Passion of the People – 1.7.2

How Do We Quantify Light Pollution?

Light pollution can be defined as the presence of unwanted, inappropriate or excessive artificial lighting. In particular, artificial light at night (ALAN) is a current phenomenon and, despite its obvious benefits, it presents various negative aspects, which are studied across multiple fields, including astronomy. Therefore, it is becoming increasingly important to be able to properly measure light pollution.

At present, there are many measurement tools at disposal. Their use is often context or field-related and, of course, they all present limitations, such as accuracy or costs. Some of the main devices for quantifying light pollution are illustrated hereafter.

Sky Quality Metres (SQM)

Sky Quality Metres (SQM) are the most widely used device to take scientific-quality measurements of sky brightness at night. This is certainly due to their affordability, but not only. Indeed, SQMs are also quite easy to use with some precautions, essentially you need to “hold the device above your head, point the photometer at the zenith, and push the button”, by repeating these steps you will be able to take multiple measurements per location.

In fact, SQMs are sensitive to visual light and they measure the brightness of the night sky in magnitudes per square arcsecond, i.e. the brightness, which is expressed using the magnitude logarithmic scale, of a square arcsecond of sky.

Furthermore, the results of these measurements, namely the SQMs readings, can be paired with the adequate level of the Bortle Scale, the latter being a nine-level numeric scale, created by John E. Bortle in February 2001, with the purpose of measuring how bright the night sky is in a location according to what celestial objects are or are not visible and regardless of the equipment at disposal. This method can be considered a marked improvement over the previous naked-eye limiting magnitude (NELM) measurements, which were less rigorous and deeply conditioned by the specific observer’s eyesight.

All-Sky Cameras

All-Sky cameras are designed to take pictures of the entire sky over a certain amount of time, usually in order to monitor astronomical phenomena. This kind of camera has recently emerged as a valid tool for measuring the brightness of the night sky and, consequently, light pollution levels.

There are several types of all-sky cameras, for instance, it suffices to combine a digital camera with a fisheye lens, to calibrate the device and to point it to zenith, i.e. the point in the sky directly above the observer, in order to create a proper ground-based imaging sensor capable to produce all-sky measurements of light pollution.

Furthermore, all-sky cameras preferably make use of a SQM software that calibrates and enhances the data obtained, thus, for instance, separating light pollution from other astronomical phenomena that can be observed in the resulting imaging.


Satellites provide another effective method for the measurement of light pollution. Indeed, light pollution can be easily perceived with the naked eye at night as a haze of light above many cities. This dome of skyglow is nothing but the scattering of artificial light at night (ALAN), which is in part scattered back to the ground and in part detected by satellites.  Hence, spaceborne sensors, by measuring upward light, can quantify the amount of light pollution obscuring the stars.

Today, there are several satellites used to research light pollution and also many public websites that show light pollution maps provided by satellite imaging, such as the World Atlas of Artificial Sky Brightness, the Dark Site Finder, and the Light Pollution Map. Nevertheless, it is important to mention that artificial satellites can be considered a source of light pollution in their turn, especially those located in low Earth orbits.

The Light Pollution Spectrum

Finally, when illustrating the measurement tools used to quantify light pollution, it is essential to identify the emission spectrum of reference, namely that of a light pollution source such as street lights. Indeed, this is a determining factor for modelling artificial light at night (ALAN).

According to NASA, the visible light spectrum can be defined as “the segment of the electromagnetic spectrum that the human eye can view.” Within the visible light spectrum, it is possible to identify different types of ALAN. For instance, the warm light of incandescent bulbs and the cold light of LEDs are quite clearly at opposite ends of the visible light spectrum.

Indeed, although LEDs are both cheaper and more energy-efficient and, therefore, widely used, they are actually worsening light pollution and making the observation of dark skies increasingly difficult, due to the higher scattering level of the blue light they emit.

