This post was written by Keith Dawson for UBM Tech’s community Web site All LED Lighting, sponsored by Philips Lumileds. It is archived here because the All LED Lighting site has gone dark. This material is Copyright 2013-2015 by UBM Americas.


Why Are US Cities So Bright?

A new, higher-resolution remote sensing satellite shows that US cities are brighter than their German counterparts. The reasons are a guessing game.

In the open-access journal Remote Sensing, German researchers dug into the data from the new Visible Infrared Imaging Radiometer Suite Day-Night Band (VIIRS DNB) satellite, which images the whole earth nightly at a resolution of about 750 meters per pixel. This resolution, while far too coarse to show individual light sources or even building-scale objects, nonetheless is able to distinguish sources of light pollution such as airports and harbors. Its resolution is 45 to 88 times sharper than that of the older satellite, the Defense Meteorological Satellite Program-Operational Linescan System, on which nearly all previous studies of global-scale artificial night light have been based.

Cultural footprints
To their surprise, the researchers found that they could pull "cultural footprints" out of the nighttime imagery data. In particular, they were puzzled at the different patterns of light coming from US cities compared with German counterparts.

The figure at right tells the whole story. The red dots are data on total light output ("sum of lights," or SOL) for 4,492 German communities plotted against population. The black dots represent 28,804 American cities. The trend lines are best fits to the two countries' data. Their slopes are unequal, which means that US cities feature more light per capita as their size increases, while German cities show the opposite trend.

The US cities clearly stand above their German counterparts in light output at all size scales. An American town with a population of 10,000 typically has an SOL about three times greater than its German equivalent. For typical American vs. German cities with 100,000 inhabitants, the disparity is about five times.

The researchers comment in passing, "Within the United States, communities with SOL/capita similar to that of Germany are generally located in the west coast and northeastern states." I will point out, though the researchers did not, that these areas of the US are more "blue" (in the political sense), more educated, and have higher per capita incomes.

East is east
The researchers found another curiosity in the data. The cities in former East Germany are brighter than those in the former West. Compared to the German trend line, 77% of former East towns are above it, but only 40% of former West ones. They speculate that the lighting infrastructure in the former East may be older and its light distribution less well controlled, sending more light into the sky, than that in the relatively more prosperous and modernized West.

What explains the US-German disparity?
The researchers go to some lengths to enumerate and discount possible sources of systematic error in their data. In the end they lean toward the conclusion that the effect they spotted is real, and offer some possible explanations.

One is the spectral sensitivity of the imaging gear on the VIIRS DNB satellite. Its sensitivity is less in the blue (< 500 nm) and greater in the near-infrared. So spectral differences in the outdoor lighting employed in the two countries could account for some of the difference.

Other possibilities are environmental. The researchers write:

If the causes of the observed disparity can be pinned down, we might discover significant opportunities for energy savings.

— Keith Dawson Circle me on Google+ Follow me on Twitter Visit my LinkedIn page, Editor-in-Chief, All LED Lighting