The use of satellite imagery for light pollution studies

We came across an article recently that used colour satellite imagery from the International Space Station in order to demonstrate levels of light pollution. The imagery is said to demonstrate a greater light pollution impact in Milan (Italy) following the change to LED technology (white light) from the previous sodium lighting scheme (yellow light).

ISS astronaut pictures of Earth used for light pollution studies

With a properly designed and implemented LED lighting scheme, it is generally possible to achieve an upward lighting ratio close to zero. Therefore, aside from the small amount of light reflected upwards towards the night-sky from the lit ground surface, there will actually be little to no upward light emitted towards the sky. However, in the case of a sodium lamp lighting scheme, upward lighting ratios are far higher due to the inability to direct the light only to where it is intended.

In fact, what you are actually observing on the satellite imagery for the areas of LED lighting is predominantly the luminance of the ground being lit and not upward light pollution at all. If you look closely, you will see that there is also no discernible light spill on the ground, as the lit roads have well defined boundaries.

Compare this with the satellite imagery for the areas of sodium lighting which will be a combination of both ground luminance and direct upward lighting. Compared with the well-defined road boundaries lit with LED technology, the roads lit with sodium lighting have blurry and undefined boundaries, indicating light trespass.

We recognise that satellite imagery could potentially be useful in assessing sky-glow, in that (albeit complex) there will be a broad relationship between sky-glow and satellite imagery for individual lighting technologies. However, in assessing sky-glow, one must consider the luminance of the night-sky when observed from earth, as it is the inter-reflected light between the light sources and the atmosphere that produces sky-glow and not the viewed luminance of the lit surface from space. That said, the level of light reflected by the atmosphere varies depending on atmospheric conditions, and so we tend to use the upward lighting ratio of a lighting installation for assessing likely sky-glow impacts. The upward lighting ratio (ULR) is also the metric adopted in the ILP Guidance Notes for the Reduction of Obtrusive Light (which is widely used to assess the environmental impact associated with exterior lighting installations in the UK).

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