Planting virtual trees: the challenge of meeting LEED requirements for light-pollution reduction (MAGAZINE)

Oct. 20, 2011
Reducing the amount of obtrusive light in an outdoor-lighting scenario while also minimizing the carbon footprint of the lighting system presents a difficult challenge, writes MICHAEL SMOLYANSKY.
Computer simulations are widely used in many aspects of our daily life. In the practice of engineering, it is critically important to verify the computer model before attempting real-world implementation. Similarly, we would expect some degree of precision to be found in computer simulations of the light distribution produced by luminaires. Sometimes, however, that is not actually the case.

Consider the key parameter regarding the thermal management of an LED device, the junction temperature, which can be simulated by using a computer model. Direct measurement of the junction temperature is often impractical. However, the junction temperature can be accurately calculated, based on a known case or board temperature and the materials’ thermal resistance.

In the practice of engineering, it is common to have information about parameters that can’t be directly measured; such information is based on other parameters which have a strong correlation to the parameter in question. Unfortunately, these engineering principles don’t seem to apply to LEED (Leadership in Energy & Environmental Design) Light Pollution Reduction requirements. In reality, we are trying to measure the immeasurable.

LEED requirements

During a recent meeting of our local IES chapter, we discussed various approaches to prevent and/or reduce outdoor light pollution. One discussion addressed LEED requirements, especially Sustainable Sites credit 8 (SSc.8): Light Pollution Reduction. Some useful strategies mentioned included the use of full-cutoff optics, spill-light shields, and the reduction of both pole height and luminaire wattage.

However, even while incorporating all of these measures, compliance with LEED requirements remains very challenging. “There is not really a problem to meet strict LEED requirements,” said one lighting designer who participated in the discussion. “Just plant as many trees and bushes along the property line as you need to reduce light pollution to the required level in your computer model.”

FIG. 1. While this "virtual trees" suggestion was offered tongue-in-cheek, even that approach would not translate to real-world results because of the effect of moonlight, which ranges from 0 fc to 0.04 fc, depending on the phase and the sky conditions. Even using an average of 0.01 fc for moonlight’s contribution, it becomes impossible to ensure that the light pollution doesn’t exceed the 0.01 fc LEED requirement.

The “virtual trees” suggestion highlights the schism between achieving results in the real world versus the virtual model. The LEED Reference Guide shows strategies to achieving Credit 8 requirements based on a layout where the property line is located 25-30 feet from the lighted area (courtesy of Clanton & Associates). Who are the property owners that would keep this expansive buffer zone just to achieve one credit towards LEED certification?

In reality, a property line is generally located very close to the parking lot or other public area. Even utilizing the best shielded optics in the industry, it is extremely difficult, if not impossible, to meet LEED requirements without at the same time compromising recommended illuminance targets for exterior applications.

Carbon footprint

LEED was developed to reduce human impact on ecological systems, reduce carbon footprints and other industrial pollutants, and to reduce global warming. Some parts of the document provide guidelines for reducing human impact on our planet through saving energy, water, land, and materials. Other sections describe how to improve the quality of living and working environments without increasing our carbon footprint. Credit SSc.8 Light Pollution Reduction provides guidelines for reducing obtrusive light, but without respect to carbon footprint.

Carbon footprintObtrusive light
Cost & impact of mining the materials usedImpact on humans
Energy used in productionImpact on the environment
Energy used during product life
Disposal/recycling costs
Table 1. Environmental impacts of outdoor lighting

According to the recently-approved Model Lighting Ordinance: “The environmental impacts of outdoor lighting fall into two categories: carbon footprint (energy used in the life of a lighting product) and obtrusive light.”

However, these two concepts, summarized in Table 1, are challenging to achieve simultaneously because they contradict one another. The strategies that have been proposed for how outdoor-lighting systems can get closer to achieving LEED Light Pollution Reduction requirements serve only to highlight the contradiction between the two objectives i.e. reducing carbon footprint and reduced obtrusive light (Fig. 1):

– Use shielded optical systems. These are inherently less efficient than unshielded systems, and therefore require more energy, more raw materials, and proportionally more greenhouse gas emission.

– Utilize more luminaries with lower wattage and lower mounting heights. From our design experience, the average LEED project requires 1.5-2 times more luminaires and poles than non-LEED projects. That means essentially more raw materials and energy must be used for manufacturing the fixtures and poles, as well as more energy consumption throughout the site.

All this is contrary to the basic tenets and goals of sustainability. The reduction of light pollution is a good idea only if its implementation doesn’t increase carbon footprint. The carbon footprint of a compliant LEED SSc.8 lighting system should not exceed the baseline performance of a non-LEED compliant lighting system. Otherwise we are simply trading one set of problems for another.

Setting boundaries

LEED requirements for boundary-line spill light are reasonable only if the bordered property has a lower zone classification. For example, if the designed property is an LZ3 zone and this property borders two other LZ3 properties and one LZ2, then it is reasonable only to do a boundary calculation where you border the LZ2 property. The request for spill-light limitation for two neighboring properties belonging to the same lighting zone is similar to establishing border customs between neighboring US states.

FIG. 2. Fig. 2 illustrates two neighboring parking lots separated by a property line, and belonging to the same lighting zone. The plan on the left utilizes luminaires with optics that eliminate back-spill for both properties. The plan includes six poles and six 150W luminaires, which consume a total of 1170W. The plan on the right utilizes three poles and three 250W luminaires, which consume a total of 930W. Both lighting systems provide similar illuminance and uniformity. Simply removing the spill requirements in this case could essentially reduce the carbon footprint of the lighting system.

In situations where the real illuminance on – and beyond – the boundary cannot be measured, where the computer model is the only avenue, the door is open for incorrect results, either purposely or in error. One common scenario is for the arm in the computer model to be too short, causing the pole to essentially shield the backlight.

We have to find another realistic and measurable approach to the LEED Light Pollution Reduction problem that allows a reduction in light pollution without additional luminaires, poles and increased energy, when compared to non-LEED projects.

Let’s stop planting virtual trees, and instead save the real ones.