Light Done Right

Tonight, after dark, go out and take a fresh look at your library. Notice how the lighting works, or works too hard. Is the walkway to the front door bright despite being empty? Is the parking lot, sans cars, aglow? Is light shining in your eyes from the building or grounds? Look up. Can you see stars? Then, imagine light when and where you need it, illuminating a path to the library entrance as you walk along it; a parking lot lit only when occupied; looking up from your screen or book to view the constellations, even the Milky Way. These are not mere futurist fantasies but realistic expectations for improved quality of life and energy savings enabled by the advancing exterior lighting strategies and technologies.

The haphazard lighting currently used to support our 24/7 lifestyles affects our natural rhythm, our physical and spiritual well-being, and our ability to see and study the stars. Continuous illumination also relates to the natural rhythms of animals, birds, reptiles, bugs, and, yes, even plants. More conscious lighting design can help us address these issues, and libraries can set the tone by modeling both appropriate architectural design and environmental stewardship.

The quality of night lighting depends on selecting appropriate light levels, reducing or, even better, eliminating glare, and minimizing stray light that involves neighboring properties and wildlife. In making fundamental design choices, visual performance, visual comfort, and energy efficiency all interrelate. If the objective is to improve public safety, security, and wayfinding, the design strategy will certainly include improving how well one can see at night. Eliminating glare will improve visual performance at lower light levels. Eliminating glare and lowering light levels will reduce our energy demand and our impact on the earth. Overillumination, glare, and stray light have an impact on our ability to see our immediate surrounds and tamper with wildlife. This is called light pollution.

Light pollution is defined by three terms: light trespass, glare, and sky glow. Light trespass is electric light that falls beyond property boundaries measured horizontally on the ground and extending vertically to ten feet above the highest structure on the site, such as a building or a light pole. Most local codes and zoning ordinances restrict the amount of light allowed to spill beyond property lines into neighboring sites. Glare includes direct or reflected light that is visually uncomfortable and in extreme cases disables vision altogether. It is a subjective response and is therefore difficult to define and measure. Sky glow results from any light in the sky, electric or celestial, that is reflected by particles in the air. Sky glow masks our clear vision of the night sky, disrupts astronomical observation, and disorients wildlife.

To understand sky glow it is important to note that we do not see light in clean air. We perceive light in the form of illuminated objects—more technically, objects that reflect light instead of absorbing it. At night, we see airborne matter such as vapor, smoke, gas molecules, dust, and other small particles reflecting the light in the sky. This is what we see suspended over large towns and cities and that defines a disturbing cycle among sky glow, air pollution, and energy production. Namely, excessive exterior lighting necessitates increased energy production. The increase in energy production raises the number of particles and the amount of greenhouse gas in the air, which in turn are amplified by reflecting the excessive light in the sky.

So, how do we light libraries at night with minimal impact on the earth and night sky? The answer is very simple…improve light quality, and switch off the lights.

Light quality doesn't equal light quantity. The most common rationale for nighttime lighting in and around public buildings is safety and security. Our natural inclination to improve security is to increase light levels. However, uniform lighting at an appropriately low intensity is better at helping us see at night than nonuniform light at brighter levels. Consider the way our eyes function: when we walk into a building from bright sunlight, our eyes take time to adjust to the change in intensity.

The same thing occurs at night as we look from highly illuminated zones such as parking lots to less illuminated or nonilluminated zones like walkways or paths. The difficulty of adapting to light and dark zones is further exaggerated by glare, which is intensified by bright light in a dark surround, much like the headlights of oncoming traffic while traveling a dark road. While glare at night is distracting to most, it is often disabling to the elderly, whose ability to adapt to bright light has diminished by the natural loss of elasticity in muscles that control pupil size.

Areas of nonuniform light create “black holes” in the visual environment. People or objects located in a dark zone might be hard to see in general and even more so as our eyes adapt from brightness to darkness. People and object recognition is further impaired by silhouetting, which results if the object or person is in front of a relatively intense light or a glare zone. Figure 1 illustrates the effects of nonuniform lighting.

