Tucson, Vienna, Metz: A tale of three cities

Image credit: John Barentine
1552 words / 6-minute read

Solid-state lighting (SSL) began replacing legacy streetlights across the globe almost two decades ago. Light pollution researchers and dark-sky advocates viewed the transition with a mix of optimism and anxiety. On paper, light-emitting diodes (LEDs) offered incredible energy efficiency, long lifespans, and precise optical control. But in practice, early retrofits often backfired. "Cheap lumens" led to over-lighting, an example of the classic "rebound effect" in economics. Blue-rich white LEDs flooded urban night skies with light that scatters strongly in the atmosphere. That intensified skyglow and disturbed biological rhythms.

Yet the narrative that LED retrofits inevitably harm the night sky is incomplete. Evidence emerged in recent years that careful municipal lighting design can slow the growth of skyglow. In some cases it can even reverse it, making the night sky marginally darker. Such outcomes demand thinking differently about how we light our cities. Such large-scale changes must be considered holistically rather than one lamp at a time.
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Three world cities — Tucson, USA; Vienna, Austria; and Metz, France — tried a different approach. Each faced a different set of starting conditions. Each implemented a different combination of levers on the problem. Together, their experiences offer a comprehensive playbook for dark-sky friendly urban lighting.

Tucson, USA: The "Lumen Reduction" Strategy

Tucson's municipal LED lighting conversion (2016-2017) was the first of these efforts. It provides an important benchmark for what happens when a city starts with a well-shielded lighting system. Tucson's pre-retrofit lighting system consisted of fully shielded high-pressure sodium (HPS) luminaires. These emit no direct light above the horizontal plane. In industry terms, they had an Upward Light Output Ratio, or "ULOR," of zero. Although the luminaires we well shielded, they were too bright. By international standards, the system emitted much more light than was necessary. The primary problem was not (wasted) direct uplight, but sheer volume.

These International Space Station (ISS) astronaut photographs capture Tucson, Arizona, at night before and after its municipal lighting transition. In 2012 (left), the urban grid was dominated by the characteristic warm amber glow of legacy high-pressure sodium streetlights. By 2025 (right), near-universal conversion of the municipal lighting system to 3000 K LEDs shifted the city's appearance to a crisp white, accompanying a massive reduction in overall lumen output across the network. (Source: NASA, ISS030-E-61700 & ISS072-E-517663)

When the City of Tucson began planning the retrofit of its nearly 20,000 municipal street lights, it made a bold promise. The retrofit would not imperil astronomy and space science, which contribute over half a billion dollars a year to the local economy. To ensure this, total light emissions had to decrease in the transition to modern LED luminaires.

​The new 3000K white LED luminaires were less bright than the HPS sources they replaced. The new lighting system reduced light output by almost 63% while keeping illumination above minimum safe levels. The City further reduced emissions by implementing an overnight dimming program. At midnight most streetlights dim from 90% to 60% of their full power. This not only reduces skyglow but lowers energy costs and improves the field lifetime of the lighting products.

Our 2018 study of Tucson’s municipal LED conversion confirmed the strategy's success. Satellite observations of upward-directed light radiating from Tucson showed a 7% reduction. Ground-based measurement revealed even steeper decreases. At remote astronomical observatory sites, night sky brightness decreased by as much as 20%.
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The Takeaway: When starting with well-shielded luminaires, further reductions in skyglow come from lumen reductions. Cities must turn down the overall "volume knob" by capping total light emissions and implementing curfew dimming.

Vienna, Austria: The "ULOR = 0 + Scheduled Modulation" Model

While Tucson’s challenge was total lumen volume, Vienna faced a classic optical shielding challenge. The city's municipal lighting department manages approximately 133,000 public streetlights. Before the current retrofit program, legacy luminaires allowed considerable upward light "spill" into the night sky. Vienna began modernizing its municipal lighting stock in the mid-2010s. The program replaced old fixtures with fully shielded (ULOR=0) luminaires. By late 2024, Vienna updated over 108,000 fixtures — roughly 80% of the network.
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Researchers established the “Licht über Wien” monitoring program to check the real-world impact of this conversion. The team deployed a network of high-precision LightMeter sensors at three key locations. They represented central urban (Wien Zentrum at Stubenring), suburban (Kuffner Observatory) conditions. A third site served as a dark-sky baseline reference (Großmugl, ~33 km away).

​They isolated the specific contribution of streetlights by analyzing sudden step-changes (Lichtstufen) in sky brightness. These occur each night during scheduled dimming events (Teilnachtschaltungen). In 2015, this dimming occurred at 11 P.M. local time. Sensors then recorded a distinct ~5% drop in horizontal irradiance in the city center. At the observatory, the drop was almost 10%.

Ground-based light measurements at Vienna’s Kuffner Observatory track how local sky brightness jumps or drops when municipal streetlights dim or turn on late at night. Between 2015 and 2024, as the city replaced over 80% of its fixtures with fully shielded LEDs, these step-changes flattened dramatically, shrinking streetlight-attributable skyglow by roughly 75% (evidenced by the morning 5:00 AM turn-on jump shrinking from a 9% increase down to just 2%). (Data: G. Wuchterl / Verein Kuffner-Sternwarte, 2025)

By 2024, Vienna achieved the conversion of 80% of its street lights. Measurements showed that skyglow was then one-quarter of the 2015 baseline level. The study projects that once the city completes the retrofit project, it will fall below the 1% detection threshold of the instruments.

