Artificial light at night (light pollution) is a growing environmental concern affecting human health, wildlife, and energy consumption. It manifests as sky glow (the dome of light over urban areas), glare (excessive brightness causing visual discomfort), and light trespass (unwanted light falling onto a property). Accurately quantifying and monitoring these forms of pollution is necessary for implementing effective conservation and policy worldwide.
Key Metrics and Measurement Units
To define light pollution scientifically, researchers use specific quantitative standards. The most common unit for measuring sky glow is magnitude per square arcsecond (mag/arcsec²). This astronomical unit uses a logarithmic scale where a higher numerical value indicates a darker sky; a value around 22.0 represents a pristine sky.
The measurement of light striking a surface, such as light trespass or glare, is quantified using illuminance, typically expressed in lux (lx). Lux meters provide a direct reading of light intensity at a specific point. Satellites and aerial monitoring systems often measure radiance or irradiance, which is the total light energy emitted upward from a source.
Ground-Based Measurement Tools
The most common instrument for localized, high-precision measurement of sky glow is the Sky Quality Meter (SQM). This handheld device measures night sky brightness at the zenith, providing a single, objective reading in mag/arcsec². Professionals use the SQM to track changes in sky darkness over time and compare sky quality across different locations.
To measure light trespass and glare from specific fixtures, researchers rely on handheld photometers (calibrated lux meters). These devices are placed at property boundaries or on surfaces to determine if light levels exceed local ordinances. Photometers allow for detailed mapping of unwanted light spillage at ground level.
Another powerful ground-based tool is the all-sky camera, which uses a calibrated digital camera and a fish-eye lens to capture the entire visible sky in a single image. These systems map the entire light dome, revealing the directionality and extent of light pollution across the horizon. Scientists analyze the visual data to create a spatial map of sky brightness, showing how light from distant cities contributes to pollution.
Satellite and Aerial Monitoring Techniques
Large-scale, global data on light pollution is collected remotely using specialized instruments on orbiting satellites. This technique measures the light emitted upward from the Earth’s surface, providing a macro-level view of light usage and urbanization. The primary instrument for this task is the Visible Infrared Imaging Radiometer Suite (VIIRS), aboard satellites like the Suomi National Polar-orbiting Partnership (Suomi NPP).
The VIIRS Day/Night Band (DNB) is highly sensitive, capable of detecting low-light emissions across the globe, including city lights and gas flares. It measures the upward radiance of artificial light, creating comprehensive maps that track the expansion and intensity of light pollution over large areas. A caveat of satellite monitoring is that it cannot accurately measure light directed horizontally, such as light trespass, or light that is shielded. Satellites are also less sensitive to certain light colors, such as blue light.
Citizen Science and Practical Application
The public contributes to light pollution monitoring through citizen science initiatives, combining technical metrics with widespread observation. The international Globe at Night campaign is a leading example where participants compare star visibility in a constellation to provided charts. This method determines the naked-eye limiting magnitude, a qualitative measure of sky darkness that correlates with quantitative data. Participants submit their observations, location, and time, often via a simple web application.
Smartphone applications, such as Dark Sky Meter and Loss of the Night, also enable citizens to contribute data. These apps use the phone’s sensor to provide a rough estimate of sky brightness or log star visibility. While not as precise as a calibrated SQM, these tools allow for rapid data collection across vast geographic areas. The collected data helps scientists and policymakers create accurate light pollution maps and advocate for responsible lighting practices.