Most people are familiar with the yellow light emitted by low-pressure sodium outdoor fixtures, and how it is nearly impossible to tell what color things are under those lights. High-pressure sodium fixtures, also common in outdoor lighting, emit yellowish light, too, although it’s less pronounced. Metal halide lamps, which emit a more neutral white light, are generally considered to be a better outdoor lighting option.
All those technologies are, however, relatively energy-inefficient, so lighting used for parking lots, garages, and streets is increasingly being replaced with energy-efficient, dimmable LED products, generally with color temperatures ranging from neutral to cool white. The higher color temperatures are achieved through a higher proportion of blue in the light output spectrum, but this blue light has raised concerns.
One concern is the possible impact on what is referred to as dark sky. Sunlight contains significant amounts of light in blue wavelengths; outdoor nighttime lighting with significant blue content brightens the night sky. The view of the night sky for city dwellers in highly populated areas can be severely limited or even eliminated because of atmospheric reflection of outdoor lighting, called sky glow. To address this concern, all outdoor lighting products, regardless of the technology used, should optimally include shielding to ensure that light distribution is confined only to where it’s needed.
As I discussed in another blog post, LEDs are easily dimmable, allowing lighting levels to be reduced when less light is required. Lights used in a parking lot, for example, don’t generally need to be on at full power all night. The use of occupancy sensors and preprogrammed dimming enables lights to dim to low levels until activity is detected. That not only contributes to the preservation of the dark sky but also lowers energy costs. Similarly, to save energy and reduce sky glow, outdoor signage using LEDs can be dimmed during the wee hours of the morning, when observers are likely to be few and far between.
Another area of concern is the potential impact of the higher levels of blue wavelengths on living things. Plant and animal life has evolved under conditions of daily periods of light and darkness, with the presence or absence of light as a resource. The cover of darkness is important for amphibian nighttime breeding rituals, for instance, for sea turtle nesting, and for plant recovery from ozone exposure and other daytime stressors. Increased nighttime light emission, especially blue light, can negatively affect such activities. For humans, blue light has been tied to the disruption of the circadian rhythm that regulates hormone production—potentially resulting in disrupted sleep cycles and compromised immune systems.
Unlike incumbent lighting technologies, LEDs are available in a range of color temperatures. While the precise biological effects of blue light are still under investigation, any potential negative impact to plant and animal life can be reduced through the installation of lower color temperature products that meet the visual requirements of the outdoor installation for nighttime lighting.
IEEE Senior Member Yoelit H. Hiebert is a professional engineer who has worked in the LED lighting field for nearly 10 years, in both manufacturing and energy management. She is a senior engineer at Leidos, where she serves as a solid-state lighting expert. She is also the chair of the IEEE St. Louis Section.