NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 15 Glare Glare is a visual condition where there is an inappropriate distribution of light creating excessive contrast that compromises the ability to distinguish details and objects (Vos et al. 2002). Glare is often perceived as excessive brightness. The perception of glare depends on the level of light/dark adaption of the viewer. Light/dark adaptation is the process of eyesight adjusting to the ambient light level. If a viewer is in a dark environment and their eyes are fully adapted to the darkness, even a relatively dim light may be perceived as glare, while that same light would cause no discomfort in a brightly lit environment in which the viewer’s eyes are adapted to higher light levels. The sensitivity to glare varies between individuals, with older people typically being more sensitive. Disability glare refers to reduction in visibility caused by intense light sources in the field of view, while discomfort glare is the sensation of annoyance or even pain induced by viewing an overly bright light source (Rea 2000). Glare is possible during the day or night. In the daytime, glare is most common as the reflection of sunlight off smooth surfaces, such as car windows, metallic roofs, or lakes. At night, natural light sources are generally too faint to cause glare. Rather, nighttime glare sources include direct views of bright artificial light sources, such as car headlights, or their reflections off specular surfaces, such as wet pavement. On BLM-managed lands, nighttime sources of glare may also include unshielded lighting at energy and other facilities. Skyglow Skyglow (or sky glow) is the brightening of the night sky that results from light pollution. Skyglow is caused by light directed or reflected upwards or sideways and reduces the visibility of stars and other celestial objects. Sky brightness has both natural and human causes. Natural components of sky brightness include moonlight, airglow, zodiacal light, integrated starlight (unresolved stars in the Milky Way galaxy), and diffuse galactic light (starlight reflected by interstellar dust) (Duriscoe 2013). Artificial light shining upward into the night sky or reflecting off surfaces is scattered by dust particles and gas molecules in the atmosphere, producing a luminous glow often with an orange or whitish cast. Skyglow is more visible in poor weather conditions and polluted skies because there are more particles in the atmosphere to scatter light. The effects of skyglow on dark night skies are greater for blue-rich light sources than for yellow-rich light sources (Luginbuhl et al. 2014). A technical memorandum by the Illuminating Engineering Society, titled “Description, Measurement, and Estimation of Sky Glow,” describes the causes, characteristics, and potential impacts of human-caused skyglow and identifies methods for quantification and control (IES 2021). Glare from direct view of light sources and reflection from wet pavement. Jesse Bower Skyglow over the lights of Las Vegas as seen from the BLM-managed Red Rock Canyon National Conservation Area.
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