At first glance, a streetlight contribution below 1% might seem counterintuitive for a major European city. Wouldn't light reflecting off streets and buildings still create substantial skyglow? The answer depends on what the meters actually measure: the hourly modulation signal. By adopting a zero ULOR standard, Vienna mitigated the component of light emissions escaping directly into the night sky. The remaining skyglow attributable to the street lights comes from unavoidable reflections from the ground and building facades. Such reflections are very diffuse and uniform. The sudden step change during late-night dimming disappears into the noisy 'background' from light sources other than street light.
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The Takeaway: Eliminating direct upward waste light delivers enormous skyglow dividends. For cities with legacy unshielded fixtures, shielding is by far the single most effective intervention.

Metz, France: A "Hybrid Optics and Ecological Spectrum" Pioneer

Tucson demonstrates the power of lumen reduction. Vienna proves the necessity of strict optical shielding. The French city of Metz illustrates how to combine both tactics. But it goes a step further with innovative spectral and smart lighting management.
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According to the Metz 2023/2024 Sustainable Development Report, the city is executing a €15 million public lighting transformation across its 16,500 municipal luminaires. LED adoption surged from 21% in 2022 to 60% by late 2024, replacing 2,500 to 3,000 fixtures per year. A top priority was the aggressive removal of unshielded, 360-degree globe lights (luminaires boules). The city replaced them with flat-glass downward directional LED units.

Metz’s results are striking. Total municipal lighting energy use dropped by 25.3% in a single year. Synchronizing late-night light reductions with the operating schedules of the local public transit network further saves electricity and reduces carbon emissions. Rather than over-lighting roads, new luminaires use precise optical beam shaping. These designs meet minimum illuminance standards without going overboard by emitting too much light.

But where Metz truly excels is in its commitment to ecological lighting design. That is particularly important in its sensitive urban ecosystem. Along the banks of the Moselle River (Île du Saulcy), Metz installed motion-activated streetlights. These products have an ultra-warm 1000 K (amber/red) light appearance. Removing the blue-component of white LED almost completely reduces the atmospheric scattering causing skyglow. It also protects sensitive nocturnal wildlife in the river and on land.

To safeguard sensitive aquatic and terrestrial wildlife while keeping pedestrian paths safe, Metz installed motion-activated, ultra-warm 1000 K (amber-red) LED luminaires along the riverbank (right). By removing short-wavelength blue light and illuminating only when human presence is detected, the system drastically cuts skyglow-causing light scatter and protects nocturnal ecosystems. (Source: Signify / Ville de Metz)

Metz experimented further. In its La Grange-aux-Bois neighborhood, the city deployed smart radar sensors. These sensors can distinguish pedestrians and cyclists from motor vehicles. The system uses this information to dynamically increase light levels when it detects human activity. When activity ceases, the lights extinguish. But it didn't stop there. It paired the infrastructure upgrades with community accountability. Volunteer teams known as Sentinelles de la Nuit ("Night Sentinels") audit Metz's commercial corridors after 1am each night. They assess illuminated storefront display compliance with France's national light pollution regulations.

The Takeaway: Modern retrofits can extend beyond standard white LED installations. Integrating ultra-warm lighting ecologically sensitive zones protects local wildlife. Using dynamic sensor controls allows cities to significantly reduce both energy consumption and nighttime light emissions. 

Synthesis: The Three Pillars of Dark-Sky Retrofits

Comparing these three cities reveals that successful public lighting modernizations rely on balancing three core variables. These are optical shielding, lumen caps, and spectral/dynamic controls.
From these empirical case studies, three key rules emerge for municipal decision-makers:

1. Zero upward light (ULOR = 0) is essential. Direct upward emissions scatter far more efficiently than reflected light from the ground. Vienna proved that eliminating direct uplight yields immediate, measurable reductions in urban skyglow.
2. Efficiency gains must not fuel over-lighting. Converting to LED without reducing total lumens or establishing curfews risks worsening skyglow. Tucson demonstrated that cutting total lumen output by over 60% maintained safe, effective lighting while lowering skyglow.
3. Spectrum and controls are location-sensitive. Blanket lighting policies across an entire city are obsolete. As Metz showed, deploying ultra-warm 1000 K amber/red LEDs in sensitive places mitigates ecological harm. And it did so without compromising public safety in transit hubs.

For decades, dark-sky advocates and municipal lighting managers were often viewed as opposing sides. One side advocated for nighttime preservation and the other for urban safety. Decision makers came to believe that a city could have one or the other, but not both. The experiences of Tucson, Vienna, and Metz prove that this binary choice is false.
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These cities employed tailored combinations of shielding, lumen caps, dimming and spectral management. They also reduced light pollution and energy consumption. Yet they also tended to the needs of their residents. The outcome is win-win all the way around. As more cities prepare to convert or upgrade their street lighting infrastructure, these case studies stand as clear proof that smart design can restore the night sky without leaving our streets in the dark.