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Measuring night sky brightness: methods and challenges
Andreas Hänel, Thomas Posch, Salvador J. Ribas, Martin Aubé, Dan Duriscoe, Andreas Jechow, Zoltán Kollath, Dorien E. Lolkema, Chadwick Moore, Norbert Schmidt, Henk Spoelstra, Günther Wuchterl, Christopher C. M. Kyba
TL;DR
The paper reviews instruments for measuring night-sky brightness and their limitations. It concludes that imaging instruments provide decisive information, while one-dimensional instruments remain particularly suited to long-term monitoring.
Problem
Recognizing artificial light’s ecological impacts makes quantifying nocturnal light levels important.
Method
The paper reviews several instruments and techniques for measuring night-sky brightness, including spectrometers, one-dimensional instruments, and imaging instruments.
Results
Imaging instruments provide information across the sky, whereas one-dimensional instruments are most strongly suited to monitoring temporal, especially long-term, changes.
Takeaways & Limitations
Combining imaging with long-term monitoring can ease interpretation of future changes in night-sky brightness.
Takeaways & Limitations
One-dimensional instruments only measure limited sky information, constraining their characterization of sites.
Abstract
from arXiv · showhide
Measuring the brightness of the night sky has become an increasingly important topic in recent years, as artificial lights and their scattering by the Earths atmosphere continue spreading around the globe. Several instruments and techniques have been developed for this task. We give an overview of these, and discuss their strengths and limitations. The different quantities that can and should be derived when measuring the night sky brightness are discussed, as well as the procedures that have been and still need to be defined in this context. We conclude that in many situations, calibrated consumer digital cameras with fisheye lenses provide the best relation between ease-of-use and wealth of obtainable information on the night sky. While they do not obtain full spectral information, they are able to sample the complete sky in a period of minutes, with colour information in three bands. This is important, as given the current global changes in lamp spectra, changes in sky radiance observed only with single band devices may lead to incorrect conclusions regarding long term changes in sky brightness. The acquisition of all-sky information is desirable, as zenith-only information does not provide an adequate characterization of a site. Nevertheless, zenith-only single-band one-channel devices such as the Sky Quality Meter continue to be a viable option for long-term studies of night sky brightness and for studies conducted from a moving platform. Accurate interpretation of such data requires some understanding of the colour composition of the sky light. We recommend supplementing long-term time series derived with such devices with periodic all-sky sampling by a calibrated camera system and calibrated luxmeters or luminance meters.
1. Introduction
Artificial light and its atmospheric effects have increased interest in measuring night sky brightness, but the expanding range of instruments, terminology, and units creates barriers for newcomers. This overview introduces visible-wavelength measurements and compares techniques, assumptions, and limitations relevant to field-site characterization.
- Motivation: Research into artificial light has grown alongside concerns about ecological and health impacts, increasing environmental light levels, and improved measurement technologies.These developments have contributed to the expanding interest in night sky brightness measurement.
- Motivation: The growing number of measurement approaches can present a barrier to newcomers because astronomical photometry, typical starlight levels, and measurement terminology are unfamiliar.The paper specifically notes challenges for interdisciplinary researchers, including biologists measuring light exposure during field experiments.
- Scope: The paper introduces visible-wavelength night sky brightness measurements for readers without field-specific background and discusses their relevance to biological field-site characterization.The authors caution that biological-site characterization also involves glare and potentially ultraviolet and infrared radiation.
- Approach: The overview compares naked-eye observations, single-channel instruments, imaging instruments, spectrometers, and astronomical photometry.Astronomical photometry is placed in an appendix because researchers outside astronomy are unlikely to use it.
- Assumptions: Measurements generally assume that direct light sources are absent from the instrument’s field of view.This assumption is stated as applying unless otherwise specified.
- Limitations: Approximate SI conversion and spectral-response mismatches can produce large errors for sources whose spectra differ from daylight-like illumination.The paper gives sodium and fluorescent lamps as examples of sources that can challenge inexpensive luxmeters.
2. Sky brightness
Night sky brightness combines natural and artificial sources, including atmospheric skyglow, and varies with viewing direction, clouds, and location. Its measurement requires distinguishing radiance-related quantities, spectral response, and whether starlight is included.
- Sources and effects: Artificial surface light can scatter in the atmosphere as skyglow, reducing star visibility and clear-sky polarization, especially near the horizon.The horizon is described as the brightest region of the glow.
- Meteorological effects: Clouds can increase urban skyglow by more than an order of magnitude, while making naturally dark skies modestly darker.The contrast depends on whether artificial light is present.
- Quantities: Irradiance describes total electromagnetic radiation falling on a surface, whereas radiance describes brightness from an area in the field of view.Spectrometers can measure radiance or irradiance across many wavelengths; radiance per unit wavelength is spectral radiance.
