Sky Quality Meter measurements in a colour-changing world

A Sánchez De Miguel,J Zamorano,J Roby,C Tapia,M Kocifaj,M Aubé

doi:10.1093/mnras/stx145

Abstract

The Sky Quality Meter (SQM) has become the most common device to track the evolution of the brightness of the sky from polluted regions to first class astronomical observatories. A vast database of SQM measurements already exists for many places in the world. Unfortunately, the SQM operates over a wide spectral band and its spectral response interacts with the sky's spectrum in a complex manner. This is why the optical signals are difficult to interpret when the data are recorded in regions with different sources of artificial light. The brightness of the night sky is linked in a complex way to ground-based light emissions while taking into account atmospheric-induced optical distortion as well as spectral transformation from the underlying ground surfaces. While the spectral modulation of the sky's radiance has been recognized, it still remains poorly characterized and quantified. The impact of the SQM's spectral characteristics on the sky brightness measurements is here analysed for different light sources, including low and high pressure sodium lamps, PC-amber and white LEDs, metal halide, and mercury lamps. We show that a routine conversion of radiance to magnitude is difficult or rather impossible because the average wavelength depends on actual atmospheric and environment conditions, the spectrum of the source, and device specific properties. We correlate SQM readings with both the Johnson astronomical photometry bands and the human system of visual perception, assuming different lighting technologies. These findings have direct implications for the processing of SQM data and for its improvement and/or remediation.

Highlights

The Sky Quality Meter (SQM) is widely used in studies related to light pollution (Falchi 2011; Pun & So 2012; Kyba et al 2012, 2015; Puschnig et al 2014)
The world is experiencing a massive transition in lighting technology, in which entire cities can change from classical high-intensity discharge (HID) lamps to white Light Emitting Diodes (LED). Under such important natural and human made changes, how can we prove that the temporal variations in Sky Brightness (SB) as detected by a broadband instrument like the SQM are representative of the variations in SB that should be detected by the human eye? how can we analyse the human made variation of the SB, excluding the natural variations of the SB? In order to provide basic quantitative tools to improve the tracking of human made or artificial SB, we will analyse the impact of each variable component on the reading of the SQM by simulating the detected signal of the SQM
Monitoring light pollution levels has become important in the last decades because of rapid industrialization and modernization, especially in densely populated regions

Summary

Introduction

The Sky Quality Meter (SQM) is widely used in studies related to light pollution (Falchi 2011; Pun & So 2012; Kyba et al 2012, 2015; Puschnig et al 2014). This change, we must have detectors with minimal spectral capabilities, like radiometers with colour filters, colour cameras, or even spectrometers. Such instrumentation is relatively expensive and cannot be spread all over the world. The SQM is very cheap and easy to use, making it a nice instrument for creating global measuring networks. They are useful to track temporal and geographical variations in Sky Brightness (SB), but without any colour information. We want to outline how the broadband spectral response of the SQM reacts to variations in the spectrum of the source for a variety of lighting technologies and for the combination of some other sources of natural origin (e.g. scattered moonlight, background stars, natural airglow)

Objectives

Findings

Discussion

Conclusion