Below is another article I wrote for the November issue of tED Magazine. Reprinted with permission.
Outdoor stationary lighting presents a substantial market and a distinctive subset of lighting design. While the market covers a variety of applications, the largest are building exterior and area and roadway lighting.
In a typical indoor space lighted during the day, the ceiling is relatively bright, the light is localized and has little or no impact on the natural environment, and the eye uses photopic vision. With outdoor lighting, the “ceiling” is relatively dark, unshielded lighting may be visible at great distances and impact the environment, and the eye may use scotopic vision (dark conditions) or mesopic vision (semi-dark).
Generally, the primary goals are to enable nighttime business, leisure, and enjoyment while promoting safety and security. A key decision is choosing what to light. Good outdoor lighting lights only what is needed without glare, light trespassing onto neighboring properties or negatively impacting wildlife, and uplight that can produce skyglow.
In this article, we will discuss outdoor lighting design considerations and the basic design process. It is based on the Illuminating Engineering Society’s (IES) RP-33-14, Lighting for Exterior Environments and other sources.
Visual factors
Visual factors relevant to outdoor lighting include light level, brightness, visual adaptation, and color quality.
Light level. Measured in footcandles or lux (metric), light level is the quantity of illumination falling on a specific area. The designer’s aim is to ensure sufficient light output to provide an average or minimum light level over time, which may be defined by the maintenance interval. IES recommends a range of light levels for accent, building entry, facade, fountain, parking deck, parking lot, pedestrian stairs, roadways, outdoor pools and retailing, pedestrian path, and other applications. A second goal is to provide a uniform distribution of light. IES recommends maximum-to-minimum and average-to-minimum light level uniformity ratios.
Brightness. Luminance is the light reflected or emitted to the observer’s eye; brightness is the subjective perception of luminance. Excessive brightness is glare, which may be disabling or discomforting to vision. A bright outdoor luminaire does not equate to sufficient light level.
Visual adaptation. The eye takes time to adjust from light to dark, including reduced visibility. Good outdoor lighting provides smooth transitions from bright to dark areas. In a large visual environment, very bright areas may result in less visibility in adjacent dim areas within view.
Color quality. At very low light levels, the eye adapts to scotopic vision, which is essentially colorblind, with an exception for very bright objects such as traffic lights. Most urban environments deliver enough ambient light to enable mesopic vision, a combination of photopic (daytime) and scotopic (nighttime) vision. For greater visibility, consider luminaires with some short wavelength (blue) output. Short-wavelength light is more likely to be judged as glaring, however, because the eye is more sensitive to this radiation at low ambient light levels, and short-wavelength light scatters more widely than other wavelengths within the eye. Outdoor luminaires should be selected with an appropriate spectral mix that provides both visibility and visual comfort.
Image courtesy of Wattstopper.
External factors
A variety of external factors influence selection of outdoor lighting, from space use to community master plans to local regulations regarding wildlife. Here, we will focus on three: energy codes, dark-sky ordinances, and American Medical Association (AMA) guidelines.
The latest generation of energy codes and standards prescribe maximum allowable power for outdoor lighting by application. These power densities are declining as more-efficient LED outdoor lighting has become viable. Codes and standards also contain detailed mandatory control requirements.
Dusk-to-dawn lighting must be turned ON/OFF using a photocell. During operation, power must automatically reduce by at least 30 percent after business operations or in response to occupancy. Façade/Landscape lighting must be operated using a combination photocell/time switch that turns it OFF between midnight or business closing (whichever is later) and 6:00AM or business opening (whichever is earlier).
Dark-sky ordinances vary but are generally designed to limit light trespass, skyglow, or both. Light trespass occurs when light spills onto neighboring properties. Skyglow occurs when uplight from nighttime lighting obscures a view of the stars. Many communities have adopted the Model Lighting Ordinance (MLO) developed by the IES and International Dark-Sky Association, which addresses both light trespass and skyglow in addition to energy efficiency and glare.
Both the major energy code standards and the MLO base requirements on a lighting zone (LZ) system (LZ 0-4). These zones range from no ambient light (e.g., wilderness parks and preserves) to high ambient light (e.g., high-activity commercial districts).
In 2016, the AMA issued community guidance cautioning against glare, which can affect safety, and LED lighting with a very cool correlated color temperature (CCT), which may suppress melatonin production. AMA specifically recommended 3000K sources, luminaire design that minimizes glare and light trespass, and dimming during off-peak operation. While cooler CCT sources remain more efficacious (lumens/W) than warmer CCT sources, the efficacy gap has been shrinking, increasing demand for warmer sources. The CCT recommendation received considerable pushback from the lighting industry, including IES, which issued a public statement saying the 3000K recommendation is insufficiently substantiated and that CCT itself is not an appropriate metric for predicting health outcomes.
Product selection factors
A wide variety of factors influence outdoor luminaire selection. Wattage, light output and distribution, glare control, color quality, ruggedness, certifications, aesthetics, maintenance, cost, and others come into play. Due to energy codes, controllability of LED sources, and advances in wireless connectivity, lighting controls are becoming much more important, offering extraordinary possibilities for global management and data collection.
The Backlight Uplight Glare (BUG) luminaire classification system developed by IES in TM-15-11 offers a useful tool for evaluating luminaire quality. Featured in the MLO. BUG delineates a luminaire’s light output by zones and tags these zones by distribution—backlight, which can cause light trespass; uplight, which contributes to skyglow; and glare, which is objectionable light. There are three zonal regions for backlight, for example (high, middle, and low). The MLO establishes maximum zonal lumens for each by LZ. Many luminaire manufacturers publish BUG ratings for their outdoor luminaires, while the MLO provides maximums by LZ.
Putting it together
All of the above must be matched to the application to provide the optimal customer solution. With the proliferation of the LED source, outdoor luminaires now offer much more expansive and robust capabilities, such as superior optical control, dimming, collecting data, and changing color by time of night. By understanding the basics of outdoor lighting, electrical distributors can add even greater value to projects.
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