DonateJoinSign In

PS-06-24: IES Position on Legislation and Ordinances for Outdoor Light at Night

Issued June 17, 2024

The Illuminating Engineering Society (IES) acknowledges recent legislation and lighting ordinances intended to restrict outdoor light at night to reduce light pollution and promote responsible lighting practices. At this time, the IES does not endorse or provide specific guidance for lighting ordinances. We are dedicated to publishing consensus-based standards developed by diverse representation of the lighting industry in support of Light to Protect the Night: Five Principles for Responsible Outdoor Lighting. In the Five Principles, light should be:

  1. Useful: All light should have a clear purpose.
  2. Targeted: Light should be directed only to where needed.
  3. Low Light Levels: Light should be no brighter than necessary.
  4. Controlled: Light should be used only when useful.
  5. Color: Use warmer color lights where possible.

Ordinances may reference our nomenclature, standards, and classification systems. While the IES develops standards, referencing our documents does not imply any official affiliation or involvement of the IES in the development of these ordinances. By equipping lighting professionals and the public with the tools and knowledge to create informed lighting designs, the IES strives to balance the needs of human activities, ecological systems, and the preservation of the night sky as a valuable resource. Through our ongoing commitment to research and standards development, the IES promotes responsible outdoor lighting practices that enhance public safety, protect environmental health, and maintain the natural beauty of our night-time environment.

Background

The IES is an American National Standards Institute (ANSI) Standards Developer for illuminating engineering. Through its standards, the IES defines and recommends controllable factors in the specification, design, and installation of lighting systems that can reduce light pollution.

Human-based light (also referred to as anthropogenic or “artificial” light) in the night sky primarily originates from outdoor light sources intended to enable or enhance the use of exterior spaces when natural light levels are insufficient. To a lesser extent, outdoor light at night also comes from building interior lighting that escapes through windows or other openings.1

Undesirable effects of using outdoor light at night, including light pollution, sky glow, light trespass, glare, and other non-visual impacts (such as alertness and ecological impacts), are increasing. The IES defines these terms as follows:

  • light pollution: The human alteration of light levels in the outdoor environment from those occurring naturally. The term is sometimes used synonymously with sky glow but is more accurately applied in a broader sense to also encompass undesired effects like light trespass and glare.1
  • sky glow: The brightening of the night sky that results from the scattering and reflection of light from the constituents of the atmosphere (gaseous molecules and aerosols), in the direction of the observer. It has two separate components: natural sky glow and artificial sky glow.1
    • natural sky glow: That part of sky glow which is attributable to natural sources. It is attributable to starlight, zodiacal light (scattering of sunlight from dust in the solar system), airglow (radiation from luminescent processes in the earth’s upper atmosphere), and (on a cyclical basis) moonlight.
    • artificial sky glow: That part of sky glow which is attributable to scattering of light from human-based sources of radiation (e.g., outdoor electric lighting), including radiation that is emitted directly upward and radiation that is reflected from surfaces.
    • light trespass: The encroachment of light, typically across property boundaries, causing annoyance, loss of privacy, or other nuisance.2
    • glare: The sensation produced by luminance values within the visual field that are sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance or visibility.2

The IES develops standards to educate the public and professionals to create lighting solutions that are context-specific, follow established consensus-based recommended practices (including recommended illuminance levels) and balance the needs of the nighttime environment. Any recommendations or policies on outdoor lighting should consider all aspects of public health, including factors that affect driver and pedestrian safety.

The IES continues to support efforts to inform the public about the Five Principles and the importance of viewing darkness as a resource. We also encourage mitigation techniques to reduce energy and lighting excess; therefore, it supports retrofitting to LED to reduce energy consumption and the use of lighting controls to reduce light levels when light is not needed. Lighting research in the following is still inconclusive, and therefore actionable recommendations have not been published by way of standards that require consensus:

  • The full range of implications associated with the spectral content of outdoor lighting that can affect: pedestrians, drivers, and nighttime ecology (specifically flora and fauna with differing unintended consequences of human-based light).
  • Applying the definition of glare to outdoor environments that is inclusive of a luminaire’s optical control, as well as contextual-driven factors such as contrast.
  • The data-driven correlation and perceived correlation of warmer correlated color temperature (CCT) light and the spectral content of a luminaire.
  • The evolution of a luminaire classification (addressing the limitations of BUG ratings) to assess the impact of an individual luminaire on the overall nightscape: glare, light trespass, skyglow, etc.

The IES remains committed to a balanced approach to outdoor light at night, considering factors such as color fidelity, spectrum, and CCT; lumen output; luminaire orientation and installation; ecological impacts; and public safety. Additionally, the IES advises considering the unique needs of individual communities, which may be influenced by the surrounding natural landscape, socioeconomic factors, and context-specific design requirements. The IES recommends consensus-developed standards (see list below) in all matters related to lighting and public well-being.

IES Resources for Outdoor Lighting:


1 Illuminating Engineering Society. ANSI/IES TM-37-22, Technical Memorandum: Description, Measurement, and Estimation of Sky Glow. New York: IES; 2022.
2 Illuminating Engineering Society. ANSI/IES LS-1-22, Lighting Science: Nomenclature and Definitions for Illuminating Engineering. New York: IES; 2022.