Over 100 years ago talk of light pollution would have made people look at you like your head was on backwards. The night was dark and threatening. The more light the better. Now we're seeing what millions of lights can do to the night environment. Cities and organizations around the world have been adding ordinances to reduce light pollution. Lighting seems like a simple enough subject but trying to make a universal lighting ordinance that gives us all the good stuff we like from lighting at night without all of the negatives is proving very difficult. The most recent attempt at a Model Lighting Ordinance to be used as a universal lighting ordinance was viewed by many as being too complicated for adoption since cities and developers wouldn't want the headache and cost of understanding it and enforcing it.
Here is an attempt at a simpler option which cities of various sizes may choose to add as their lighting ordinance. It is not intended as a perfect, bullet-proof ordinance. Rather one which is simple and effective in 90% of the situations.
At 10 feet (3.0 m) beyond the property line the vertical light at 5' above grade level shall not exceed 0.10 fc, except at drive entrances to the site. Average illumination of hardscape shall be between 0.5 and 1.5 fc. No point on the site shall exceed 10.0 fc horizontal at grade level. All site fixtures with lamps over 50W to have cutoff optics. Limit total site uplight to 5% based on initial lamp lumens.
It's best to understand what each part of the ordinance is doing. There are 4 main types of light pollution: light trespass, over illumination, glare, and skyglow.
Light Trespass[edit | edit source]
At 10 feet (3.0 m) beyond the property line the vertical light at 5' above grade level shall not exceed 0.10 fc, except at drive entrances to the site.
Stating "zero light at the property line" is too vague. Absolute zero means that even if a light fixture is a mile away and the light source is visible, it is in violation, and would require hoods to be placed over every light fixture. What is realistic may vary according to whether an area is residential or industrial, urban, suburban or rural. The credit offered by LEED provides limits at the property line and 10-15 feet beyond it. At the 10-15 foot distance LEED limits light to 0.01 fc. (For comparison, a full moon provides 0.03 fc and a moonless night 0.004 fc). This is a very difficult limit to comply with while providing even light on a parking lot and driveway. Most light poles would need to be at least 70' away from the property line. How is the light to be measured? Horizontal measurements are common for interior and exterior lighting calculations. However, for light trespass the concern is how much light shines into a neighbors property or window. Since light into the window causes all of the complaints let's focus on the vertical light measurement rather than the horizontal measurement. Window heights and locations vary so we'll measurement at approximate eye level (5' high) of the vertical light level facing into the site. Exceptions might be allowed where drives enter the street. This would permit street lights at the drive entrance to make cars more visible as they pull into traffic. As ideallistic as we want to be limiting light pollution there's going to be some spill onto a neighboring site. We're trying to keep it down to a realistic level. Lighting technology may have improved a lot in recent decades but there are no magic wands to stop all spill light.
Over Illumination[edit | edit source]
Average illumination of hardscape shall be between 0.5 and 1.5 fc. No point on the site shall exceed 10.0 fc horizontal at grade level.
Another effective ordinance is to limit the maximum light on the site. Businesses are in an arms race with their neighbors. The brighter the building or parking lot the more business it attracts. Putting a limit on the arms race allows the lighting designer to tell the developer 'No' when he demands more light. Commonly a parking lot is lit to get an average of 1.0 fc at night. When our eyes are adjusted to the night environment 1.0 fc is more than adequate light. But this is an average. The light is always brighter directly under the pole than between poles. Since there's more area between the poles than under the poles the result is hot spots under the poles quite a bit higher than 1.0 fc. Most parking lots should be able to get good coverage without going over 5 or 6 fc. Allowing up to 10.0 fc would be for the larger shopping plazas. Any more and we are reaching indoor, daytime light levels. Pay attention to the Light Loss Factor (LLF) they use in the submittal. Light levels go down as lamps age and dirt gathers on the lens. A LLF of 0.65 (35% light loss) is common for MH and HPS lamps to estimate light output after several years of use. Anything lower and they're manipulating the calculation to get a brighter site while allowing you to think it's not so bright.
Also it might be good to know what we're measuring. 1 fc means 1 foot-candle. A foot-candle is a measurement for light levels on objects. There's another unit in the metric system but in the US everyone is familiar with foot-candle. To get an idea of how much 1 fc is think of a dark room with a single candle in the center as the only light source. Hold your hand next to the candle 1 foot away from the flame. The amount of light you see on your hand is 1 fc. 1 foot away from 1 candle. 1 fc. But light level decreases quickly with distance. 2 feet away from the candle your hand only has 0.25 fc.
If you'd prefer the metric system the conversion rate is One footcandle ≈ 10.764 lux. Many people use a quick 1:10 ratio for conversion. So either 5 to 15 lux or 6 to 18 lux could be used as acceptacle ranges for average illumination on the site. Maximum not to exceed 100 lux or 108 lux. People tend to like using round number and the difference between the numbers would not be noticable by people. Think of the lux unit as the amount of light 1 meter away from a candle.
Glare[edit | edit source]
Covered in Light Tresspass and Sky Glow. Many cities have traditionally limited pole height to reduce glare. This was before fixtures were designed to "cutoff", "semi-cutoff", and "full cutoff" standards. With the old glare bombs like street lights and yard lights the best way for a city to reduce the glare was to limit the height so trees and other buildings would quickly block the light before it traveled too far. These newer cutoff style light fixtures not only limit the light going up into the sky but they also limit light between 80-90 degrees above nadir. This area causes the most problems with glare. And it's being improved at the same time we're limiting sky glow. If cities want to limit pole heights it should be for asthetics rather than trying to limit light pollution.
Sky Glow[edit | edit source]
All site fixtures with lamps over 50W to have cutoff optics. Limit total site uplight to 5% based on initial lamp lumens.
Allowing 50W lamps to aim up is mostly for things like flag lights or decorative wall sconces on the building. Some ordinances choose to limit uplights based on lamp lumens. People are much more familiar with lamp wattage than lamp lumens. By focusing on wattage it encourages the designer to choose more efficent lamps. If technology improves significantly this can always be updated with a lower wattage. The second sentence is to prevent people deciding to put (1000) 50W lamps all aimed up. Lots of little lamps do add up. So if the site lighting levels are limited and this limits the percentage of uplight we now have a cap on the maximum uplight for the site. It's a somewhat flexible cap but does provide a limit. To give an idea of what 5% up light is.
- 10 * 250W MH parking lot lights with full cutoff optics: 20,000 lm each = 200,000 lm
- 3 * 50W MH flag lights, all of which is aim up: 3325 lm each = 9,975 lm
- 9,975 / (200,000 + 9,975) = 4.75% uplight for the site.