About Light Pollution

What is Light Pollution?

Light pollution is unwanted or unnecessary light. It falls into four basic categories:

Light trespass is where a light illuminates an unwanted area, such as a streetlight illuminating a bedroom. Sky Glow is the illumination of the atmosphere by unshielded lights. The light thrown up into the air is dispersed in the atmosphere, causing the stars to be almost entirely blotted out. Bright lights, such as security lights or floodlights, are examples of light waste, where there is too much light for the job. This can cause glare which dazzles the eye so it is unable to see clearly. The effects of poor lighting are show in the diagram below.

Why should I be concerned about Light Pollution?

Light pollution affects everyone, not just astronomers. Yet until the 1980s few people complained about the glare caused by streetlights or security lights, assuming it was the price paid for better lighting. This changed in 1988 when David Crawford, Tim Hunter and others founded the International Dark Sky Association (IDA) in the USA. The British Astronomical Association went on the following year to set up the Campaign for Dark Skies (CfDS), which fights for the same causes in the UK.

Many people think that astronomers want to switch off all their lights so they can see the stars better. This isn’t true. The IDA and CfDS merely encourage good lighting practices that minimise light pollution and are beneficial to everyone. These benefits include being able to see the Milky Way, doing less harm to the environment and increasing safety.

It is a sad fact that according to a survey done by the CfDS 90.3% of people are affected by sky glow, 87.1% rating it as "severe". In fact the only sign of civilisation that can be seen from space is not the Great Wall of China, but the glow of badly designed lights.

Image from NASA GSFC

All primary school teachers say there are two things children love learning about in science: dinosaurs and space. Yet one of these sources of wonder that could encourage them to study science, is being denied to them through carelessness; a shame considering how few people there are in Britain now studying in this field.

But it’s not only children who are affected. Britain is foremost in the world for the amount of amateur observing done: between 100-120 nights a year. According to Guy Hurst this "astonishes our overseas’ colleagues, who have better conditions but do not observe anywhere near as much as in this country." Yet we allow the levels of light pollution to grow until 4.6% of people say the pollution is so bad that no stars are visible.

Let us also consider the cost to the environment. Every year 330 million Watts of energy is wasted by streetlights in the UK where much of the light is directed above the horizontal. This extra energy results in 400 million tonnes of Carbon Dioxide being emitted from power stations every year. A single security light left burning for 12 hours every night in a year will produce 1.75 tonnes of carbon dioxide.

In Northern Ireland £1.8 million is spent on purchasing and installing new street lights per annum, whilst £2 million is wasted on paying for energy we wouldn’t need if full cut-off streetlights, which direct all the light down, were used. For every streetlight in one year £16 is spent on the energy it uses. Yet if the excess light from these lamps were removed £8 of this money could be saved. Surely halving the electricity bill for streetlights would make economic sense.

Another large source of light pollution is the security lights used in homes. The Institute of Lighting Engineers clearly state in their Guidance Notes for the Reduction of Light Pollution that "a 150W tungsten halogen lamp is more than adequate," yet most people have lamps of between 300-500W. To put this into perspective, the brightest lighthouse in Britain has a 1000W light, which can be seen for 25 miles away. Therefore many people effectively have half a lighthouse in their back gardens.

The irony of this situation is that, although these lights are installed to make people safer, they have the opposite effect. Because the bulbs are so bright objects cast very dark shadows, which facilitate hiding. An on-looker's eye will be dazzled, making it difficult for them to see. Their pupils will contract so much that they will see almost nothing in the shadows in which a potential burglar would hide. This is illustrated in the photographs below. The light is very bright, so that when the woman stands in the shadow against the lamp-post she is almost invisible; the contrast is too great for the eye to detect anything in the dark. By using a lower wattage and directing the light downwards, away from the eyes of a potential witness, the difference between light and dark is minimised. There are far fewer shadows, and since the contrast is much less it is easier for an on-looker to see into them.

 

We should also consider the cost of this unnecessary light on animals. Birds in particular are very sensitive to light, a good example being chickens, who can’t differentiate between night and day if the illuminance is greater than 0.5 lux, and therefore don’t know whether or not to roost. However some of the most shocking examples have been with migratory birds. Birds are naturally attracted towards light, especially when it is dark and even more so if the weather is bad. They will fly into a beam of light, and, if they don’t collide with the light source, continue to flap around in it until they collapse from exhaustion. In one example 50 thousand birds died in two consecutive nights at Warner Robins Air Force Base, Georgia, in 1954 when birds flew straight into the ground, following the light.

Turtles are also very sensitive to light. They always bury their eggs so that the moon is over the ocean. Hatchlings instinctively follow the brightest light they see, so traditionally they all move towards the ocean. However, if the brightest light they see is not the moon, but the lights from a town they become disoriented and head towards that light instead. This almost always leads to their death: they are eaten by predators. One study showed that there were 46,845 hatchlings disoriented due to bad lighting in Florida in the space of only one year. But, by introducing the Marine Turtle Protection Act in Florida which forces lights to be turned off around the time of the hatchings, the number of disoriented hatchlings fell to 28,587 within a year.

 

 

How can I minimise Light Pollution?

Light pollution can be minimised by following good lighting practices, such as those recommended by the Institute of Lighting Engineers (ILE).