Light Pollution – Image by Getty Images
All-Sky Camera – Image by Thomas Jacquin
SQM – Image by IDA
All-Sky Camera Photo – Image by Alcor System
The Bortle Scale – Image by Dark Sky Diary
Light Pollution in Paris – Image by International Space Station NASA
The Visible Light Spectrum – Image by oncelighting
LEDs – Image by PhysOrg
Incandescent Bulbs – Image by HeiSolar

Further Resources

Links below will redirect you to external websites. In accordance with the European data protection declarations, we would like to point out that by clicking on these links you may send data to external providers. We cannot prevent that. 


Light Pollution 101 – National Geographic

Where Are the Stars? See How Light Pollution Affects Night Skies – National Geographic

Why you should care about light pollution – DW Planet A

Huge loss of stars visible in night sky due to light pollution say scientists – BBC News

Italia maglia nera per l’inquinamento luminoso – MEDIAINAF TV

Troppi satelliti in orbita e inquinamento luminoso impattano sulle osservazioni astronomiche – Università di Padova

Gli spettri di emissione – Edizioni Atlas

Online Resources

How to measure light pollution—A systematic review of methods and applications

Sky Quality Meter measurements in a colour-changing world

How Dark is My Sky? An Astronomer’s Guide to the Bortle Scale – Love the Night Sky

Sky Quality Meter Reading – Astrotourism Western Australia

How Dark is Your Night Sky? – Stargazers

How To Conduct A Night Sky Quality Survey – International Dark-Sky Association (IDA)

Eyes In The Sky: Exploring Global Light Pollution With Satellite Maps – International Dark-Sky Association (IDA)

Picture imperfect: light pollution from satellites is becoming an existential threat to astronomy – The Guardian

Beyond All-Sky: Assessing Ecological Light Pollution Using Multi-Spectral Full-Sphere Fisheye Lens Imaging – MDPI

All Sky Camera Observations of Cloud and Light Pollution at Thirty Meter Telescope Candidate Site

Emission spectra of light-pollution sources determined from the light-scattering spectrometry of the night sky

Tour of the Electromagnetic Spectrum – Visible Light – NASA Science

The environmental consequences of LEDs – Spectrum News 1

Bright LEDs could spell the end of dark skies – MIT Technology Review

These Energy-Saving Bulbs Are Making One Pollutant Much Worse – National Geographic

Effects of the COVID-19 lockdown on urban light emissions: ground and satellite comparison

Measurements of Night Sky Brightness in the Veneto Region of Italy: Sky Quality Meter Network Results and Differential Photometry by Digital Single Lens Reflex

Luci e buio: tecniche e dati per lo studio dell’inquinamento luminoso – ARPA Veneto

Inquinamento luminoso e insetti: un nuovo studio scientifico

Non riusciremo più a riveder le stelle – Valentina Guglielmo INAF

L’inquinamento luminoso è in rapido aumento – Il Post

E’ vero che i satelliti in orbita causano inquinamento luminoso? – ANSA

Satelliti e inquinamento luminoso: osservazioni astronomiche sempre più difficili – Università di Padova

Sulla Terra non c’è più un luogo immune dall’inquinamento luminoso – Focus

Quanto è Buio il Cielo di Notte? – Meteo Planet

L’inquinamento luminoso – Cielo Buio

Spettro di emissione – Chimica online

Further Readings

Light Pollution : Responses and Remedies – Bob Mizon

Teaching Materials

A Collection of Light Pollution Lesson Plans for Educators – International Dark-Sky Association (IDA)

Innovazioni tecniche per la riduzione dell’inquinamento luminoso negli impianti di illuminazione esterna – Università degli studi Guglielmo Marconi

Parametri tecnici per la limitazione dell’inquinamento luminoso – Mario di Sora

For Kids

Light Pollution | Reasons and Effects | Video for Kids – Learning Junction

Save the Sky by Bethany Stahl | Children’s Animated Audiobook | A Story About Light Pollution – Bethany Stahl