Get lost? Check the glossary!
A useful tool for understanding the most relevant terms and definitions related to light pollution. Here you will find simple and clear explanations of key concepts that will help you navigate our website more easily.
A
Adaptive controls. Devices such as motion sensors, timers, and dimmers that are used alongside outdoor lighting systems to adjust the light brightness or to control how long the lights stay on.
Agenda 2030. A global plan adopted by all United Nations member countries in 2015 that aims to make the world a better place by ending poverty, protecting the planet, and ensuring peace and prosperity for everyone. At the heart of Agenda 2030, there are the 17 Sustainable Development Goals (SDGs), a roadmap that encourages governments, businesses, organisations, and individuals to work together for a more sustainable future.
ALAN. Acronym for Artificial Light at Night. It refers to the presence of artificial lighting during the nighttime, which disrupts the natural darkness of the night sky. This includes any human-made light sources such as streetlights, buildings, vehicles, and other forms of nighttime lighting.
All-sky cameras. Digital (and usually commercial) cameras equipped with fisheye lenses to capture all-sky images, which are images of the whole visible hemisphere of the sky from a given location. While All-sky cameras provide detailed insight into night sky brightness, such as the spatial information about the sources of light pollution in the surrounding area, SQM and TESS instruments are more cost-effective and user-friendly for general use.
Artificial light. Light produced by human-made sources such as electric lights, campfires, candles, and fireworks.
ASI. Acronym for Agenzia Spaziale Italiana, namely the Italian Space Agency. It is responsible for coordinating and managing the Italian space exploration, research, and satellite missions.
Astrophotography. A type of photography that involves taking pictures of astronomical objects, celestial events, and the night sky. This includes photos of stars, planets, the Moon, the Milky Way, galaxies, comets, and more.
Astrotourism. A type of travel focused on experiencing and observing the night sky, stars, planets, and other celestial phenomena. It is usually experienced in places with little light pollution, such as remote deserts, mountains, or specially designated Dark Sky Parks.
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Calibrated data. Data acquired by scientific instruments, especially by means of photometers, are often given in terms of an internal measurement unit (e.g. “counts”). On the other hand, calibrated data are reported to a standard measurement unit, internationally recognised by the scientific community.
Candela. A unit of luminous intensity, which measures the perceived brightness of a light source in a specific direction. It quantifies the amount of light emitted by a source in a particular direction, taking into account the human eye’s sensitivity to different wavelengths of light.
CAS. The Chinese Academy of Sciences (CAS), namely China’s national research institute for natural sciences. Under CAS, the International Research Center of Big Data for Sustainable Development Goals (IRCBDSDG) was established to focus on using big data technologies to support the monitoring and achievement of the United Nations Sustainable Development Goals (SDGs).
CMOS camera. A digital camera that uses a CMOS (Complementary Metal-Oxide-Semiconductor) sensor to capture images. CMOS sensors are the most commonly used in modern camera modules, such as those of smartphones.
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Dark sky. A dark sky refers to a night sky that is free from light pollution, which makes it ideal for stargazing and observing celestial objects such as stars, planets, and the Milky Way.
Data availability. Whether or not the data can be accessed (e.g. online, upon request, etc.).
Data Format. The digital format in which data are available, e.g. spreadsheet (csv, xml), text file (txt, dat), image (jpeg, png), etc.
Data publishing delay. The time delay, if any, after which the data is published (e.g. a month, a year, multiple years, etc.).
Data publishing frequency. How often the data owner updates and shares its data with the public or other stakeholders.
Data policy. The policy regulating how data can be used, reproduced, and shared, e.g. Creative Commons (CC), Open Access, etc.
Data visualisation. How the data are displayed on the owner’s website.
Day Night Band (DNB). A low-light imaging sensor on the VIIRS instrument on board satellites like Suomi NPP that is capable of capturing visible light at night. It detects faint light sources such as city lights, wildfires, auroras, and moonlit clouds, providing valuable nighttime imagery for weather monitoring, disaster response, and environmental studies.
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Earth Observation. It refers to the collection, analysis, and interpretation of data about the Earth’s physical, chemical, and biological systems using remote sensing technologies, typically from satellites, aircraft, drones, or ground-based sensors.
Ecosystem services. The many benefits that natural ecosystems provide to human societies, both directly and indirectly, although they are often unrecognised or undervalued. Assigning them a measurable value helps make the importance of nature more tangible and visible, especially when dealing with policy and economic decisions.