- Quantities: Photometric instruments are designed to match human visual response, so perfectly matched measurements are reported as luminance or illuminance rather than radiance or irradiance.Restricted-band devices require their spectral sensitivity to be reported with the radiance value.
- Measurement definitions: Sky measurements should specify whether they represent sky background between visible stars or sky brightness including starlight, while irradiance includes all light sources.This distinction is important when interpreting sky radiance.
- Units and bands: Astronomical sky brightness is commonly reported in mag/arcsec2, where larger values indicate darker skies.The Johnson UBV system provides standardized observations in ultraviolet, blue, and green bands, with V approximately related to visual photometry.
- Typical values: 22 magV /arcsec2 characterizes the darkest places on Earth, compared with 16–17 magV /arcsec2 in bright cities.These representative values show the range described for terrestrial sky backgrounds.
- Spatial variation: Sky brightness is often estimated using a uniform-sky approximation relating luminance and illuminance by π, although brightness is generally greatest near the horizon.The approximation is described as conservative, particularly for clear nights in predominantly artificial-lit areas.
3. Constraining the night sky brightness by visual observations
Visual observations constrain night sky brightness through the faintest visible stars or standardized star fields, providing accessible measurements with precision that depends on the observation method. Smartphone-assisted observations can improve precision, but their catalog limits their useful range.
- Visual metric: Limiting magnitude is the magnitude of the faintest star visible to the naked eye and is used to qualify night sky quality.The relation between limiting magnitude and sky brightness depends on the observer and field factor.
- Observation methods: Observers can estimate limiting magnitude by checking sequences of stars near the celestial North Pole or counting stars in designated fields.Public projects also use comparisons with star charts showing integer limiting magnitudes.
- Observation methods: Chart-based observations reduce precision to ±1.2 magnitudes but enable broad participation by non-experts.This trade-off supports large-scale citizen observations despite coarse individual measurements.
- Smartphone-assisted observations: The Loss of the Night app uses inertial sensors and a live star map to direct observers toward individual stars and record their visibility.It distinguishes visible, invisible, and otherwise unobservable stars, including averted-vision reports.
- Smartphone-assisted observations: The app can achieve precision as good as 0.05 magnitudes when many stars of differing magnitudes are assessed.Individual-observation precision can be estimated from data self-consistency.
- Limitations: The app’s star catalog extends only to about magnitude 5.2, restricting its useful use to urban and suburban areas.This scope boundary limits application at darker sites where fainter stars would need to be evaluated.
4. One dimensional instruments
One-dimensional instruments typically measure sky brightness through a single channel, often near zenith, with the Sky Quality Meter as a widely used example. Their interpretation depends on spectral response, calibration, viewing geometry, and susceptibility to nearby or stray light.
- One-dimensional devices measure sky brightness through a single channel, typically observing only at zenith, and may include stars within the viewing field.
- The Sky Quality Meter: The SQM combines a sensor, infrared filter, lens, and weatherproof screen, producing a spectral response that differs across photometric systems for different sky colours.
- The Sky Quality Meter: 100 µcd/m2 (∼22.5 magSQM/arcsec2) is measurable with the SQM, whose lensed SQM-L version has a 20° FWHM field of view and is normally aimed near zenith.
- The Sky Quality Meter: Nearby lamps can affect SQM readings because residual sensitivity at large angles or scattered lamp light may dominate the natural sky signal.
- The Sky Quality Meter: 0.15 to 0.2 mag/arcsec2 systematic brightness differences occur between newer LED-calibrated SQMs and older instruments calibrated with earlier sources.
- The Sky Quality Meter: The SQM-L is recommended over the un lensed version because its approximately 20° field of view produces more consistent readings near nearby light sources.
5. Two dimensional (imaging) instruments
Two-dimensional imaging instruments map sky brightness across wide fields, often the whole upper hemisphere, using CCD or CMOS cameras. ASTMON illustrates calibrated, multi-filter all-sky monitoring, while consumer cameras offer portable, information-rich measurements but require careful calibration.
- Imaging approaches: Wide-angle CCD and CMOS imaging systems can map the whole upper hemisphere and report sky luminance or sky background luminance.Sky background luminance yields darker values, expressed as larger mag/arcsec2, than one-dimensional instruments.
- Instrument deployment: ASTMON versions range from permanent robotic stations to portable and compact systems, but portable units are less safe outdoors and manufacturer software limits user control.The Lite version can operate outdoors for a few weeks, whereas the Full version is designed for continuous protected monitoring.
- ASTMON: An ASTMON sequence using five filters takes around 8 minutes, allowing about 70 independent observations during a standard night.Exposures must balance signal-to-noise requirements against smearing caused by Earth’s rotation.
