Friday, January 15, 2016

The nights are still blue

A few years ago (20 September 2012 to be exact), we had written about our blue nights. We still did not get used to them, but we let the rhododendrons outside the picture window grow all the way to the roof.

Yesterday, I was catching up on my reading and I came across the interesting article LED light pollution: Can we save energy and save the night? by Mark Crawford in the January 2016 issue of SPIE Professional. When we buy light bulbs for the home, we look at the spectral distribution of the light they emit and buy models that have natural spectra. Crawford reports that LEDs designed for street lighting are optimized differently and have typically a correlated color temperature of 6500ºK and are dominated by a narrow, short-wavelength emission band together with a broader long-wavelength emission band.

This results in excessive light pollution, as illustrated in the figure below. This image of Milan was acquired after the transition to LED technology in the downtown area. The illumination levels appear to be similar or even brighter in the city than the suburbs, and the amount of blue light is now much higher, which suggests a greater impact on the ability to see the stars, human health and the environment. Since the European Space Agency’s NightPod device was installed on the ISS in 2012, astronauts have been taking systematic night images. It incorporates a motorized tripod that compensates for the station’s speed and the motion of the Earth below. Before that motion could blur images even though astronauts compensated with high-speed films and manual tracking. This NASA/ESA image was taken by Samantha Cristoforetti.

City center of Milano. NASA/ESA image was taken by Samantha Cristoforetti

Do we really need so much street light at night? When I was a kid, all establishments had to close before midnight and half an hour later the street lighting would go off, until six o'clock in the morning. It does not have to be that drastic, but I do not think our lives would be any different if the street lights would be dimmed to well under 0.25 lux after midnight. We no longer walk, and all vehicles have headlights by law. For the few pedestrian a faint light is more than enough because we adapt to the light level: the pupils dilate and eventually we switch to scotopic vision. When I was a kid and walked home after the street lights were off, I could see extremely well at full Moon, which is just 0.25 lux. When the Moon is not full, you just walk slower and enjoy the Milky Way.

I am not a Luddite. In Palo Alto, the heart of the Silicon Valley, in the Barron Park neighborhood everybody turns off the porch lights at night, and these people are the cream of the digerati. The neighborhood's park is called Bol Park, after Stanford physicist and research associate Cornelis Bol, the inventor of the high-intensity mercury vapor lamp.

Vincent van Gogh: Nuit étoilée (Saint-Rémy-de-Provence), 1889