American Electric Autobhan Streetlight

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.