08How streetlights work, from gas lamps to LEDs

The short version: Street lighting has gone from oil lanterns to coal-gas lamps, electric arc lamps, incandescent bulbs, the orange glow of sodium lamps and, since the 2000s, LEDs. Most streetlights switch on by themselves, using a small light sensor or a timer that calculates sunset. LEDs cut energy use sharply but raised new arguments about glare, blue light and dark skies, and a batch of faulty fixtures turned streetlights purple across North America. Boston owns about 71,000 streetlights, including about 2,800 gas lamps.

How it changed

Oil. Ancient Greek and Roman cities used oil lamps, and France introduced an improved oil street lantern, the réverbère, in 1745.¹⁵⁹

Gas. On June 4, 1807, Frederick Albert Winsor gave the first public demonstration of gas street lighting on Pall Mall in London.¹⁵⁹ The display marked King George III's birthday, and the pipes were old musket barrels.¹⁷¹ The Gas Light and Coke Company, the world's first gas company, received a royal charter in 1812, and by the 1820s London had about 50,000 gas lamps on roughly 300 miles (480 km) of pipe.¹⁷¹ Gas lit Westminster Bridge on December 31, 1813, and the main streets of Paris from 1829.¹⁵⁹ Lighting all those lamps was a job: by the London Museum's count, the Victorian city had more than 25,000 lamplighters.¹⁷¹

Electric arcs. Pavel Yablochkov's arc "candle" dates to 1875. Paris installed electric street lights on the avenue de l'Opéra on May 30, 1878; Cleveland demonstrated 12 electric lights around Public Square on April 29, 1879; and Wabash, Indiana, hung four Brush arc lamps over its courthouse on February 2, 1880.¹⁵⁹ An arc lamp makes light from a spark jumping between two carbon rods. Arc lamps were harsh and needed frequent electrode replacement, and late in the 19th century incandescent lamps, often wired on high-voltage series circuits, replaced them.¹⁵⁹

Discharge lamps. In the 20th century, cities moved to lamps that make light by passing current through a gas or vapor. Low-pressure sodium became common after World War II, and high-pressure sodium became the preferred type in the late 20th century.¹⁵⁹ High-pressure sodium (HPS) is the orange-pink light that many people still picture when they think of a city at night.

The high-pressure mercury-vapor lamp came out of work in the 1920s and was widely used for street lighting by the 1930s.¹⁷² High-pressure sodium was demonstrated in a laboratory in 1959, and the first commercial HPS lamps came from companies in the US, UK and the Netherlands in 1965.¹⁷³

LEDs. A light-emitting diode (LED) is a semiconductor light source that uses far less energy than older lamps and lasts much longer. Ann Arbor, Michigan, is cited as the first US city to fully implement LED street lighting, in 2006, and Mississauga, Canada, converted more than 46,000 lights between 2012 and 2014.¹⁵⁹ "First" claims like Ann Arbor's are hard to pin down, and others may dispute it.

How do streetlights turn on at night?

In most places, nobody flips a switch. Most streetlights control themselves in one of a few ways:¹⁵⁹

• Photocell. A small light sensor, usually a button or puck on top of the fixture, switches the lamp on at dusk and off at dawn. It also turns lights on under heavy cloud.
• Astronomical timer. A controller that calculates local sunrise and sunset for each day of the year and switches the lights accordingly.
• Solar dials. An older mechanical method that tracked the changing length of the day.
• Networked controls. Newer systems put a communicating node on each light, letting a city monitor lights remotely, dim them and detect faults automatically.

Some councils, notably in Britain, use "part-night" lighting, switching lights off between about midnight and 5:30 a.m. to save energy.¹⁵⁹ London's surviving gas lamps still run on mechanical clock timers.¹⁷¹

A streetlight that burns all day often has a failed photocell or a controller stuck in the on position. On a networked system, the city sees that fault on a screen; on an older one, it waits for someone to report it.

Why are LED streetlights so harsh and blue?

The switch to LEDs was driven by energy savings. Boston says its LED conversion cut energy use and emissions by 60%, and that LEDs last about three times longer than the lamps they replaced.⁷²

The first wave of LED streetlights, though, gave off a cold, blue-white light, and residents in many cities complained about glare. The measure for this is color temperature, given in kelvins (K): lower numbers look warm and yellow, higher numbers look cool and blue-white. The debate in the 2010s was over fixtures at 4000K and above.

At its June 2016 annual meeting, the American Medical Association (AMA) adopted guidance urging communities to minimize blue-rich lighting and to choose 3000K or lower rather than 4000K.¹⁷⁴ It cited glare, reduced visual acuity and suppression of melatonin, a hormone linked to sleep.¹⁷⁴ The AMA did not tell cities to avoid LEDs, and the US Department of Energy continued to promote LEDs for their energy savings.¹⁷⁴ In our reading, the AMA's health claims are best treated as guidance rather than settled science, and the practical outcome has been a push toward warmer fixtures rather than away from LEDs.

Industry and government replied within weeks. The DOE published "Get the Facts: LED Street Lighting," saying low color temperatures "may be beneficial for reducing nonvisual impacts, but they may also reduce the effectiveness of the lighting."¹⁷⁵ The National Electrical Manufacturers Association (NEMA) agreed that spectral content should be one factor in outdoor lighting but said "a single solution is simply not appropriate for all situations."¹⁷⁵

Dark-sky advocates make a related argument about wildlife, astronomy and wasted light. DarkSky International sets out five principles for responsible outdoor lighting: it should be useful, targeted, low level, controlled and warm-colored.¹⁷⁶ The principles do not name a specific kelvin figure.¹⁷⁶ In practice that means a full-cutoff or shielded fixture that sends light down rather than up or sideways, at no more brightness than the task needs, on a timer or dimmer, in a warm color.