Electromagnetic radiation. Energy that travels through space in the form of waves made up of electric and magnetic fields. It includes a wide range of wavelengths and frequencies, forming the electromagnetic spectrum, i.e. from radio waves (longest) to gamma rays (shortest).
ESA. The European Space Agency (ESA) is an intergovernmental organisation founded in 1975 to coordinate space exploration and research among its 22 member states. It conducts missions in Earth Observation, satellite navigation, human and robotic exploration, and space science.
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Field-of-view. The portion of the sky observed by the ground-based instrument (e.g. 20°, 20° x 20 °, all-sky, etc.) / The extension (e.g. 100 km2) or the dimensions (e.g. 10 km x 10 km) of the area observed by the satellite.
Frequency of measurement. The time between two consecutive measurements (e.g. every 5 seconds, every minute, once per day, etc.).
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Glare. A visual condition where excessive or uncontrolled brightness, usually from artificial light or sunlight, makes it difficult to see clearly. It can cause discomfort, eye strain, or even temporary blindness in extreme cases.
Glimmer and Multispectral Imager. A type of instrument for satellites that uses sensors to capture data in multiple spectral bands (e.g. red, green, blue, near-infrared).
Ground-based instruments. Scientific tools or devices that are located on the Earth’s surface and can be employed to collect data about various environmental or atmospheric conditions. These instruments are typically used to measure air quality, temperature, light levels, or even geological activity.
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High-sensitivity panchromatic optical camera. A grayscale imaging system optimised for low-light, high-detail photography using visible light. It is widely used in satellites, drones, and aerial surveillance systems for its clarity, resolution, and versatility.
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INFN. The Istituto Nazionale di Fisica Nucleare is Italy’s leading research institute in the field of nuclear and particle physics, advancing our understanding of the fundamental components of the matter.
Interreg. A European Union (EU) funding programme designed to promote cross-border cooperation among EU member states, as well as non-EU countries, in different regions of Europe. It supports projects that address shared challenges and foster economic, social, and environmental development in these regions.
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LAADS-DAAC. The Level-1 and Atmosphere Archive and Distribution System Distributed Active Archive Center (LAADS-DAAC) is a NASA data centre located at the Goddard Space Flight Center in Greenbelt, Maryland. Established in 2007, LAADS DAAC specialises in archiving and distributing atmospheric and geophysical data products derived from satellite instruments.
Light pollution. The excessive, misdirected, or poorly controlled artificial light that interferes with the natural darkness of the night sky. It can come from streetlights, buildings, and other sources and it can disrupt the environment, human health, and our ability to see stars and celestial objects.
Low Earth Orbit (LOE). An orbit around the Earth that is located between approximately 160 km and 2,000 km above the Earth’s surface. Satellites in LEO move quickly and can complete an orbit in about 90 to 120 minutes, making this orbit ideal for applications like Earth Observation, weather monitoring, telecommunications, and space research.
Luminance. The measure of the perceived brightness of light emitted or reflected from a surface, as seen by the human eye. It quantifies how much light is visible and how bright a light source or illuminated surface appears. Luminance is typically measured in candelas per square meter (cd/m²).
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Magnitude. A measure of the brightness of a celestial object, such as a star, a planet, or a galaxy. The magnitude scale is a negative logarithmic scale, meaning that a smaller magnitude number corresponds to a brighter object. As a rule of thumb, a difference of 5 magnitudes between two celestial objects means that the brighter object has a flux 100 times higher than the other fainter one.
Magnitude per square arcsecond. The unit used to describe the brightness of the night sky or the intensity of light emitted by celestial objects, such as stars or galaxies, as observed in a specific area of the sky. It represents the magnitude of the sky normalised by the unit of solid angle, which measures, for example, the extension of the portion of the sky observed by a specific instrument.
Measurement point. The type of location where the instrument is situated (e.g. city centre, suburban area, rural area, isolated place, astronomical observatory, etc.).
Measurement unit. The unit in which the data, calibrated or not, are available (e.g. mag/arcsec2, lux, counts, etc.).
Mini-EUSO. Mini-EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory) is a satellite instrument, a wide-field telescope designed to perform observations of the UV light emission from the Earth. Since its launch in August 2019, it has been installed in front of the UV transparent window of the Zvezda Russian module on the International Space Station.
Monitoring and Evaluation (M&E). The processes used to track the progress and assess the effectiveness of a project, a programme, or a policy. Monitoring involves ongoing data collection and analysis to ensure that activities are being implemented as planned and that goals are being met. Evaluation involves a more in-depth assessment that happens at specific points to determine the overall impact, success, and outcomes of the project or programme.
