else the inner tube is made of the same material as laboratory beakers.

Mercury Vapor.  Dating from the early part of the last century and refined in the 1930s, mercury vapor gives a bluish-green light, unless a red phosphor has been added on the color enhanced version, which gives them a reddish tinge.  Mn/DOT has long since eliminated the last of these, but older installations still exist on some city maintained lighting and on older Minneapolis parkway fixtures. Mercury Vapor, while extremely long lived, is relatively inefficient compared to other HID bulbs. New ballasts have been banned for several years, and new bulbs will be banned starting in 2016. Although not describe as such, standard fluorescent lighting can be though of as “low pressure mercury vapor”.


Mercury Vapor Light

Metal Halide is a refinement of mercury vapor.  Developed in the 1960s, Metal Halide adds additional substances to improve color rendering and efficiency at the cost of bulb life. They last 10,000 hours, or half that of other HID lamps.  Metal Halide was always the choice for car dealerships and indoor applications where color was important; in the 1990s as the designs improved and before the LED invasion it became much more common for general outdoor use. Newer Minneapolis parkway fixtures are metal halide. A further refinement is ceramic metal halide that produces a warm, pleasing light. They’re not used here on highways here but Chicago had been adopting them.

Metal Halide decorative lights on the Blatnik Bridge, now replaced with LED lights that illuminate the bridge internally.

Metal Halide decorative lights on the Blatnik Bridge, now replaced with LED lights that illuminate the bridge internally.

Fluorescent and Induction Street Lights

The ubiquitous fluorescent tube (which can also be thought of as “low pressure mercury vapor) was also once used for streetlighting. Municipalities were eager to discard the old fashioned looking and inefficient acorn lanterns and “teardrop” overhead lights in their downtown districts in favor of these bright, streamlined luminaires. However their bulk, lack of control over the broad diffuse light sources, and poor cold weather performance eventually led to their extinction for streetlighting. All the major streets in Minneapolis were once lit by fluorescent luminaires but these were all gone by 1990 and no lighting enthusiast knows of any in service or in private collections.  The last ones I saw in the field were in Waconia in the late 1990s.


Fluorescent Streetlights in Spooner

A modern development has been induction lights. These basically use a high frequency magnetic field to put energy into the lamp. They’re very efficient, more compact than standard fluorescents, and last a very, very long time because they lack a starting electrode, which is the usual failure point on conventional fluorescents. Like fluorescents, they give a very diffuse, omnidirectional light. They’re good for lanterns, good cold weather performance, less good for overhead lighting where you want it focused down. There’s a test installation in the lanterns on 46th street west of Hiawatha, and a few cobraheads near the river in Hudson. Had LED technology not progressed we would be seeing a lot more of these.

Jersey Series Induction Light

Jersey Series Induction Light

Control Gear for HID Lighting

With the coming of High Intensity Discharge (HID) lamps (mercury vapor and it’s relative metal halide, and high-pressure sodium vapor) these were much more efficient, but required other components in or near the luminaire to operate. The problem is that they appear as a dead short when run directly off the mains voltage, so a device to limit that current and reduce the voltage is needed. Originally this was just a ballast, but with newer HID lamps also requiring ignitors, the term “control gear” is sometimes used.

The simplest ballast is a two-wire choke ballast, a coil of fine wire wrapped around an iron core many times. This both limits current and reduces the voltage by a substantial, fixed ratio. More complicated ballasts are 4 or more wire autotransformer types. They are more flexible in input vs output voltage and often have multiple taps for different input voltages. When designing the lamp plus ballast combination, there is a trade-off vs the optimal arc voltage for the lamp vs what voltage is obtainable from the mains with a simple choke ballast. Mercury vapor lamps were designed in the UK with 240 volt mains. With an arc voltage around 100 volts, they require an autotransformer in the US on 120 volts, although many older streetlights used chokes with a 240 volt supply. Here’s a 240 volt choke mercury ballast from my 1960s streetlight.


