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| Light Pollution - An Overview | | | The skyward portion of outdoor lighting is utter waste: a waste of energy, a waste of fuel resources, a waste of utility money, and a waste of the dark night sky.
Urban Skyglow
Many people who are living in cities have never seen the Milky Way. The true beauty of the heavens is usually blotted out by urban skyglow due to excessive or ill-designed lighting. In the immediate vicinity of the city, this skyglow is so intense that even the brighter stars become invisible to the eye. Only the very bright stars and planets outshine the city sky. The dimmer star clusters, nebulae, and galaxies most important to astronomy are lost in the glare. Since the turn of the Century, urban astronomers have noticed that, when the last glow of dusk disappears, the sky fails to darken completely. Rural areas have not escaped the problem because skyglow from large metropolitan regions can be seen from more than 150 kilometres away.
 | Flood-lights in multitude are used to light up vast areas of greens as well as the sky above golf-courses.
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Astronomical observatories around the World are plagued by light pollution too. Today there is almost no observatory on Earth where illumination from distant cities has not or will not become a problem. One of the well-known examples is the sad case of the 254-cm telescope atop Mount Wilson in California, US, which has been the site of an astonishing number of major astronomical discoveries. The telescope was recently decommissioned in part because of severe skyglow from the Los Angeles basin. In the past few years, night golfing activities are becoming more popular in Singapore. Flood-lights in multitude are used to light up vast areas of greens as well as the sky above golf-courses. It has become one of the major contributing factors to local skyglow. The unsightly aura around one of the country clubs here is bright enough to obscure the brightest stars in the sky.
Waste of Energy
Light pollution is a general glow in the night sky hanging over most cities. This light does not contribute to safety, utility, or security, yet it significantly obscures our view of the celestial objects. The problem is mainly due to the wasteful pattern of illumination from many lighting fixtures, and from careless placement of fixtures.
 | Outdoor lighting is the major source of light pollution. Skyglow from metropolitan regions, industrial or commercial areas can be seen from far away. |
Outdoor lighting is the major source of light pollution. It has sprawled enormously in recent decades. If you look down from an airplane at night, it is easy to tell commercial centres from surrounding residential areas by their distinctive patterns of waste illumination. What you normally notice is not the illuminated ground, since the reflectivity of urban surfaces is typically under 15%. The light you see aloft comes directly from lamp fixtures which fail to direct all their light onto the ground where it is needed. The skyward portion is utter waste: a waste of energy, a waste of fuel resources, a waste of utility money, and a waste of the dark night sky.
Lamps for Lighting
Most lighting sources emit light throughout the visible spectrum. These continuous-spectrum lights include incandescent (ordinary light bulbs), fluorescent, and pinkish high-pressure sodium vapour lamps. Other types emit their light in a few specific spectral lines. These are called emission-line lights which include low-pressure sodium vapour and mercury vapour lamps. The energy required to run these lamps comes from electricity.
The electric power it takes to run a lamp is measured in watts, while the amount of light we get from a lamp is measured in lumens. Thus the efficiency of a lamp in converting electric energy into visible light can be compared by noting how many lumens per watts it produces. The more lumens we can get out of every watt, the less it will cost to produce light with that lamp. The table below shows the typical efficiencies of various lamp types:
| Lamp Type | Lumens per watt | | Incandescent | 20 | | Mercury Vapour | 60 | | Metal Halides | 80 | | Fluorescent | 100 | | High-pressure Sodium | 140 | | Low-pressure Sodium | 200 |
Clearly, low-pressure sodium lamps are the most efficient form of lighting from the point of view of operative costs. These lamps glow with a characteristic yellow colour. Because the human eyes are most sensitive to yellow emission lines, low-pressure sodium lamps produce the best outdoor lighting for visual acuity. Low-pressure sodium lamp is strongly preferred for use in the vicinity of observatories, for its light is monochromatic. For certain types of observation, scientists can overcome some of the effects of light pollution by using filters designed to block all but a portion of the spectrum or to filter out specific wavelengths.
Mercury vapour lamps, besides squandering electricity, have the disadvantage of giving off harmful ultraviolet rays if not properly filtered. The use of these lamps became widespread with poor lighting fixtures designed in the "old days" of cheap energy. Unfortunately, because they are inexpensive, many of these fixtures are still sold today. However, these lamps are not cheap to operate, as the table above indicates.
Dollars and Sense
Light pollution is a large, complex problem. It will not go away overnight, and it will certainly get worse in most areas before it gets better. Nevertheless, viable solutions exist. It is important to bear in mind that the problem is totally controllable. Furthermore, precious energy resources are being wasted.
Note that it takes about 290 litres of crude oil to produce 1,000 kilowatt-hours of electricity. A 200-watt dusk-to-dawn mercury vapour lamp uses, with ballast, about 1,000 kilowatt-hours of electricity and costs $135 per year to operate. If we estimate conservatively that 30 percent of illumination at night is wasted, going elsewhere but on the ground where it is needed, then every year some 90 litres of crude oil are consumed needlessly by each such fixture. As an alternative, a 40-watt low-pressure sodium lamp with proper fixture would cost less than $40 a year to operate and provide better light.
The Night Sky
The single most important consideration for those who have a deep love for the infinite beauty of the night sky is that outdoor lights be shielded properly. If this practice were more wide-spread, there would be a significant reduction in light pollution. The resulting darkening of the night sky would be truly dramatic. There is an overwhelming aesthetic imperative in the pure beauty of the star-filled sky. This is a sight currently denied to city-dwellers.
 | The unsightly aura around country clubs is bright enough to obscure the brightest stars in the sky.
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Poorly shielded fixtures produce glare, light trespass, and loss of dark adaptation. There is no excuse for light trespass. All lights can be properly shielded. Actually a fixture with properly designed reflector or refractor elements has no need for a shield, as long as the light is aimed at the ground. Enclosing a poorly designed fixture is only a temporary solution. The only real cure is to retrofit bad housings with full-cutoff fixtures which throw no light skyward.
However, light reflecting off buildings and floor in large cities is much more difficult to overcome. We want well-lit streets and parking lots, but we also want dark skies. In fact, both are readily achievable. If we learn to use well-designed lighting, and use the lowest wattage necessary for the task, than the skyglow in even the largest cities could be reduced to acceptable levels.
Public Education
Understanding and controlling light pollution is important - not just for astronomers but for everyone. Poor lighting wastes literally billions of dollars each year across the world. Everything that is done to minimise light pollution also saves energy by improving the efficiency and utility of the nighttime lighting.
The light pollution problems created by ineffective, inefficient light fixtures can be solved; clearly the first step toward that solution is education. We would like to believe that no one deliberately wants to install and use inefficient and bothersome outdoor lights. It is up to concerned individuals to inform the public about the problems of bad nighttime illumination and to help establish workable solutions.
At present, a lack of awareness rather than resistance is generally the biggest problems in controlling light pollution. It is our hope that this article will help to increase that awareness.
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