N
Nadir. The point directly beneath an observer, opposite to the zenith, which is the point directly above.
NASA. The National Aeronautics and Space Administration (NASA) is the U.S. agency responsible for space exploration, from sending astronauts to the Moon to studying distant planets and galaxies.
NOAA. The National Oceanic and Atmospheric Administration (NOAA) is a U.S. government agency that focuses on understanding and protecting the Earth’s natural environment, its activities include weather forecasting, ocean conservation, and climate monitoring.
Non-sun-synchronous inclined. The orbit type used by satellites that do not follow a Sun-synchronous path and are tilted (inclined) relative to the Earth’s equator.
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Observed area. The area of the Earth observed by the satellite (e.g. from -60° to +60° of latitude).
Off-nadir. The angle away from the vertical (nadir) direction at which a satellite or a sensor observes the Earth. Off-nadir viewing allows satellites to observe areas not directly beneath them, which is useful for targeting specific locations without waiting for a direct overpass. However, it can lead to image distortion and reduced spatial resolution compared to nadir views.
OLI-2 pushbroom sensor with photosensitive detectors (Landsat-9). A pushbroom imaging instrument that uses thousands of photosensitive detectors arranged in a line to collect multispectral imagery of the Earth’s surface as the satellite passes overhead. It enables precise, high-quality Earth Observation.
Orbit type.The orbit type of a satellite depends on its average altitude while orbiting the Earth, e.g. Geostationary Orbit (GEO), Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Polar Orbit (PO), Sun-synchronous orbit (SSO), etc.
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Photometer. A scientific instrument used to measure the intensity of light. It quantifies the amount of light in a specific area or how bright a light source appears.
Photometre network. A group of instruments, called photometers, set up in different locations in order to measure the brightness of the sky or light levels over time. These instruments work together to collect consistent data across a region and, often, to study light pollution, atmospheric conditions, or night sky quality.
Photometry. A general term used across many different fields (e.g. physics, optics, astronomy, etc.), but that generally refers to the measurement of brightness, the amount of light perceived by an instrument or by the human eye.
Phototaxis. The movement of an organism in response to light. It can be positive (towards the light) or negative (away from the light), thus helping organisms with activities such as finding food, avoiding predators, or optimising their growth conditions.
Pointing direction. The direction the instrument is pointed at in standard working conditions (e.g. ground-based instrument: zenith (upwards), to the North (Polaris); satellites: nadir (downwards), off-nadir (at a specific angle); etc.)
Polar Orbit (PO). A satellite path that passes over the Earth’s poles, allowing it to travel over almost every part of the planet as the Earth rotates beneath it. This type of orbit is ideal for global coverage and is commonly used for weather, environmental, and reconnaissance satellites.
PRISMA. Acronym for Prima Rete Italiana per la Sorveglianza sistematica di Meteore ed Atmosfera, namely the First Italian Network for the Systematic Monitoring of Meteors and the Atmosphere.. This Italian network of all-sky cameras observes bright meteors (i.e. fireballs and bolides) for the purpose of determining the orbits of the celestial objects that generated them and of accurately defining the areas where fragments may fall, in order to be able to recover freshly-fallen meteorites.
Pushbroom sensor. This type of sensor captures images line by line as the satellite moves forward in its orbit. Instead of using a scanning mirror like the older “whiskbroom” sensors, it has a linear array of detectors (thousands of photosensitive elements) that collect data simultaneously across a wide swath. This allows for better geometric accuracy, less moving parts, and higher radiometric performance.
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Radiance. The measure of the total amount of light emitted, reflected, transmitted, or received by a surface per unit of solid angle and per unit of area. It describes the intensity of light travelling in a particular direction and is typically measured in watts per square meter per steradian (W/m²·sr).
Radiometric imaging sensor. A device that measures the intensity of electromagnetic radiation (e.g. visible or infrared light) to create detailed images. Unlike regular cameras, it quantifies how much energy is emitted or reflected from surfaces, thus allowing an analysis of temperature, vegetation, or atmospheric conditions. These sensors are commonly used in satellites, drones, and scientific instruments for Earth observation and climate research.
Reflected light. The light that bounces off a surface rather than being absorbed or transmitted through it. When light hits an object, some of it is reflected back in different directions, depending on the object’s surface characteristics (smooth or rough).
Repository. An online system where data, documents, images, or other digital materials are safely stored and made accessible.
Roadrunner method. This method involves mounting a Sky Quality Meter (SQM) onto the roof of a car and driving at night to collect light level data. While this method allows for quick surveys of large areas, it can show measurement biases (e.g. interference from car headlights).