The intensity of light required to illuminate objects adequately at night depends on the site. Exterior lighting at a rural site will appear brighter than the same lighting system installed in a fully developed suburb or city center with a higher ambient light level. The contrast between illuminated and nonilluminated zones at a rural site makes it more difficult to see safety hazards than does the same lighting in a city. The international lighting commission (Commission Internationale de l'Eclairage, CIE) developed the lighting zone classifications shown in Table 1 (p. 7) to define areas of high and low ambient light. This classification system has been adopted by the International Dark-Sky Association (IDA), U.S. Green Buildings Council (USGBC), and professional lighting organizations to identify appropriate nighttime light levels for various demographic regions and lighting applications.

Achieving light quality requires putting light only where it is needed. This practice addresses light trespass and sky glow and will be realized primarily through proper selection and placement of light fixtures. Selection of well-designed light fixtures will minimize glare, improve uniformity, reduce energy demand, and minimize uplight (the amount of light that a fixture emits directly into the sky). The Illuminating Engineering Society of North America (IESNA) classifies light fixtures as full cutoff, cutoff, semicutoff, and noncutoff based on the amount of light that is distributed above 80°, which is optimal (see Figure 2).

The Leadership in Energy and Environmental Design (LEED) program developed by the USGBC and IDA in their Outdoor Lighting Code Handbook combine the CIE zone system with a luminaire classification system similar to that of IESNA's to identify criteria for minimizing light trespass and sky glow. Table 2 (this page) provides a summary of LEED criteria for light pollution reduction credits based on these two design metrics.

There are numerous guidelines and much basic information available about dark sky design. When planning for a specific site, it is important to keep the big picture in mind as each library type has its own set of use and location circumstances that will direct the outcome of the final design. Local codes and organizations such as IESNA, IDA, and USGBC provide prescriptive criteria for generic installations and light fixture types. Focusing on the restrictive requirements of local codes and LEED criteria will result in an energy-efficient dark sky design. Yet this isolated approach will not ensure light quality, public safety, or an aesthetically appealing design.

In terms of exterior lighting performance, LEED addresses three measurable quantities that contribute to light pollution and energy reduction. Meeting these LEED criteria will ensure that energy demand and lighting controls meet current codes, light levels at a site boundary do not exceed a specified value, and exterior light fixtures emit little or no light above 90°, which is less stringent than the IESNA classifications. However, it is not the intent of the USGBC to define light quality, so it doesn't address the quality of light within the site boundary as expressed in terms of glare, visual comfort, uniformity, safety, security, and other library- and site-specific use requirements. Nevertheless, LEED has created a common vocabulary and set of tools for libraries and design teams to use to evaluate a design and its impact on the environment and the community.

The traditional “shoe box”—the black or dark bronze rectangular light fixtures mounted on a 25' to 40' pole—is without a doubt the most common and cheapest light fixture available offering full cutoff performance, but it is not the only installation type that will meet dark sky or LEED design criteria.

Take St. Cloud Public Library (SCPL), MN, for example (see p. 6). Decorative post-top pedestrian luminaires define a daytime site aesthetic and line the allée along the site entry, while shoe box fixtures mounted on 25' poles are relegated to the parking lot. Post-top luminaires are also located on the pedestrian plaza, downlights are installed in the entry, and pedestrian canopies and recessed step lights define the front edge of the building illuminating the walkway leading to the main entry. This library was designed to be a high-performance, low-energy building. The exterior lighting demands less operational energy than allowed by code, and the decorative luminaires are managed by the building lighting control system, which switches them off one hour after the library closes.

These strategies exceed the intent of a basic LEED design. All of the light fixtures, with the exception of the decorative post-top luminaires and step lights, emit zero percent of their initial fixture lumen into the sky. Each of the 17 post-top luminaires emits 26 percent of its initial fixture lumens above 90°; each of the ten low-wattage step lights emits 50 percent of its initial lumens above 90°. The result of this light fixture mix is that seven percent of the total site lumens are distributed above 90°, above the limit for LEED criteria.

