Glossary

Artificial light at night (ALAN): Human-made sources of light that shine after natural darkness. When not used responsibly, ALAN can contribute to light pollution and have negative impacts on humans and the environment. 

Blue light: A short-wavelength light that produces higher amounts of energy. For humans, exposure to blue light is beneficial during the day and is associated with better attention span, reaction times, and mood. Exposure to blue light after dark is detrimental to our health because it interferes with the circadian rhythm, the disruption of which can cause various health issues and sleep problems.

Bortle Scale: A method for measuring the quality (brightness) of the night sky in a particular location using astronomical observations. The scale features 9 classes of sky, from Class 1 Excellent Dark Sky Site (darkest skies, and least affected by light pollution) to Class 9 Inner-city sky (brightest skies, and most affected by light pollution). Learn more here.

Canadian Guidelines for Outdoor Lighting: Best practices for outdoor lighting created and promoted by the Royal Astronomical Society of Canada’s Light Pollution Abatement Committee, and implemented at all designated Dark-Sky Preserves, Nocturnal Preserves, and Urban Star Parks.

Clutter: Bright, confusing, and excessive groupings of light sources.

Correlated colour temperature (CCT): The colour appearance of a light, measured in degrees Kelvin (K). Higher temperatures, such as 4000K or 5000K, appear blue or white. Lower temperatures, such as 2700K or 3000K, appear yellow or orange. High-pressure sodium lights tend to have a CCT of 2200K, while LEDs can range the entire spectrum.

[Rick’s definition: The temperature of a glowing surface with a given colour. A very approximate reference to the colour of HID and LED light sources, measuring the equivalent "colour" of lamp light measured in spectral colour (in Kelvins), with very blue lamps being 5000K CCT, less blue 4000K, cream-white 3000K, yellow-white 2700K, orange 2200K. HPS lights are 2200K. LED lights range the entire spectrum. Though specified as a certain CCT, LED lights can produce an enormous Blue Leak, so LED lights specified at low CCTs may still look very blue.]

Cut-off (CO): A type of luminaire shield that limits light emission to 2.5% above an angle of 90° above nadir, and 10% at or above an angle of 80° above nadir.

Dark-Sky Preserve: One of three designations in the Royal Astronomical Society of Canada’s Dark-Sky Sites program. Dark-Sky Preserves offer public observing sites, programs, and events.

Dark-Sky Site: A site that has been recognized by the Royal Astronomical Society of Canada for their commitment to protecting the nocturnal environment, educating the public about the importance of dark skies, and practicing responsible lighting. An all-encompassing term for the Dark-Sky Preserves, Nocturnal Preserves, and Urban Star Parks that participate in the RASC’s Dark-Sky Sites program.

Fixture: The part of a light (generally metal) that holds the lamp, luminaire or lightbulb.

Full cut-off (FCO): A type of luminaire shield that limits light emission to 10% of the total light within the glare zone between 80° and 90° above nadir, and does not emit any light above an angle of 90° above nadir.

Glare: The effect of light emitted from a luminaire at an angle and intensity that affects vision. Glare may occur through reflection from surfaces.

High-pressure sodium lamp (HPS): High intensity discharge luminaire whose visible light emissions appear yellow or amber.

Illuminance: The amount of light falling onto a unit area of surface, measured in footcandles or lux.

Light emitting diode (LED): A semiconductor light source that converts electrical energy directly into light, making it more efficient than conventional light sources. White LED bulbs emit large amounts of blue light and have higher Correlated Colour Temperatures, but LEDs in warmer colours with lower Correlated Colour Temperatures are also available.

Light pollution: The inappropriate or excessive use of artificial light, where light does no useful work by shining where it is not intended or is too bright for its purpose. 

Light pollution abatement (LPA): The reduction of light pollution and mitigation of its negative impacts.

Light trespass: Light that spills into an area where it is unintended or unwanted. 

Luminaire: The general term for the enclosure, lamp, socket, mounting hardware and associated circuitry and controls of a light.

Luminance: The intensity of light reflected or emitted from a unit area of surface, such as a sign face. Measured in nits and lumens.

Nadir: The point that is vertically downward from a particular location or observer, and directly opposite the zenith.

Nocturnal Preserve: One of three designations in the Royal Astronomical Society of Canada’s Dark-Sky Sites program. The primary goal of Nocturnal Preserves is to protect the nocturnal environment, so they may not be open to the public after dark.

Responsible lighting: Lighting that is designed and installed without causing adverse effects. Responsible lighting principles include turning off non-essential lights, using motion sensors or timers, shielding lights or aiming them downward, and choosing light bulbs with low Correlated Colour Temperatures.

Scotobiology: The study of the biological need for periods of darkness, as opposed to photobiology, which describes the biological effects of light.

Semi cut-off (SCO): A type of luminaire shield that limits light emission to 5% at or above an angle of 90° from nadir, and 20% at or above an angle of 80° above nadir.

Sharp cut-off (Sh-CO): A type of luminaire shield that limits light emission to 1% of the total light within the glare zone between 80° and 90° above nadir. 

Shield: A cover that restricts the path of light as it exits a luminaire. Shields are used to directly control glare and light trespass.

Skyglow: The diffuse luminance of the night sky, apart from discrete light sources such as the moon and visible individual stars. It is commonly seen over urban centres. Skyglow is composed entirely of wasted light energy that has escaped light fixtures in non-useful directions.

Sky Quality Meter (SQM): A light meter developed by Unihedron, Inc. to measure the brightness of the sky. Measurements are a number reflecting magnitude per square arcsecond (mag/arcsecond2), a standard astronomical sky brightness measurement.

Unshielded (non-shielded): A light fixture that has no shielding or partial shielding that does not project lighting into a useful direction.

Urban Star Park: One of three designations in the Royal Astronomical Society of Canada’s Dark-Sky Sites program. Urban Star Parks are located in urban or suburban areas, so they typically have brighter skies than Dark-Sky Preserves or Nocturnal Preserves. They are a great opportunity for public education because they are easily accessible.

Uplight: Light that shines upward into the sky at any angle beyond horizontal.

Zenith: The point that is vertically upward from a particular location or observer, and directly opposite the nadir.