Measuring night sky brightness: methods and challenges

TL;DR

This paper reviews various methods for measuring night sky brightness, emphasizing the practicality of calibrated consumer digital cameras with fisheye lenses for comprehensive, rapid, and multi-band sky assessments amidst challenges posed by artificial lighting.

Contribution

It provides an overview of existing measurement techniques, discusses their strengths and limitations, and recommends combining all-sky imaging with traditional sensors for accurate long-term monitoring.

Findings

Calibrated consumer cameras effectively sample the entire sky in minutes.

Single-band devices may misinterpret long-term changes due to spectral shifts.

Combining all-sky imaging with traditional sensors improves data accuracy.

Abstract

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.