If St. Cloud were a major metropolitan, high–ambient light zone LZ4, this design would easily meet the LEED-allowed uplight criteria of ten percent. However, St. Cloud is a moderately active college town with an ambient light level assigned to LEED zone LZ3, which allows five percent of the total initial designed fixture lumens to be emitted above 90°. If this library had been designated as a LEED project, the design team might have selected a step light with full cutoff shielding from the many aesthetically pleasing styles available. The team might also have selected a decorative post-top luminaire with 19 percent uplight in place of 26 percent, or reduced the lamp wattage of the decorative fixtures, which in turn would reduce the individual fixture lumen, or reduced the number of decorative light fixtures in the design. Any of these minor adjustments would have led to the LEED criteria easily being met without impacting the visual appearance of the site during the day or the light quality at night.

The application and construction costs for LEED-certified buildings can sometimes exceed the cost of buildings designed to meet basic high-performance and/or energy- efficiency criteria. This is not necessarily so for site lighting. While high-performance light fixtures will cost more than mediocre light fixtures, the overall site costs may be lower owing to the number of fixtures required for equal light performance of the two fixture types. For instance, while the shoe box is not the most aesthetically pleasing lighting instrument available, it is certainly one of the least costly; if properly designed, a shoe box system will provide good performance without trespass or sky glow.

It is very difficult to estimate general costs for lighting systems because of how they are specified, purchased, and bid. Just as energy costs vary by client, site, and region, so do light fixtures. There are numerous costs added to the price of a basic light fixture such as overhead and profit (O&P) for the manufacturer, the distributor, the electrical contractor, and the general contractor; delivery charges based on location; secure site storage; and the bidding environment. To make the system even more complex, the cost of the hardware is difficult to extract from the total system of installed costs after the bid has been accepted. The exact price of an individual fixture is often buried in the lump sum of the interior and exterior lighting package. Packaging is a lighting industry practice developed to protect the distributor's ability to provide all of the lighting specified for a project and to safeguard distributor and electrical contractor profitability. Lighting professionals request manufacturers' guaranteed pricing in an effort to document cost comparisons and remove the fixture from the lump sum price. However, in the end, prices are not guaranteed until the actual bid has arrived.

The cost savings for implementing dark sky strategies are significant. Though exact cost savings will vary by installation, the sources of the savings are universal. Limiting light intensity and locating light fixtures only where they are needed will reduce costs of energy and initial installation. Switching lights off will reduce energy and long-term lamp replacement expenditures.

Light within and without libraries finds its way into the night sky in a variety of ways. Interior light emitted through windows either directly or indirectly contributes to the measured light in the exterior surrounding. After-hours activities such as community gatherings and Internet café operations require parking, paths, and walkways to be adequately lit for nighttime use. Lighting above entry doors and at loading docks offers safety for staff and security of the building. After-hours book drops should be well illuminated to help safeguard people and library property. Signage, flags, and architectural details are often illuminated at night to reinforce the importance of the library within the community.

With the exception of illuminating signage, flags, and architectural details, exterior lighting is typically designed to direct light downward to horizontal surfaces such as parking lots and walkways. Even fully downward-directed lighting systems create a potential of adding reflected light into the night sky. Flags, signage, and architectural details are often lit by floodlights on the ground and aimed upward. Most building codes don't require exterior lighting or interior egress lighting to remain on when the building is unoccupied or closed to the public, and herein lies a significant strategy for reducing both energy demand and light pollution.

Switch lights off This will reduce the amount of distracting night light released into the environment, save energy, and reduce toxins such as air pollution and greenhouse gases. People are naturally attracted to illuminated areas. Switching interior and exterior lights off signals to the public that the library is closed, discouraging pedestrian and vehicular activity on library grounds after hours. Controlling exterior lights via occupant sensing systems illuminates people when they are present, providing safe paths and parking, alerting neighboring properties and security personnel to site activity. Where switching lights off is not appropriate, consider dimming the lights when unoccupied. Occupant sensors and time clocks can be used to switch lights on, change light intensity, and trigger a camera response to the location.

Improve light quality Set goals for light quality based on location, climate, and public use. Lowering light intensity to appropriate levels, improving light uniformity, and reducing glare will enhance security and safety around the library at night and reduce the environmental impact. If designed properly, this can also reduce energy demand. Minimum light levels for parking and pedestrian paths are often identified in local building codes. In its most recent library lighting recommendation RP-4 (to be published in spring 2010), IESNA will document minimum light levels and uniformity ratios for parking areas (see Table 3, p. 9). It is important to note that adequate light must occur at least 60 inches above the ground surface in order to illuminate people and other vertical objects. Note that lighting energy requirements measured in watts per square foot are not the same as light quality measured in intensity and uniformity. The exterior lighting at Carmel Clay Library, IN, on the cover, provides an example of glare-free uniform light on the library grounds.