6. Spectra of the night sky
Spectrometers measure the night sky’s spectral power distribution, helping separate natural and artificial contributions to sky radiance. SAND-4 provides automated, high-resolution spectral measurements, but requires much longer integrations at dark sites than in urban environments.
- Spectral analysis: Spectral power distribution measurements decompose night-sky light and help identify different contributions to sky brightness.Because artificial light sources often have different SPDs, nighttime spectral evaluation can indicate the kinds of sources responsible for sky brightness.
- SAND-4: SAND-4 is an automated long-slit spectrometer using a CCD imaging camera, with 2 nm resolution across 400–720 nm.It can operate independently with minimal human intervention.
- Spectral analysis: Spectrometers can relatively easily separate total sky radiance into major contributing sources.Examples include spectra from minimally polluted sites without moonlight and highly light-polluted sites where artificial sky brightness dominates.
- Measurement conditions: Urban SAND measurements typically integrate for a few minutes, whereas sites without artificial skyglow require about two hours.In highly light-polluted conditions, the natural contribution can be effectively neglected when the Moon is down.
7. Conclusions and outlook
Night-sky measurement requires trade-offs among spectral detail, angular coverage, temporal monitoring, cost, and calibration. The review concludes that calibrated fisheye cameras often offer the best overall compromise, while single-channel instruments remain useful for long-term or mobile measurements when supplemented by periodic broader sampling.
- Measurement goals: The ideal measurement would provide spectrally resolved radiance at fine angular resolution in every direction, but current instruments cannot achieve this at typical non-urban nocturnal luminance levels.Instrument choice must therefore follow the experimental task, such as long-term monitoring or characterizing a field site.
- Instrument classes: Instruments span one-dimensional versus imaging designs and broadband versus spectrally resolving measurements, with differing strengths and weaknesses for specific tasks.The review organizes commonly used systems into these classes and summarizes them in Table 4.
- Single-channel instruments: One-dimensional instruments are well suited to temporal, especially long-term, monitoring because they are relatively low-cost and can be deployed widely, including by citizen scientists.Their limitations include zenith-only or single-direction sampling, broadband responses that do not match standard photometric bands, and difficulty distinguishing natural airglow from scattered artificial light.
- Imaging instruments: Imaging instruments provide all-direction information and three broadband RGB channels, supporting intuitive interpretation, animal-relevant measurements, and monitoring of changing artificial-light spectra.Fisheye lenses can obtain all-sky information in one image, but vignetting, distortion, calibration, and orientation constrain accurate measurements, especially near the horizon.
- Calibration and stability: Long-term stability remains insufficiently known for most instruments: cameras appear stable, whereas SQM housing windows can lose transmission over time, at least in tropical climates.Periodic laboratory recalibration or intercomparison campaigns, together with databases of device and lens properties, would support future comparison and calibration.
- Conclusions and outlook: Calibrated digital cameras with fisheye lenses usually provide the best compromise among cost, ease of use, and information obtained in many situations.The review identifies universally accepted camera calibration and display software as a high priority, while recommending periodic imaging or spectrometer validation for long-term SQM measurements.
Appendix. Classical Astronomical Photometry
Classical astronomical photometry uses calibrated standard stars to derive atmospheric and instrumental parameters, then converts sky-background counts into sky brightness. Measurements require suitable nighttime and atmospheric conditions and careful photometric processing.
- Calibration with standard stars: The classical method uses standard astronomical stars as reference candles for estimating night-sky background brightness.Stars are observed in defined photometric systems and across different altitudes, magnitudes, and airmasses.
- Observing conditions: Observations are conducted after astronomical twilight, with the Moon below 10° under the horizon and preferably during photometric nights.Photometric nights provide atmospheric stability that helps keep calibration conditions constant.
- Airmass and stellar measurements: Airmass is determined from the selected star’s zenith angle, while measured stellar counts are corrected for sky contribution and divided by exposure time.The observation strategy covers the altitude range of the sky using standard stars at different airmasses and magnitudes.
- Photometric calibration: Catalogue magnitudes, airmass, and detected counts per second are combined to determine the instrumental zero point and atmospheric extinction coefficient.These parameters remain constant during the observation period when the atmosphere and instruments are stable.
- Sky-brightness calculation: Sky brightness is calculated from background counts in a star-free image region, normalized by angular area and exposure time, and reported in magnitudes per arcsec2.The resulting sky-background quantity is denoted Isky in counts per second and per arcsec2 before conversion to sky brightness.
- Application: The technique has been used to measure sky-background brightness across extended regions of the Netherlands.It can be applied to images obtained during the night after calibration parameters have been established.