3 Modelling light pollution for sky-luminance reduction based lighting standards.

A comprehensive mathematical model has been developed that calculates how light emitted by street-lamps is reflected by surrounding surfaces, such as roads, grass, walls and trees, and then scattered through the atmosphere back down to the ground. In rural areas, it is shown that improved street-lamp design could cut skyglow by a factor of three to five, depending on the lamp's elevation and distance of view.

In the UK, glow from towns illuminates the sky even in rural areas and the majority of young people have never seen the Milky Way. The research shows that they are being deprived needlessly of this inspiring sight. With changes to street-lamp design, we can not only reduce skyglow significantly, but also save energy in lighting our streets roughly equal to the continuous output of two power stations.

The model tracks how the scattering of light is affected by atmospheric conditions, such as cloud cover, the concentration of dust and water droplets and the variation in density with altitude. It also takes into account the angle and the wavelength of light emitted and the viewing distance, allowing comparisons of the contribution to skyglow by different, commonly found designs of street lamps.

The research shows that close to towns, skyglow is dominated by reflections from surrounding surfaces to the roads, such as grass in suburban areas. At a distance from towns, direct radiance from lighting above the horizontal is the main cause of skyglow.

Full cut-off lamps (FCOs), where the light is only emitted downwards through a flat glass panel and no light escapes above the horizontal, can cut skyglow by a factor of eight compared with traditional low-pressure sodium lights. FCOs produce around 15-30% less skyglow than lamps with shallow bowls. A light with a polycarbonate bowl will cause about 15% more skyglow than the same light fitted with a glass cover.

The study also shows that the trend towards white light sources is substantially increasing reflections off vegetation and scattering in the atmosphere. Shorter wavelengths, towards the blue end of the spectrum, are scattered more than longer wavelengths which is why we see the sky as blue during the day. White lights emit across the visible spectrum and the blue and green components are adding significantly to light pollution, compared to yellow or orange light. This is something that most lighting engineers have never thought about.

This work has been published by the Institution of Lighting Engineers, and the UK Highways Agency has incorporated the findings into their new standards. The findings have been accepted by our national organisations, but we also need to ensure that the message reaches the people that are designing and commissioning light fittings. We are currently working to produce some educational tools intended for use in professional lighting engineering qualification courses.

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