The LED Streetlights Are Here

The Dawn of Light Emitting Diodes (LEDs)

Previously I covered the cobrahead streetlights of the Twin Cities. Well, they’re almost gone. The thousands of high pressure sodium MnDOT lights on the freeways have been converted to LED (light emitting diodes) and the 90,000 Xcel energy lights are in the process of being converted. So it’s time to revisit the topic. But first a background in LEDs.

H.J Round (1888-1966)

Just about any material will produce light if you put energy into it. The heat of fire makes surrounding air glow. The heat of an incandescent filament. An electrical arc through certain gasses in fluorescent tubes. In the electrical world, most materials either conduct electricity very well (ionized water, metal) or very poorly (plastic, glass). But there exists materials between these that sort of conduct electricity called semiconductors. These of course are the basis for transistors and integrated circuits, but as early as 1907, H. J. Round, an employee of Marconi Labs of the UK, noticed a certain semiconductor would light up when electricity was applied. He simply wrote a few paragraphs about the curiosity and moved on with his work.

Oleg Losev (1903-1942)

The first real interest was shown by Oleg Losev of the Leningrad Central Radio Laboratory in 1927. He proposed the theory on how LEDs worked and proposed some practical applications, as well as inventing solid state amplifiers. However this was greeted with yawns by the Soviet government and scientific community. Semiconductor material at the time needed to be imported from the United States while light bulbs and vacuum tubes could be made domestically. And it as hard to see how a quirky way to produce a dim glow could have military, industrial, or propaganda value. Losev died during the siege of Leningrad and his burial place is unknown.

Nick Holonyak (1928-    )

We now jump across the ocean to Syracuse, NY in 1962.   Nick Holonyak, an engineer at the GE research lab, was trying to create a red laser, and instead created an LED. This time the inventor, the company, and the scientific community knew what they had and it’s potential. (And Nick Holonyak is still alive to see what it has become — he’s currently on the faculty at the University of Illinois-Urbana Champaign). Shortly afterwords, the National Association of Science Writers wrote

The latest dramatic laser discoveries, made by General Electric, may someday make the electric light obsolete… If these plans work out, the lamp of the future may be a speck of metal the size of a pencil-point which will be practically indestructible, will never burn out, and will convert at least ten times as much current into to light as does today’s bulb.”

Or as Holonyak himself more bluntly put it

“We knew [expletive] well what happened and that we had a very powerful way of converting electrical current directly into light. We had the ultimate lamp.”


“I knew that it was a very powerful thing and that these materials will become a source of white light. I thought it might be a decade. Little did I realize that it would take much longer than that.”

It did indeed take a while, but it came. For technical reasons, longer wavelengths such as red and infrared are easier to make (that’s why red is still the default color when you buy say a $1 USB adapter). So although yellow and green shortly followed, that was all that was available for decades.  Blue LEDs did not come until the early 1990s. Back then, I recall paying several dollars each for a couple and being amazed by them. The inventors, Isamu Akasaki and Hiroshi Amano of Nagoya University and Shuji Nakamura, employee at Nichia chemical, were award the Nobel prize in 2014.

But what about white? The issue is white is basically all the colors, or at least the mix of several of them, and LEDs can only produce one color. But there exists things called phosphors, which absorb light and then re-emit it at a longer wavelength. These had been used for many years; a fluorescent tube produces almost all ultraviolet light so phosphors are used to shift it into the visible spectrum. That’s why you never see a clear fluorescent tube (aside from special germicidal lamps). Shifting some of the blue light to yellow makes an LED look white.

Potential Health Effects of LEDs

The conversion to LEDs has not come without controversy. Some people subjectively prefer the warmer orange tones of incandescent or high pressure sodium lighting, but more serious issues have been alleged. Remember that LEDs emit a lot of blue light? The American Medical Association claims LEDs are hazardous to our health!

High-intensity LED lighting designs emit a large amount of blue light that appears white to the naked eye and create worse nighttime glare than conventional lighting. Discomfort and disability from intense, blue-rich LED lighting can decrease visual acuity and safety, resulting in concerns and creating a road hazard.

In addition to its impact on drivers, blue-rich LED streetlights operate at a wavelength that most adversely suppresses melatonin during night. It is estimated that white LED lamps have five times greater impact on circadian sleep rhythms than conventional street lamps. Recent large surveys found that brighter residential nighttime lighting is associated with reduced sleep times, dissatisfaction with sleep quality, excessive sleepiness, impaired daytime functioning and obesity.

