The Universe in the Classroom

Light Pollution

Villains and Heroes of Lights and Lighting Fixtures

If we are to reduce the effects of light pollution, we must learn about the types, both good and bad, of fixtures used in outdoor lighting. Some of these are heroes: They save energy, direct light to where it is needed, and add to the ambiance of our nighttime environment. Others are villains that waste energy, spill light in all directions, produce glare, and ruin the beauty of the night.

The Villains

Globes and similar fixtures. These usually consist of a transparent or translucent "ball" or cylinder with a lamp bulb inside, sitting atop a pole. They look attractive in the daytime when they are not turned on. In fact, this type of fixture is often chosen for this very reason.

These lights are villains because when turned on at night, they radiate not only in the direction where illumination is needed, but in all directions - more than half the light is directed upward to add to the sky glow and sideways to cause glare. Much of the rest is not doing any useful work in helping us see the things we need.
L.A. at night
So many billboards, so much wasted light.

Billboards and signs lit from the bottom. Look at the billboards around town. How are they lit? In most cities, it is from below and not from above. If one sees the boards at night, especially in a fog, one can easily see all the up-going light, a lot of it not hitting the board surface at all, the rest bouncing off the board and going up into the sky. These are villainous lights, making a major adverse impact on urban sky glow. Can we make a "hero" of these kinds of installations? Easy, illuminate them from above so that nearly all the light hits the billboard: The light is reflected once from the board to the ground and then again off the ground before going up. This reduces the amount of up-going light a great deal.

Mercury vapor lamps. Although these lamps are not very energy efficient, and they are often found in older, inefficient fixtures that enhance light pollution, they still exist in great abundance in most locations and are still sold by the millions. Why? They are deceptively cheap. They don't cost much to buy, but they cost a lot to operate, more than any of the more modern energy-efficient lamps; over their lifetimes, they end up costing the owners much more than the efficient lamps. In addition, the style of lighting fixtures used for mercury vapor lamps is usually obtrusive, glary, and inefficient.

How do we recognize these villains? Just look for their harsh blue-white light. Mercury vapor lamps are often the illumination sources in glary "dusk-to-dawn" security lights or the so-called "barn lights" seen in rural locations.

Badly aimed or misdirected lights. Otherwise hero lights can become villains if they are badly aimed or mis-installed. Spot lights often become villains for this reason, putting their light output where it is not wanted or needed - through our bedroom windows or directly into an automobile driver's eyes. One must carefully install and aim lights to get the best performance and least glare out of them. All too often lights are installed in the daytime and never checked or aimed at night for good performance.

Wall-mounted, non-directional lighting fixtures. We have all seen these villains in action. Such a light, whatever kind of lamp it has in it, is usually mounted on a wall, often above a door or simply on the side of a building. The building's owner wanted to illuminate the entrance or the area around the building to help with visibility and to deter criminals, but he goofed: The glare makes it difficult for visitors to see well and, thus, aids the criminal who lurks in the nearby darkness. And lights of this kind do nothing to add to the ambiance and attractiveness of the facility; they probably repel more customers than they attract. However, such lights can be made into heroes by adding good shielding that eliminates glare and directs the light output to where it is useful.

Dropped refractor fixtures. These fixtures are everywhere. Our common street lights are dropped refractor "cobra heads," so named because of the supporting "neck" and the flaring "head" where the lamp is mounted. They have a glass lens or refractor which is "dropped" (hangs down below the fixture) and from which the light output is spread onto the street surface in both directions. Some fixtures of this type have reasonably good control of the light output, but most do not, and most all are very glary.

Other inexpensive fixtures often have nothing but a sort of cylinder of glass which directs the light out horizontally from the fixture; these are usually great sources of glare. In some cases, the "refractor" is nothing but an ineffective glare shield helping somewhat to diffuse the light output from the bare lamp. Light coming from a street lamp configured like this is almost useless because, by the time it strikes any surface (including the ground), it is too faint to provide much illumination and becomes wasted light.
Before low-pressure sodium lights After low-pressure sodium lights
Righting the problem. Local changes can substantially reduce the problem of light pollution, as these images of the University of Arizona in Tucson demonstrate. Before (left), the University used 400 watt mercury vapor lamps; after the University agreed to replace the glary, energy-inefficient lights with 135 watt, low-pressure sodium lights (right), the light levels in dark areas were increased, less energy was used, and sky brightness waas greatly reduced.

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