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Satellite instruments. They are used to measure light pollution by detecting the scattering of artificial light, which obscures the stars. Various public websites, like the World Atlas of Artificial Sky Brightness, use satellite data to provide light pollution maps.
Sky brightness. The amount of light that is present in the night sky, either from natural sources (e.g. the Moon and stars) or artificial ones (e.g. streetlights and buildings).
Skyglow. The brightening of the night sky caused by artificial lighting, particularly in urban areas. It is a form of light pollution where the light from streetlights, buildings, and other sources scatters in the atmosphere making the night sky appear lighter than it naturally would. This can reduce the visibility of stars and other celestial objects.
Sky Quality Meter (SQM). An affordable device used to measure night sky brightness, popular among researchers and amateur astronomers. It is one of the most widely used instruments in light pollution research, offering the possibility of making data accessible in the Unihedron repository.
SP (Shipin) and GF03C Series (Gaofen 03C). The terms SP (Shipin – 视频, meaning “video” or “video imaging”) and GF03C Series (Gaofen – 高分, meaning “high resolution”) refer to Chinese Earth Observation satellites, which are part of broader satellite families used for imaging, surveillance, and scientific purposes.
Spatial resolution. The level of detail visible in a satellite image (i.e. the spatial dimension of one pixel). A resolution of 10 m means that each pixel covers a 10 m × 10 m area on the ground.
Spectral radiance. The radiance of light at a specific wavelength or frequency. It provides a detailed measurement by breaking down the light intensity across different wavelengths of the electromagnetic spectrum. Spectral radiance is usually measured in watts per square meter per steradian per wavelength unit (e.g. W/m²·sr·nm) or watts per square meter per steradian per frequency unit (e.g. W/m²·sr·Hz).
Sub-meter observation. Satellite or aerial imagery with a spatial resolution of less than one metre, meaning that each pixel represents an area smaller than 1×1 metre on the ground. This allows for very detailed images, capable of distinguishing small objects such as vehicles or individual trees.
Sun-synchronous low altitude orbits (SSO). This type of orbit is a near-Earth path that allows the satellite to pass over the same region at the same local solar time each day. This ensures consistent lighting conditions for high-resolution Earth observations, useful for monitoring environmental and surface changes over time.
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TESS-W. A newly developed photometer for measuring light pollution, equipped with a Wi-Fi connection and using a photodiode similar to the SQM. It offers a comparative accuracy of around 10% and provides data accessible through the Stars4All TESS dashboard, used in fixed monitoring stations of the STARS4ALL network.
Time range. Time frame of the measurement (e.g. from year to year).
Type of instrument. The type of ground or space-based device used to monitor light pollution.
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VIIRS. The Visible Infrared Imaging Radiometer Suite (VIIRS) is one of the key satellite instruments on board the Suomi National Polar-Orbiting Partnership (Suomi NPP) spacecraft, launched on October 28, 2011. It performs moderate resolution imaging with 22 channels, allowing environmental monitoring and weather forecasting. The very high sensitivity of the Day Night Band (DNB) of VIIRS allows global daily measurements of the visible and near-infrared light, in low light conditions.
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Wavelength. The distance between consecutive pics of a wave. It is typically used to describe waves, such as light, sound, and other types of electromagnetic radiation.
Wavelength range. The specific interval of wavelengths (e.g. 300-600 nm) describing the observed range of the light spectrum or the spectrum range itself (e.g. ultraviolet, visible, infrared, etc.).
Wide-field detector. A high-sensitivity sensor designed to cover a large area in one shot, particularly useful for sky surveys, Earth imaging, and environmental monitoring.
Wide-field telescope. A type of telescope designed to capture a large area of the sky in a single image, rather than zooming in on a small region. It uses special optics and sensors to maintain image clarity across a broad field of view, making it ideal for surveying the sky, detecting asteroids, tracking satellites, or observing large-scale cosmic structures.
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Zenith. The point directly above an observer, meaning the highest point on the celestial sphere relative to the observer’s location.
This repository is managed by INAF as part of the Interreg Central Europe project DARKERSKY4CE, co-funded by the European Union. The views and opinions expressed herein are those of the authors and do not necessarily reflect those of the European Commission. Whilst INAF takes every precaution to ensure that the content is accurate, INAF cannot guarantee that all information or data on the website is correct. Therefore, INAF accepts no responsibility or liability whatsoever with regard to the information or data on the website or for any use which may be made of it.