Being both a “bulb geek” and working in the health insurance industry, my email and Facebook were blown up by people wanting to know what I thought of this when it came out. My official response then and now is “I don’t know”. I’m not a doctor or scientist. My initial take was to note there’s a lot of blue lights around- the fluorescent lights in your office and at the store, even the computer screen or phone you’re reading this on.  And prior to the mid 1980s, just about every streetlight in the Twin Cities was the bluish mercury vapor or fluorescent. Surely we’d know by now if it was harming us. Then again, we thought asbestos was an OK idea too.

First a primer on color temperature. How “warm” or “cool” a light source is measured in “K”, short for “Kelvin”, with higher values equaling cooler tints. 

Remember, due to the need to use phosphors to convert some of the light from blue to yellow, the bluer the light is, the more efficient and inexpensive it is. Unlike an incandescent, where the color temperature was set due to unrelated technical reasons, LEDs could be made at any color temperature, but for commercial reasons are usually set to mimic legacy light sources. Pure blue light is obviously unacceptable, so the first LED streetlights were 5000K. But they quickly compromised to 4100K or “cool white”, which is what the bulk of the new streetlights going up in this area are.

At the same time there’s many dozens of household sockets for every streetlight, and consumers for the most part want LEDs to mimic the yellowish incandescent lamps even thought there’s no technical reason to do so. Thus there’s an intense commercial motivation to develop better warmer phosphors. (And for that matter, consumers also want LEDs to mimic the look of a glass bulb too even though there’s no technical reason to. The newest LEDs even come in [breakable] glass the exact shape of an incandescent).

Thus the penalty for using warm white streetlights is not nearly as much as it used to be. San Francisco has adopted warm white as policy; Shakopee is allowing developers an option to use them. Minneapolis is testing them out. Davis, CA is even yanking 650 cool white fixtures and replacing them with warm white. But what we have is inertia as well as the fact that if you’re buying thousands of fixtures for conversion projects and paying to run thousands of fixtures, every little bit helps.

Warm White Streetlights being tested on Plymouth Ave.

Warm white streetlights being tested on Plymouth Ave. Compare the tint on the streetlights to the lights on the building to the right and in the back center.

A Spotter’s Guide to LED Streetlights

LED streetlights vary by shape — some of them are round to mimic the cobraheads they replaced, even though there’s no technical reason to. Cobrahead reflectors were round, so the most efficient use of material to enclose them was rounded; nevertheless they were sometimes made square for aesthetic reasons. By contrast, an LED array is fundamentally square. Some fixtures use exposed LED panels with no further optics, while some add internal or external optics.

American Electric Autobahn ATBM

These are rounded fixtures with the emitters clustered into groups with supplemental glass optics that resemble large beads, one of two fixture types used by MnDOT for the mass freeway upgrade. An optional plexiglass diffuser can convert it into an omnidirectional light for security applications or as a replacement for agencies that prefer NEMA area lights. I personally do not like these, as these seem to glare much worse than other designs. With MnDOT, the sticker denotes the mounting height in feet, either 40 (250 watt equivalents) or 49 (400 watt equivalent).

American Electric Autobahn ATBM on I-35W, Bloomington

American Electric Autobahn Streetlight with refractor in place

American Electric Autobahn ATBM streetlight with refractor in place, southern MN


Cooper Navion NVN streetlight, I-35W Bloomington

The alternate MnDOT standard fixture, these have a more traditional layout of individual emitters, no supplemental optics, and a square shape. The vast majority of the freeway fixtures are these or the Autobahns.

GE Evolve

These are rounded use supplemental optics. These are the ones being used by Xcel for their mass conversion of their Minnesota wood pole lights. The sticker is a code for the wattage equivalent: “B” = 100 watt, “C” = 150 watt, “D” =200-250 watt equivalents.

GE Evolve Streetlight

GE Evolve streetlight, Bloomington

Leotek GreenCobra

I’m not a fan of the aesthetics of these; they bear an unfortunate resemblance to a common plumbing fixture and using a rounded fixture to mimic a cobrahead with an LED array results in wasted space on the bottom and blank areas. But there’s nothing wrong with them technically and the output can be field adjusted so the same fixture can replace multiple wattages of HPS and thus a city has fewer fixture types to stock. These are used by the city of Minneapolis and Richfield.

