NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 13 3.1 Basics of Light and Light Pollution Light pollution happens when the night sky is brightened from the use of artificial lights so that stars and other celestial objects are washed out from view. It can affect natural ecological systems dependent on patterns of light and dark (Longcore and Rich 2004; Cinzano 2001). Not all lighting that is installed for night activities causes light pollution. Light pollution generally refers to excessive, misdirected, or intrusive outdoor light sources such that it causes adverse effects (Henderson 2010). Longcore and Rich (2004) make a distinction between astronomical light pollution and ecological light pollution. Astronomical light pollution affects the visibility of astronomical objects in the night sky. The negative effects of astronomical light pollution are often referred to as night sky impacts or effects. Night sky impacts primarily (but not exclusively) affect human values and activities, such as aesthetic values, cultural values, economic values, nighttime recreation, and astrotourism. The loss of natural darkness caused by light pollution is often referred to as ecological light pollution, because it primarily affects the health and functioning of ecosystems, which are biological communities of interacting organisms and their physical environment. Ecological light pollution can have a variety of negative effects on health and behavior of individual organisms (including humans), the interactions between organisms, such as predator-prey relationships, and reproductive behavior. Note that the terms astronomical light pollution and ecological light pollution should not be interpreted too narrowly. Night sky effects can apply to ecological resources, and loss of natural darkness can negatively affect human values. For example, loss of visibility of the night sky can negatively affect certain birds and insects that use the stars for navigation. Moreover, humans value natural darkness for reasons beyond its benefits to human health. The following section provides basic information about light qualities, how they are measured, and types of light pollution. This information is useful to introduce the artificial light at night principles (Section 4) and lighting BMPs on BLM-managed lands (Section 5). 3.1.1 Light Qualities The qualities of light that influence visibility are based on the following factors: • Light intensity • Physical form, direction, and dissemination of light • Color of light Light Intensity Light intensity is the amount of light falling on a surface. The more intense the light source, the more light falls on an object. Illuminance is measured by a light meter (corrected for the curve of the human eye) in footcandles (square foot) or lux (square meter) (Green Business Light UK 2021). The lumen is a measure of the total amount of light produced by a light source. Differences in light intensity are perceived by people as a change in brightness. Physical Form, Direction, and Dissemination of Light Light broadcasts across surfaces of objects and moves through space to the eye where it is translated into a sense of form, location, movement, and appearance. Light allows the eye to identify an object in terms of its shape, scale, proportion, and position. Lighting from multiple directions, angles, and colors can be duplicative, causing reflectivity, glare, and shadows that may impact visibility. Light pollution from a manufacturing plant near Salt Lake City, Utah, illuminates the adjacent hillside (light trespass), altering the natural dark environment. This may affect ecological health and function.
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