Sports Field Lighting
Sports lights can be placed into one of three categories: fully shielded, shielded, and unshielded. One might think of them as good, bad and ugly. Fixtures used on courts are very similar to those used on fields and in some cases may be used interchangeably. The major difference is the area that needs to be lighted. Fields are generally wider than courts, so lights mounted on the sides of a field need to reach a longer distance. This is probably why field lights are rarely fully shielded.
To understand more, first consider what light pollution is. The components of light pollution include: ® Glare, which can blind pedestrians and drivers, harming visibility. Glare is never good. ®Light trespass, which is outdoor lights that trespass onto other’s land.
®Urban sky glow, which is destroying our view (and our children’s view) of the universe (it would be a shame if our children thought that the Milky Way was only a candy bar). Sky glow is the atmospheric phenomenon caused by stray ground-based light being scattered and reflected by airborne particles suspended in the atmosphere. The result of this scattering diminishes the view of the cosmos, much like turning on the lights in a movie theater while the film is showing.
®Energy waste, shown to cost over one billion dollars a year in the USA alone.
To qualify as fully shielded, a luminaire must have zero light emitted above 90 degrees from straight down. This means that fully shielded lights contribute very little to sky glow.
Light emitted above 90 degrees only illuminates the bellies of birds and airplanes. However, fully shielded lights can still cause glare and light trespass. It is best if the light emits zero light above 70 degrees. Light emitted above 70 degrees (as in the top half of the NEMA fixture diagram) is wasted: by the time the photons finally reach the ground (if ever) they do no good.
The lower half of the NEMA diagram shows a light that is limited to no more than 70 degrees from the vertical. This is the ideal type of light for outdoor use. The second figure shows how a flat bottomed luminaire can be configured such that no light goes above 70 degrees. However this could still cause glare or light trespass, depending on where the light is located, the terrain, and the height of the pole.
Bainbridge municipal code states that all light trespass is prohibited, and that “All light sources shall be hooded or shielded so the lamp is not visible from adjacent properties or public rights-of-way.”
How far would a person have to be from a fully shielded lamp so that it would not be visible? It depends on several factors, but for a light pole of height h, and a luminaire that emitted light no more than 70 degrees above vertical (angle ? in the following diagram), then distance d is equal to the tangent of 70 degrees times the height h. For example, if the height is 60 feet, then d would be almost 165 feet. If the height is 90 feet, then d would be a bit over 247 feet.
These examples assume level terrain. The above computations demonstrate why sports fields need to be carefully located, to avoid glare and trespass, even with full cutoff luminaires. And if light is emitted above 70 degrees, and especially above 90 degrees as with most sports lighting, then it may not be possible to comply with the municipal code.
A few examples of fully shielded and shielded luminaires may be worth a thousand words of explanation. A fully shielded luminaire has a flat bottom, and looks like this example.
On the other hand, shielded luminaires, which emit light above 90 degrees, look like these examples. These lights have shielding, but because the fixture is at an angle, some light escapes into the sky. This precludes them from being called fully shielded. And, since sports lights need to be very bright, using unshielded luminaires, or partially shielded luminaires, leads to excessive light trespass and glare. Nearby buildings will be illuminated as if it were daytime.
See http://www.darksky.org/fixtures/sports.html for lists of sports and field lighting that have the IDA Fixture Seal of Approval. The web page http://www.darksky.org/infoshts/ is027.html describes a situation at Wesleyan University, near the site of a disused observatory. The replacement of inefficient lighting both reduced nighttime crime and allowed students to see some constellations for the first time in many years.
When done wrong, a sports field is bad for everyone: players, neighbors, pedestrians, drivers, and those that love the night sky.