LIGHTING/ Reducing light pollution through specifying, design
Specifiers typically design highway lighting systems the way they have always designed them, meeting the requirements of seven minimum footcandles and stringent maximum/minimum and average/minimum ratios. However, quality-of-life concerns are resulting in specifiers getting more complaints today from citizens and local officials than they used to. That is because they not only have to meet required lighting levels, they have to do it in a manner that eliminates light pollution and trespass and “skyglow.”
Drivers demand lighted highways for reasons of safety and security, but property owners do not want stray light in their homes. The easy answer for preventing light pollution (which is a perception of brightness and not the same as the measureable and quantifiable problem of light trespass) is the house-side shield – a large sheet of metal fastened to a light fixture that has already been installed near the house. Skyglow, on the other hand, is light that extends above 90 degrees and is, thus, wasted. Skyglow is the primary culprit addressed by local Dark Sky ordinances and requires different methods of control than do light pollution and light trespass.
New roadway lighting designs with integral internal louvers and shields to control stray light now are available. They are designed for roadway lighting with optical systems that set new standards for light control, and they are economical to install and maintain. Additionally, they are constructed to comply with Dark Sky ordinances.
Although they are not equally as efficient at reducing light pollution, numerous roadway lighting systems and fixtures exist:
– Highmast fixtures. Commonly used and economical for interchange and roadway lighting, highmast systems are aesthetically pleasing and offer low operating costs. They do contribute to light pollution and light trespass.
– Dark Sky fixtures. At best a compromise, “Dark Sky” fixtures cut off all light at and above 90 degrees. Typically, they have flat glass lenses as opposed to prismatic glass refractors. They do not address light trespass, or, more importantly, light pollution, but they can minimize the problem of skyglow.
– 80-degree shield. The fixtures feature total cut-off above 80 degrees from the vertical with 360-degree radial control. Light above 80 degrees never reaches the ground, but it is the most objectionable source of direct glare and fixture brightness.
– 45- and 75-degree louvers. Fixtures with internally mounted light control options can help designers optimize fixture placement and reduce maintenance costs. The fixtures provide for maximum spacing along roadways and offer light control off the right-of-way. They are generally considered the most efficient way to minimize light pollution and light trespass.
– Off-set fixtures. A contemporary version of the cobrahead fixture, the off-set fixture provides good light distribution on the roadway, but it is responsible for the greatest glare and light pollution of all roadway fixtures in use today. The fixture is basically a cobrahead mounted at a 30- to 45-degree angle.
– Cobrahead. The “workhorse” of roadway lighting, the cobrahead typically features a prismatic glass lens that creates a source brightness visible from several miles away. To reduce light pollution, cobraheads must be fitted with internal louvers, and the median master style remains a better alternative.
– Median master style. The most economical solution and aesthetically pleasing alternative to common cobrahead or off-set fixtures for roadway lighting, the median master style fixtures offer internal shielding options. It prevents skyglow, and, with internal louvers, it can prevent light trespass and light pollution.
– Ed Kramer, Metrolux Lighting, Richfield Springs, N.Y.
