Light travelling through the atmosphere scatters to various view angles

If the atmosphere had the same density all the way up to space, it would only be 7 km thick. The density drops exponentially with height and most of the particle aerosol scattering is in its lowest part. Light just above the horizontal travels the greatest distance within the atmosphere, and scatters most, as most is forward scattering. That which travels upwards goes through a smaller distance in which to scatter and a larger angle to scatter to in order to be seen, and so has little effect on a viewer. Particulate scattering is very forward and backward directional (Mie scattering) and hardly at all at right angles. The upper atmosphere does allow sideways scattering but not a great deal and is very blue biased (molecular Rayleigh scattering). The scattered light from luminaires is effectively channelled near the horizontal.

Contributions from scattered light from a luminaire illuminating successive sections along along the observers view path

Sky glow luminance from a source 10 km away, for elevations from horizon to horizon. Luminaires types SOX (orange) and cutoff SON (light orange) and full cutoff (pink). (normal incidence reflectivity 0.1). Scatter just from molecules (dots) and molecules with aerosols (lines) respectively. Aerosols dominate at low elevations. Skyglow at close distance, <2 Km is dominated by ground reflection, but at increasing distance it becomes dominated by low angle light from above the horizontal. Scatterring at small angles is mostly from aerosols. Scatter at large angles is mostly by air molecules, maximally in the blue. Little is from the upper parts of the path, as there the scattering is orthogonal or back scatter and the aerosol density is much The diagram also shows which part of the view path is mostly illuminated from ground reflection, and which from direct transmission from the luminaire. Which is why different designs of luminaire have such a big difference in forward, overhead , and backward illumination of the sky!

Scatter into line of sight for at an view elevation

Sky glow luminance from a source 10 km away, for elevations from horizon to horizon. Luminaires types SOX (orange) and cutoff SON (light orange) and full cutoff (pink). (normal incidence reflectivity 0.1). Scatter just from molecules (dots) and molecules with aerosols (lines) respectively. The road illuminance is the same in each case.. Note on this logarithmic scale, the great difference between typical low pressure sodium Sox with its high percentage of light to the sky, shallow bowls, which are better, and of flat glass so much better still. The differences increase with elevation view angle, especially when viewing away from the source, as the latter has no light components to cause back scatter.
Luminaires types SOX (orange) and cutoff SON (light orange) and full cutoff (pink). (normal incidence reflectivity 0.1). Scatter just from molecules (dots) and molecules with aerosols (lines) respectively. Aerosols dominate at low elevations.

Modelling a Lamppost

Diagram to show relative impact of a luminaire’s output with regards to contribution to skyglow.


A 180-100° Critical area for skyglow from within urban areas but proportionally less impact to rural areas.

B 100-95° Significant contributor to skyglow, especially in rural areas where it is most aerosol dependent. Less likely to be obstructed.

C 99-90° Critical zone for skyglow and obtrusion seen at 10s of km (in rural areas) where it is strongly dependent on aerosol scattering.

D 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.
E 70-0° Ideal light distribution.