Leeds Astronomical Society LAS Meetings Observing Membership



What is Light Pollution?



Image Courtesy: National Geographic

In a nutshell Light pollution is essentially light which is in the wrong place or at the wrong time, or the wrong brightness, or even the wrong colour.

It is often categorised as follows[1]:

  • Sky Glow - A combination of reflected and refracted light from the atmosphere. A major effect of sky glow at night is to reduce the contrast in the sky. This is the most pervasive form of light pollution and can affect areas many miles from the original light source.
  • Glare - The excessive contrast between bright and dark areas in the field of view.
  • Light Trespass or Light Spill - Unwanted light, for example from adjacent properties and activities.
  • Light Clutter - The excessive grouping of lights, where multiple sources of light are in close proximity and compete against each other. eg. roadside LED advertising which can be a dangerous distraction to road-users.
  • Light Prolifigacy - Over-illumination which wastes energy & money as well increasing our Carbon emissions.
  • An Absence of Darkness - Artificial light makes experiencing natural night-time lighting conditions impossible in many areas of the country & most definitely in Leeds.


Sky Glow

Lighting in cities has far reaching consequences. Just as light from the Sun reaches us from 147 million km away, light from the centre of Leeds affects the night sky for many miles out into the countryside.

Sky Glow is created by light scattering in the atmosphere and reflecting off the underside of clouds. The extent of the problem is also dependant on the spectrum or colour and the brightness of the light. The extent to which light scatters of air is highly wavelength dependant. Blue light scatters about 3 or 4 × that of red light, which incidentally is why the sky in the daytime appears blue - at least on a good day! Another scattering mechanism occurs when light interacts with water vapour in the atmosphere (clouds), which doesn't have the same wavelength dependancy, so clouds appear to be varying shades of grey depending on how dense they are.

The effects of the spectral profile of light sources, is especially concerning with new and replacement LED products, which typically have more blue light than older types of lighting.

One of the main factors in Sky Glow, is light which is emitted above the horizontal, called 'uplight'. This effect can be minimised by careful choice of lighting and installation, particularly with street lights. In Leeds the Council is about 30% through it's LED street lighting program, which is due to be completed in October 2023. There are about 92,000 street lights here in Leeds with 1,900 being replaced with LED's each month. What effect this will have on light pollution is uncertain. Modelling in the US by the Department of Energy[2] suggests that there may be a modest reduction, based on the 3000K CCT (Correllated Colour Temperature) lights that are being fitted in most residential areas, and the 4000K CCT lamps on the main roads. In Tucson AZ, a study of sky quality measurements, before and after their LED street light replacement program suggested that there may be a ≈7% reduction in skyglow[3], but this is far from certain. Another study[4] in Tucson, calculated that their street lights only accounted for 20% of the light that was visible from space. This is likely to be much higher in UK cities like Leeds, where lights aren't already being dimmed at night. Tucson may also not be a good city to use as an example, after all the IDA headquarters are based there!

Sky glow will only be mitigated if there is better lighting guidance from the Government, mandating maximum permitted levels of street lighting & encouraging variable light levels in residential areas. Ideally, we need legislation to address all sources of light pollution, similar to the French "Decree of 27 December 2018 Relating to the Prevention, Reduction and Limitation of Light Pollution"[5].



Glare can be thought of as objectional brightness. It occurs where bright light sources are directly visible to the eye, contrasting with darker surroundings. For example, car headlights can easily dazzle pedestrians; outdoor floodlights make it uncomfortable to view places like car-sales forecourts & outdoor sports facilities; and poorly positioned & over bright home security lighting often seem more intent on blinding people, rather than providing useful light.

Discomfort Glare, is the sensation or annoyance induced by overly bright light sources.

Disability Glare, is a type of glare which affects older people, where light scatters within the eye and causes loss of visiblity from stray light.

To minimise glare, lights should point downwards with shielding to prevent stray light.


Light Trespass

Light Trespass or 'light spill' occurs when lighting illuminates more than the intended area. For example, street lights which shine into bedroom windows, or exterior lighting which illuminate adjacent gardens or properties.