Why did streetlights turn purple?

Starting in early 2021, people across North America began noticing streetlights glowing violet. The cause was a manufacturing defect.¹⁷⁷

A white LED is usually a blue-violet LED chip coated with a phosphor, a material that converts some of the blue light into yellow so the mix looks white. In some LED packages the phosphor layer degraded or peeled away, letting the raw blue-violet light through.¹⁷⁷ The affected fixtures were American Electric Lighting (AEL) products from Acuity Brands, made roughly between 2017 and 2019.¹⁷⁷

By 2022, reports had come from at least 30 US states.¹⁷⁷ Places named include Massachusetts (Boston and Interstate 93), South Florida, Milwaukee, Schaumburg in Illinois, Los Angeles, Bend in Oregon, Vancouver and Ireland.¹⁷⁷ Acuity's warranty and recall expenses were $32.3 million in 2021, $47.0 million in 2022 and $52.4 million in 2023.¹⁷⁷ Those totals cover all of the company's warranty and recall costs, not only purple lights, so they are an upper bound on what the defect cost rather than an exact figure.

Residents can report purple lights through utility portals so the fixtures are replaced under warranty.¹⁷⁷

Who owns the streetlights?

It depends on the place, and it determines who you call when one goes dark. In Boston the city owns and maintains its streetlights through the Public Works Street Lighting Division, and residents report problems through the city's 311 service (more on 311).⁷² Where a utility owns the lights, residents report problems, including purple lights, through the utility's portal.¹⁷⁷

In Massachusetts, Chapter 164, Section 34A of the General Laws, adopted as part of the 1997 Restructuring Act, lets municipalities buy their streetlights from the utility. The Metropolitan Area Planning Council says buying the lights alone can save 30-60% of streetlighting costs, and up to 70-80% when combined with an LED retrofit.¹⁷⁸

In Boston

The system. Boston has about 71,000 streetlights, including about 2,800 gas lights and 1,500 fire-alarm lights, in 19 fixture types, all maintained by the Public Works Street Lighting Division.⁷² The fire-alarm lights likely mark the city's street fire-alarm boxes (more on those in From fire alarm boxes to 911). An older, undated city page gave a smaller total of about 67,000 lights and described what is underneath them: 4 million feet of conduit, 18 million feet of cable, 1,600 control boxes and 32,000 pull boxes.⁷⁶ That page also says fixtures designed since 2000 are "medium cut-off," aiming 95% of their light down.⁷⁶

LEDs. Boston began converting to LED in fall 2010.⁷² By 2022 about 85% of the city's lights were LED.¹⁷⁹ Purple LED streetlights were reported in Boston and along I-93 during the early-2020s phosphor failures.¹⁷⁷

Reporting a dark light. Use 311 (311.boston.gov) or call 617-635-5810.⁷²

The gas lamps. Boston was early to gas (the pipes are covered in Gas under the street, and why steam comes out of it). The Boston Gas Light Company was chartered on January 22, 1823, and the first public gas street lamp was lit in Dock Square, near Faneuil Hall, on January 1, 1829.¹⁷⁰ Growth was slow at first: by 1839 the city had only about 180 gas lamps.¹⁷⁰

Gas lamps survive today in larger numbers than most visitors expect. In 2022 Boston had about 2,800, more than 1,000 of them on Beacon Hill.¹⁷⁹ Many are 1960s replicas rather than 19th-century originals.¹⁷⁹ The cost adds up: nearly $1 million a year in gas plus about $200,000 in maintenance, and nearly 5,000 metric tons of greenhouse gas emissions a year, about 3.4% of the city government's operational emissions.¹⁷⁹

In 2022 the city began testing a replacement. The first LED "faux gas" lamp, built to look like a gas lantern, went up in Bay Village in March 2022.¹⁷⁹ Not everyone wanted the change: the Beacon Hill Civic Association objected in January 2022.¹⁷⁹ The city frames the issue as emissions and cost; preservation-minded residents frame it as the character of historic streets. Both are describing real things.

For comparison, London has about 1,300 surviving gas lamps, maintained by five British Gas lamplighters.¹⁷¹ On those figures, Boston has roughly twice as many.

What's changing now (2026)

As of the city's May 6, 2026 update, Boston still lists about 2,800 gas lights among its 71,000 streetlights, and describes LED conversion as a program that began in 2010.⁷² That is the same figure the city gave in 2022, so on its own count the gas-to-LED conversion that began with the Bay Village pilot has not yet much reduced the number of gas lamps.

When the conversion was announced in March 2022, the city said the 2,800 gas lamps were 4% of its streetlights but 37% of streetlight emissions, and cost nearly $1 million a year in gas plus about $200,000 in maintenance. At the time, it was also exploring smart lighting for non-historic streetlights.¹⁸⁰ Boston's Smart Utilities standards list energy management for street light networks among the technologies for projects that rebuild sidewalks.¹⁸¹

Three broader trends run through the sources above:

• Warmer color. After the 2016 AMA guidance and dark-sky campaigns, cities increasingly specify warmer LEDs than the first wave.¹⁷⁴¹⁷⁶
• Controls. Networked nodes allow remote monitoring, dimming and automatic fault detection, so that the city, not a resident, can be the first to know when a light fails.¹⁵⁹
• Cleanup of the purple fixtures. Acuity's warranty and recall costs rose each year from 2021 to 2023, and affected lights are replaced under warranty when reported.¹⁷⁷

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