Leotek GreenCobra in Minneapolis

Leotek GreenCobra streetlight in Minneapolis

Philips Roadfocus

If you travel to our neighbor to the east, you’ll see a lot of these. They were used on the Wisconsin side for the Xcel conversion.

Philips Roadfocus streetlight, Hudson WI

Philips Roadview

Another one that uses LED arrays with no supplemental optics. MnDOT ceased installing these since they use substantially more energy than the alternatives, but these are still used some by local agencies including the city of Shakopee. Quite a few did make it onto the metro highways before the current mass conversion, particularly in the northeast quadrant of the metro. These look very similar to the Cooper NVN. The way to tell is that Coopers are more rectangular and when looking at them straight on have three vertical fins on top instead of two; the Cooper emitters are broken up into three sections arranged in a row while the Philips appear as four sections arranged in a square, and the top of the electronic housing of the Philips is curved but the Cooper is squared.

Philips Roadview Streetlight near the St. Croix Crossing

Philips Roadview streetlight near the St. Croix Crossing

The Dawn of a New Era

And finally, a mention of the lights of the I-35W bridge. These were the first LEDs used on an interstate highway system anywhere (and they had to space the poles closer together than was standard). I’m old enough to have seen the transition from the bluish mercury vapor lights to the gold of the high pressure sodium, and now I’m seeing the nightscape of the city go back to blue. I still remember a few warm white incandescent lights in Saint Paul and small out-of-the way towns; perhaps I’ll see the current crop of LEDs be replaced with warm white too.

I-35W Mississippi River Bridge streetlight


About Monte Castleman

Monte is a long time "roadgeek" who lives in Bloomington. He's interested in all aspects of roads and design, but particularly traffic signals, major bridges, and lighting. He works as an insurance adjuster, and likes to collect maps and traffic signals, travel, recreational bicycling, and visiting amusement parks.

9 thoughts on “The LED Streetlights Are Here

  1. Walker AngellWalker Angell

    Great post Monte.

    One potential bit of the health problem isn’t necessarily just too much blue but gaps in large bits of the visible and invisible spectrum. Sunlight (and reflected moonlight) produces continuous light across the full spectrum. Incandescents mostly do so as well. Fluorescent lamps lack bits of the spectrum near magenta and have dropouts elsewhere which can cause headaches and other problems for some people. Some of the headache problems are direct from the light but most are believed to be from significantly increased eye strain attempting to distinguish colors.

    LED’s are much better than fluorescent but are still rarely full spectrum. There is a huge difference in a 2700k incandescent and a 2700k LED (and between manufacturer A’s 2700k LED and manufacturer B’s 2700k LED). Both have the exact same color temp but the LED is lacking bits of spectrum (except for a few very expensive units used for cinematography). These spectrum gaps can cause headaches and other health issues as well as make some colors and surfaces look unnatural.

    There have been attempts at using CRI (Color Rendering Index) to allow better comparisons but so far not CRI measurement has been found to suffice. This proved particularly problematic when LED manufacturers tuned their lamps to specific CRI tests. Other tests such as the TM-30 series are hoped to provide for better comparisons for both consumers and commercial applications.

    Another issue is that some people with poor vision can have difficulty seeing some things under LED light. Some hues and saturations can be indistinguishable from others. Some people with macular degeneration who can still see fairly well cannot distinguish curbs or other undulations under some LED’s for instance.

    1. Monte Castleman Post author

      I tried to avoid getting sidetracked into a libertarian rant against the incandescent light bulb ban, but yes, I agree that LEDs are problematic for some people. An aging friend from California, where due to energy prices and environmental regulations LEDs came sooner and are more pervasive complains that he “can’t see as well” with the LED streetlights. And household LEDs give both my sister and I headaches, as did CFLs. As a bulb geek I went out and bought a screw-in LED for $40 as soon as they came out, but was reluctant to actually use them.

      We’ve kind of settled on using LED and fluorescent for places where they’re left on a lot but people don’t spend time (interior hallways), or where we need a lot of lumens a lot of the time but supplemental incandescent or halogen task lighting is available (home office, kitchen). Places where a moderate amount of light is needed, and they aren’t turned on a lot, and dimming is desired like the bedrooms, the dining room, and living room are incandescent or halogen.