To minimise light spill/trespass, the placement and angle of lighting needs to be considered & shielding added if need be. Timers or motion activated sensors can also be used to minimise problems.

In Leeds the Council will fit a baffle onto a street light, if the light from it shines into a bedroom window & disturbs your sleep. Extreme problems with light trespass/spill may fall under the 1990 Environment Protection Act, as nuisance lighting, although there are several restrictions.


Light Clutter

Light Clutter is the kind of combination effect of multiple light sources, that are in close proximity to each other. This can occur at roundabouts & junctions, where there is conflicting light from signage, LED advertising displays etc. and is a hazard to road users and pedestrians. Light clutter also describes the lighting arms-race that occurs with adjacent light sources from shops & businesses have ever increasing bright facades, in an attempt to be noticed above the background level of light noise.


Light Prolifigacy

This is the trend towards over-illumination, with levels of illumination increasing beyond what is needed. The average road illumination in Britain, for instance, has more than doubled in intensity since the 1970s.[6]

Domestic security lighting is another example, where 500W halogen floodlights (and now 50W LED), provide complete overkill in many instances where much less wattage is needed.

The advent of cheaper lighting technologies like LED's has also led to an increase in decorative lighting, which serves no real purpose.


An Absence of Darkness

This is a bit of a difficult one to describe, unless you have already had a proper dark sky experience... In Leeds we live with a very artificial view of the night sky. Most stars are lost in a sea of light, so we never experience the full majesty of the naturally dark skies.

To get a proper 'dark sky' experience, where the sky is awash with pinpoints of light set against a jet black background, you need to travel a long way away... For a list of some possible UK destinations see the Dark Places page.

[1] Royal Commission on Environmental Pollution - 'Artificial Light In The Environment' (2009) https://www.gov.uk/government/publications/artificial-light-in-the-environment
[2] US Dept Of Energy – "The Impact Of LED Street Lighting On Sky Glow" (2017) https://www.energy.gov/sites/prod/files/2017/05/f34/2017_led-impact-sky-glow.pdf
Explanatory webinar: https://www.energy.gov/eere/ssl/potential-impacts-led-street-lighting-sky-glow
& the slides from the webinar: https://www.energy.gov/sites/prod/files/2017/08/f35/Sky-Glow-Webinar_7-27-17.pdf
[3] Cornell University – "Skyglow Changes Over Tucson, Arizona, Resulting From A Municipal LED Street Lighting Conversion" (2018) https://arxiv.org/abs/1802.03474
[4] https://www.sciencedaily.com/releases/2020/10/201029135503.htm
'Direct measurement of the contribution of street lighting to satellite observations of nighttime light emissions from urban areas' (2020) https://journals.sagepub.com/doi/10.1177/1477153520958463
[5] International Dark Sky Association (IDA)– "France Adopts National Light Pollution Policy Amongst Most Progressive In The World" (2019) https://www.darksky.org/france-light-pollution-law-2018/
[6] Fotios & Gibbons "Road lighting research for drivers and pedestrians: The basis of luminance and illuminance recommendations" (2018) https://journals.sagepub.com/doi/full/10.1177/1477153517739055



The Good, The Bad & The Ugly




Image Source: US Dept of Energy / Acuity

CCT - Correllated Colour Temperature is often used to describe how 'white' a light appears to the human eye. The figure in Kelvin (K) relates to the emission spectrum of a perfect black-body when heated to that temperature. For example, when first heating a tungsten filament it emits a dull-red light & as it heats up will glow orange/yellow & at higher temperatures will become ‘white’ hot. This is why higher Kelvin CCT lights appear to have a blue-white light appearance and lower Kelvin lights a softer yellow hue.

CCT is quite a crude measure of the actual spectrum of light, but in general it is best practice to use lamps with a CCT 3000K or preferably less.

Paradoxically we perceive the lower CCT lights as 'warmer' and the higher CCT lights as 'colder'...



Light Trespass



Light Prolifigacy