# Sky Quality Meter measurements in a colour changing world

## TL;DR

This paper analyzes how the spectral response of Sky Quality Meters affects sky brightness measurements across different artificial light sources and environmental conditions, highlighting challenges in data interpretation and conversion.

## Contribution

It provides a detailed analysis of the spectral effects on SQM measurements and discusses the difficulties in converting radiance to magnitude under varying conditions.

## Key findings

- Spectral response of SQM varies with light source and environment.
- Conversion of radiance to magnitude is complex and often unreliable.
- Implications for improving SQM data processing and interpretation.

## 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.