Require two point-by-point lighting calculations from your lighting professional One will show initial light levels, the other will show maintained light levels reduced by age, dirt, and light source depreciation. To document light trespass, request an Outdoor Site Performance (OSP) calculation to show light levels on a horizontal plane located ten feet above the highest element on the site and vertical light levels at the site boundaries. This calculation should be provided for both initial and maintained light levels as well.

Reduce ground reflectance Lowering the ground reflectance in and around your library will reduce sky glow caused by indirect light emitted from surfaces. Light-colored, hard surfaces such as concrete are typically specified for walks, steps, ramps, and parking areas. The light color is attractive during the day, and because it reflects rather than absorbs radiant energy from the sun, it will reduce heat islands. A natural concrete surface reflects 40 percent of the sunlight, but it also reflects 40 percent of the electric night light, thereby increasing the amount of light that has a potential to become sky glow. Dark surfaces reduce the amount of light reflected into the night sky but increase the temperature at night by radiating the sun's energy absorbed during the day. This catch-22 requires a balanced response. Trees and bushes can be used to shade dark surfaces during the day in an effort to limit the amount of radiant energy reaching and absorbed by the dark surface. Inversely, plants could be used to block indirect electric light reflected from light-colored surfaces. This might include locating trees and lighting standards side by side against parking lot medians or along walkways. Trimming tree branches up to seven feet and pruning bushes to three feet will clear sight lines for pedestrians and address the public safety issues typically associated with planted material.

Identify design trade-offs If it is important to illuminate the building exterior, trade that lighting for signage lighting and allow the building to become the sign. Instead of illuminating a flag, retire it at night, once standard practice for public buildings especially in rural communities. Consider lighting only those areas that require nighttime illumination, such as a book drop and the parking spaces immediately around it.

Incorporate light shelters Night lighting located below overhangs and canopies has many benefits. These structural elements provide daytime shade and shelter for pedestrian travel, exterior events, and reading/lounge spots. They also provide locations that fully shield direct and indirect light from the sky. The pedestrian plaza and entrance canopy at SCPL are examples of lighting in people shelters.

Scrutinize luminaire choices Not all light fixtures are created equal. The key criteria to look for when presented with light fixtures are position, cutoff classification, distribution patterns, and efficiency. Lamps installed vertically in pole-mounted area lights cannot be appropriately shielded and at the same time provide efficiently distributed illumination. Vertical lamps are usually identified by a drop or sag lens. The result will be visible lamps, bright reflectors, or both—almost always resulting in visual and sometimes disabling glare.

Increase mounting heights Mounting light fixtures on tall poles and using lower wattage lamps will diminish low-level glare, improve horizontal and vertical uniformity, and reduce the number of locations required. By using low-wattage lamps and concentrating on light uniformity, the installation will provide better light and reduce the overall energy demand. For white light, consider metal halide light sources between 150W and 250W mounted on 20' to 25' poles. If more light is required, consider high-pressure sodium in the same wattage range and at the same pole height. New LED sources designed for pole mounting are becoming more readily available. However, the lighting industry has not yet developed a consistent method of documenting LED performance. If performance information is not available, request a sample, a mock-up, or a site visit to verify glare and distribution performance.

Influence public policy Library administrators and staff are representatives of one of the most important public buildings within each community. As such, they should influence public policy regarding exterior environmental criteria, and they should certainly make their voices heard in any debate regarding public policy that directly affects library grounds. Each town, city, region, and state has a unique set of exterior lighting and environmental criteria. To begin any discussion, these criteria must be identified, thoroughly understood, and documented. The IDA's Outdoor Lighting Code Handbook presents detailed discussions on lighting code topics, issues, and choices to be made when it comes actually to writing and influencing exterior lighting codes. Being informed and joining in the debate is a first step toward improving the night environment.

A library's influences extend beyond its walls. By taking lighting to the next level, we will enhance our libraries and our environment in many ways…seen and unseen.