Sky Quality Meter measurements in a colour-changing world

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 skys 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 skys radiance has been recognized, it still remains poorly characterized and quantified. The impact of the SQMs 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.

Monitoring transition: Expected night sky brightness trends in different photometric bands

“…In this paper we further generalize Sánchez de Miguel et al results [48] to the more frequent case of measurements made after the light emitted by the luminaires has interacted with the surrounding surfaces (pavements and façades) and has propagated throughout the atmosphere, undergoing absorption and scattering in the molecular and aerosol atmospheric constituents, before being detected by the observer. Our goal is to model the expected evolution of the night sky brightness, measured in different photometric channels and at different distances from the sources, along the transition process of substitution of traditional gas-discharge lamps by LEDs.…”

“…It can be expected that the detected evolution trends may depend on the specific observation channels, so the possibility of observing seemingly diverging results regarding the sign of these trends shall not be discarded. A convincing case has been made in a recent paper by Sánchez de Miguel et al [48], who have shown that, under appropriate circumstances, the scotopic and photopic luminances associated with the close-range direct-viewing of lamps in the transition from high-pressure sodium lamps (HPS) to high correlated color temperature (CCT) LEDs may show an opposite behavior.…”

Variations on a classical theme: On the formal relationship between magnitudes per square arcsecond and luminance

“…As it is evident from the above section and from previously published works [3][4], these conversion formulae must be considered only approximate, since sources with the same J V m will generally have different C V m , and consequently will give rise to different luminances, depending on their particular spectral composition.…”

“…the star Vega) or in terms of a prescribed spectral irradiance distribution, as the AB (absolute) magnitude system [3][4]. Environmental studies require the use of a wide set of specific action spectra and photometric bands [5][6][7][8][9][10][11][12], to account for the variety of physiological and behavioural processes of the different species sharing an artificially illuminated area.…”

“…The all-sky system ASTMON [15], for instance, reports angularly resolved measurements of the brightness across the celestial hemisphere above the observer, in magnitudes per square arcsecond in the Johnson-Cousins B, V, and R bands. The widely-used low-cost radiometers SQM (Unihedron, Canada) [4,[16][17][18][19] and TESS [20] use device-specific photometric bands. Other popular detectors, like the DSLR cameras, provide radiance readings in the RGB color space, from which the Johnson V magnitudes can be approximately estimated.…”