Light pollution is said to occur when artificial sources of light interfere with the natural night environment as a result of the glare that such lights create. This phenomenon is essentially a function of the increased, and increasing, level of urbanization worldwide. Thus, light pollution is especially a problem in the world's urban sprawls where the pollution from the lights of the city diminish the view of the stars and planets. Night-time satellite photographs of the world's cities shows this kind of pollution as glowing regions around them.
Light pollution is caused by outdoor lights, of which the cities of the world are plentifully provided, aimed upward or sideways. When the light hits the atmosphere, it scatters and is reflected back to the ground; it is this reflected light that shows up as the characteristic glow that is seen in night-time satellite photographs of the world's cities.
Light pollution is also related to other forms of pollution; the more polluted the air of a city is, the more the problem of light pollution, as those particles in the atmosphere which are responsible for air pollution also cause an increase in light pollution by increasing the scatter-properties of the atmosphere. Indeed, a study by the U.S. National Oceanic and Atmospheric Administration suggests that light pollution may worsen the effects of other forms of pollution such as smog. Apparently, light pollution destroys nitrate radicals which normally bring about a night-time reduction in the smog that has been emitted into the atmosphere during the day by cars and factories. Further, light sources that emit blue light cause more light pollution than other types of lights, as blue light is more prone to scattering than other light colors.
Light pollution has quite a disruptive impact on the planet and, like other forms of pollution, its effects can be felt very far away from the source of the pollution; according to the U.S. National Park Service, city lights can affect the natural night environment as far away as 200 miles (320 kilometers). The pollution can also affect the natural behavior of animals and plants. Sea turtles, migrating birds, as well as nocturnal animals, are easily disoriented by the glare and the lights are also an attraction to insects, whose distraction may result in a reduction of the pollination of nocturnal flowers. Not even humans are unaffected; light pollution can affect the natural circadian rhythms of sleeping and waking by affecting the production of melatonin, a chemical that plays a major role in the regulation of human sleep patterns, low levels of which have been linked to an increased risk of cancer.
But it is in its effect on astronomy that the insidious nature of light pollution really shows. Astronomy is, essentially, a night science as the overwhelming presence of the daytime sun limits what may be done during daylight hours. Further, clear skies are necessary if the best results are to be gleaned from astronomical observations. What the increasing light pollution in more and more of the world's leading cities has meant is that it has become harder, and it is becoming increasingly moreso, to do any meaningful astronomical work in urban areas. The night skies, as viewed from a city, most often bear no resemblance whatsoever to the skies when viewed from a dark sky area, i.e. an area that is less susceptible to light pollution. The glow from scattered light reduces the contrast between the stars and galaxies, on the one hand, and the sky, on the other. As a result, it becomes more difficult to view fainter objects than it would be in a place that has less pollution.
Apart from the background glow, which affects the objects astromers attempt to view, there is also the possibility that light sources in urban areas are more likely to directly impact astronomical observation, as when light finds its way directly into a viewing instrument such as a telescope and its reflection from the non-optical portions causes a glow across the instrument's field of view which can reduce the contrast between the viewed object and its background. This optical pollution, as it has been called, also has another effect, to wit; the urban observer finds it more difficult to become properly adapted to the dark, thereby increasing the difficulty of observation.
For these reasons, it has become increasingly common for new observatories to be located in isolated areas far away from the bright city lights, e.g. some Chilean observatories are located deep in the Atacama Desert, far away from the bright city lights and new ones are springing up. But, of course, not all observatories can be located in isolated areas. Long established observatories can hardly be moved lock, stock and barrel, from Cambridge, Massachusetts, for instance, to the wilds of the California deserts. As a result, urban observers have devised some approaches that help to reduce the harmful effect of light pollution.
Thus, nebula filters, which allow only certain wavelengths of light known to be emitted by the object that is being viewed, and broad-band light pollution filters, which help to reduce the hazards of light pollution by filtering out those wavelengths that are commonly emitted by the sodium/mercury vapor lamps that are most common in the world's urban areas, have been developed so that contrast is improved and dim objects are better seen. Similarly, when the observer is concerned about optical pollution and a direct elimination or reduction of the offensive light is not possible, the telescope tube and it's associated accessories may be flocked. Flocking is a process whereby the inside of a telescope tube is painted or lined by a light absorbing paint/material. Although the insides of most telescopes are painted black, flocking may still be needed in order to reduce the danger of stray light affecting a viewing to the barest minimum.
Additionally, a light shield, which can also double as a dew shield, may be used with the telescope, thereby reducing light entering into the telescope from angles other than those near the object being viewed. Many astronomers will also carry out their viewing under a black cloth so that they are properly adapted to the dark.