the cities are no longer yellow

Astronauts who have been lucky enough to travel to space more than once in the last decade are privileged witnesses of a chromatic change on a planetary scale. From their vantage point 400 kilometers high, they have been able to see that the cities, previously faint spots of amber, now shine with an intense white light.

It’s not a metaphor. It is the visible trace of one of the most rapid and widespread infrastructure transformations in recent history: the great replacement of public lighting. We have retired the old sodium vapor streetlights and massively embraced LED. This change, driven by regulation in favor of energy efficiency, has redrawn the night map of the Earth, a phenomenon that can be seen more clearly from space.

The invention that earned a Nobel Prize in Physics. Old sodium vapor lamps, especially low-pressure ones, were monochromatic in nature. They emitted light in a very narrow band of the spectrum, resulting in that characteristic and ubiquitous yellow-orange hue that tinted our streets and skies. LED lights work in a completely different way.

His breakthrough, which earned Isamu Akasaki, Hiroshi Amano and Shuji Nakamura the 2014 Nobel Prize in Physicswas the invention of the high-efficiency blue LED. By combining this blue LED with a phosphor coating, it was finally possible to generate a bright and affordable white light. This diode is not only more efficient (exceeding 300 lumens per watt, compared to 16 for an incandescent bulb), but it offers a much broader spectrum.

Southern Europe in 2025 from the International Space Station. Image: Don Pettit

The cities changed color. To the eyes of a night observer in space, cities have gone from being yellow to glowing bluish white. Milan is the paradigmatic case: it completed its transition to LED in 2015, and appears in an ESA comparison with before and after photos taken by astronauts André Kuipers and Samantha Cristoforetti. But it is by no means the only case.

Los Angeles was a pioneering city: it ordered the replacement of 140,000 streetlights in 2009. Buenos Aires modernized its lighting with smart LED streetlights between 2013 and 2016. New York finished replacing 500,000 bulbs in 2023. Barcelona plans total remote management of public lighting by 2028. But India is the country that carries out the largest replacement in the world, with more than 13 million LED streetlights already installed.

The b side of this transformation. Like any revolution, the LED has a dark side. Light is cheaper, so cities are not only replacing old streetlights, but also increasing the number of light points or their intensity. The result is that we are leaving a brighter planet, where it is most difficult to escape of light pollution.

The statistics indicated otherwise, but it must be taken into account that light pollution is measured by satellites, and satellites are partially blind to blue light. This means that the actual increase in light pollution, especially that perceived by human beingsis much higher than official figures indicate. To make matters worse, blue light is the one that interferes the most with our biological clock, and can affect the quality of sleep, in the same way that disorients migratory birds and the moths.

The future is adjustable. The solution is not to go back to sodium. The efficiency of the LED is indisputable. The key, as with any technology, is in its application. The next phase of this transition is not about changing light bulbs, but about installing smart streetlights. It is estimated that almost one in four streetlights will be smart by 2030. When connected, they can regulate their intensity depending on the time or traffic, detect faults in real time and collect environmental data.

This remote management will allow one of the new lighting maxims to be applied: using only the necessary light, when and where it is needed. In parallel, other solutions have emerged to protect biodiversity, such as red light streetlights being tested in Nordic cities so as not to disturb the bats. and the idea of ​​bioluminescence as an organic way of generating light without any electrical consumption and with minimal environmental impact.

Image | The Iberian Peninsula in 2012, by astronaut Don Pettit

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