240 Volt only choke type ballast

High pressure sodium lamps were designed in the US and the smaller ones (150 watts and less) can use choke ballasts with an arc voltage of 55. That would be inefficient in Europe, where 120 volts is unavailable, so they designed their own sodium lamps for 100 volts (and for good measure called them “SON” lamps and used smaller, higher strength glass envelopes). Larger sodium lamps  are designed to use autotransformer ballasts only to have the most efficient arc voltage for each size lamp regardless of mains voltage and the additional cost of an autotransformer ballast is less of an issue.

Even though there’s little resistance when a lamp is hot, there is significant resistance when it is cold. Mercury and older metal halide lamps use a starting electrode; when the lamps starts, the arc forms between two electrodes very near each other on one end, and eventually the end heats up enough to form the arc from one end to the other.  Newer lamps use a several thousand volt zap to initialize the arc, generated by the ignitor.

Here’s the gear of an American Electric model 315 cobrahead. The  ballast is at lower left, the ignitor is the circuit board at the lower right. At top is a capacitor for power factor correction. This is important to utilities and businesses (who the utilities charge if it’s not corrected) but not strictly necessary for lamp operation and is omitted in inexpensive fixtures for home use.


American Electric HPS Control Gear

The most modern ballasts use switching power supplies rather than heavy magnetics to produce the desired current and voltage. These are now standard for fluorescent lamps, but were never adopted for HID use in the US except for interior ceramic metal halide fixtures.


As fixture costs have gone down the energy efficiency and long life of LED fixtures are becoming attractive. The I-35W bridge was notable as the first use of LED lighting on an interstate highway in the country. At the time fixtures that were direct replacements for conventional luminaires weren’t available so they had to space the poles closer together.


Early Mn/DOT LED light,

With Xcel Energy and Mn/DOT converting most of their fixtures to LEDs, the last remaining ones will be  high mast fixtures,  where an LED fixture that can put out enough light that has been fully evaluated and approved by Mn/DOT does not exist yet. (A LED high mast setup is being tested at US 61 and I-494, but it does not equal the brightness of the conventional fixtures).  Maine has had a retrofit made that does equal the light output, but some agencies are leery about modifying existing fixtures due to UL approval issues.


High Mast LED test setup, US 61 and I-494.


High Mast LEDs, Houlton, Maine

Also excluded are the tunnels, which are relatively new installations using fiber optic light pipes to provide a very even light.

There have been some issues. At one point I noticed about a fifth of the lights on the I-35W Minnesota  River Bridge were out. Mn/DOT told me that they had been hit with a power surge and the power supplies were blown out. Being a test installation they had to have more shipped in from the manufacturer. HPS fixtures with their magnetic ballasts are essentially immune to power spikes. With the old sodium fixtures they would not go around chasing individual burned out bulbs, but their intent is to address individual LED fixture failures due to warranty issues.

The Health Effects of LEDs.

Recently an article came out claiming LEDs were detrimental to human health. My friends kept emailing me about the article, and my official response is “I don’t know”. First of all I’m a bulb geek, not a doctor. Secondly, it’s not clear if they’re objecting to the light blue “ice white” tint of LEDs, or that pure blue makes up a large part of the spectrum. (LEDs are essentially blue emitters with yellow phosphors, the mix of those appears white to us. If it’s the former, we have a lot of other bluish lights around, in our offices and computer screens and whatnot, to say nothing about the fact that mercury vapor streetlights were once extremely common, so I’m not sure why they’re suddenly sounding the alarm. If it’s the portion of blue in the spectrum, they seem to be advocating the use of “warm white” LEDs, which also contain pure blue, just more yellow to mask it.


LED Spectrum. Older mercury based light sources had more greens and purples instead of blues and yellows.

I’m old enough to have remembered the transition from the moonlight glow of mercury vapor to the golden hue of sodium lights, and now back to the blue of LEDs.

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