Skyglow in the countryside is dominated by stray light from towns, even 20km or more away. This is still derived mostly from street lighting, although other lighting is on the increase. Direct light emitted from G5 or lower class luminaires just above the horizontal travels through long path lengths in the lower atmosphere, and it is this that is mostly responsible for skyglow seen from rural areas. Curbing these low angle emissions must therefore form the focus of our efforts to minimise skyglow.

These low angle beams of light are preferentially scattered by aerosols (dirt particles) in a forward direction, while air molecules scattering dominates at high elevations. The widespread use of G3 luminaires could reduce the rural skyglow problem. G6 luminaires have a positive impact over large distances particularly beyond 20km, and should continue to be encouraged, especially in rural (or open to rural view) projects.

The change from high pressure sodium to white light enhances scattering significantly where the higher reflectivity from grass and leaves and the strong blue bias of Rayleigh scattering from molecules is dominant.

This is a diagram to show relative impact of a luminaires output, with regards to contribution to skyglow. 180-100° Critical area for skyglow experience from within urban and local areas but minimal impact to rural viewing areas. 100-95° significant contributors to skyglow, especially in rural areas where it is most aerosols dependent. Less likely to be obstructed. 95-90° Critical zones for skyglow seen at tens of km where it is strongly dependent on aerosol scattering. 90-70° Significant contributor to skyglow seen at a distance through reflection but reflected light more likely to be obstructed by buildings, trees and topography. 70-0° Ideal light distributions.

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