      1. Walker AngellWalker Angell

        At a former office we played around a lot with the problems people were having with fluorescent. Interestingly just a few 100w incandescent ceiling lamps made a huge difference for everyone. For those with significant problems, being located directly below an incandescent or having a 60w lamp on their desk often solved any issues. We also worked with a local architectural engineering firm to install some solar tubes to bring more sunlight in during the day.

        Cree and LIFX are both working on LED lamps with more uniform spectrum and fewer spikes. It’d be nice to see this with street lamps as well.

        BTW, on a somewhat related note… The Netherlands does a lot of experimenting with lighting. How to light paths and bikeways so that they are safe for riding, socially safe, and don’t negatively impact those nearby. One of their better solutions is a light green LED. Does a good job of lighting bikeways, objects can be fairly well seen by users, and has less negative impact on neighbors than other colors.

  2. melissa schmitt

    My son is very interested in street lights, LED’s in particular since our city is switching them out this summer. Thank you for writing this article and the others on this site. It has answered all the questions he keeps asking me that I have no answer for. Your pictures are great and the thoroughness is wonderful. He and I were making a shutterfly book about street lights this afternoon and stumbled upon this site while we were researching. What a gem!

  3. GlowBoy

    My Portland neighborhood got converted to LED streetlights about the time we moved here, so we got to see the difference it made.

    Of course the more neutral light color was an improvement.

    One oddity was that because the fixture heads had 6 (or maybe 8?) separate emitters, every tree branch and wire made six shadows on the ground. Not really a problem, just a little weird until you get used to it.

    A larger annoyance was that they were quite a bit brighter than the sodium lights they replaced. We were on a corner lot adjacent to a streetlight that shone directly into one of our bedrooms. If we hadn’t been getting ready to move, we’d have had to upgrade our fairly standard drapes to room-darkening ones, the increase in light was so strong.

    But the most serious problem was that the greater brightness strongly increased the contrast between the areas directly under the lights – spaced half a block or so apart – and the areas in between or under tree cover. Your eyes adjust to the bright areas. I found myself tripping on sidewalk cracks that I didn’t before, because it was harder to see in the dark areas. It just seemed like a whole lot more light than was necessary on a quiet residential street.

    I know these fixtures use less energy than the sodium lights they replaced, but it seemed to me they could have achieved even greater savings – and reduced the safety problems caused by increased contrast – by using lightheads that produced about the same amount of light as before.

    1. Monte Castleman Post author

      With the sharp cutoff that you’re seeing, the fixture manufacturers regard it as a feature not a bug since less “wasted” light means greater efficiency specs. That municipal agencies rely on the light waste rather than use proper fixture spacing isn’t seen as the manufacturers’ problems.

      The lights are brighter because LEDs get dimmer as they age. The extra brightness is to allow them to still remain in spec after a period of years.

    2. Rosa

      that’s my beef with LEDs in general – since it’s so cheap to go bright, people just go brighter and brighter. It’s annoying when it’s a neighbor’s security light, but on the bike paths it’s a safety hazard – people with upward tilted front lights, or headlamps set for visibility through car windows, blind the oncoming cyclists at night.

      I use headlamps at my summer job and it’s great that they’ve gotten so cheap and the batteries last forever, but it’s really hard to train people NOT to shine it in everyone else’s eyes and they’re about twice as bright as they were 10 years ago.

  4. Matt SteeleMatthew Steele

    I’ve sent this general message to the City of Minneapolis lighting folks as well as others, but have not received traction. The conversion of Xcel fixtures to LED marks an opportunity to significantly improve lighting consistency, reduce glare, and improve safety. Take alley lights, for example. They space alley lights usually every other telephone pole, so there are two interior lights and one at each end of the alley. The fixtures are about 25 feet up, and are not full cutoff. There are definite dark spots in the alleys, and there’s significant glare (which harms the safety effectiveness) due to the current type of fixture. It would make far more sense to use some of the long-term energy savings on these to put full-cutoff fixtures on every telephone pole down an alley, but at a lower height – maybe 15 feet or so – enough to ensure even distribution of light down the alley, significantly less glare into neighboring windows, and more safety in the alley.

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