# Sky quality meter

A sky quality meter (SQM) is an instrument used to measure the luminance of the night sky, more specifically the Night Sky Brightness (NSB) at the zenith, with a bandwidth ranging from 390 nm to 600 nm. It is used, typically by amateur astronomers, to quantify the skyglow aspect of light pollution and uses units of "magnitudes per square arcsecond" favoured by astronomers.

## Structure and design

The SQM is equipped with a silicon photodiode functioning as detector which is partially covered by a rejection filter for the near-infrared wavelength. The system has a high response to wavelengths up to the near-infrared (from 350 nm to 2500 nm), thanks to a converter from light to frequency. This structure tries to mimic the human eye spectral response under the photopic regime.
A final spectral response is provided by the combination of the photodiode and the transmission near-infrared cut-off filter. This response overlaps the Johnson B and V bands, well known in astronomical photometry, in the wavelength range between 320 nm and 720 nm, which includes visible light spectrum.
Beyond amateur astronomers, the SQM photometers have become very popular among researchers from different fields of study, including associations involved in fighting light pollution. The instrument has a systematic uncertainty which is quoted of 10% (0.1 mag arcsec−2). The aspect of uncertainty is also related to the stability of these radiometers: the variation of the instrument behaviour (mainly due to sensor ageing, the influence of the air temperature and atmospheric conditions and internal temperature) could be confused with changes of the sky brightness, especially when NSB tracking is performed over a long time interval.

## Models and Production

There are several models of SQM made, offering different fields of view (i.e. measuring different angular areas on the sky), and various automatic measurement and data logging or data communication capabilities. The current versions has only one band of observation, that can produce misinterpretations if the light pollution changes from sodium-vapor lamp to LED.
The SQM-L, or "Sky Quality Meter - L," is a model with an additional integrated lens, offering a narrower measurement range of 20° compared to the 84° range of the standard SQM model.
The SQM is produced by the Canadian company Unihedron in Grimsby, Ontario.

## Scale

The values reported by the SQM are in units of magnitudes per square arcsecond (mag arcsec$^{−2}$). Typically, the data provided by SQMs are recorded in magnitudes, denoted as m or mag, specifically in mSQM (or magSQM), where the subscript SQM indicates that the measured radiance is calculated by weighting the electromagnetic radiation according to the spectral responsivity of these instruments. As astronomical magnitudes are a negative logarithmic scale, smaller values indicate a brighter sky and a difference of 5 mag arcsec$^{−2}$ corresponds to a difference in luminance of 100 times. Typical values will range between around 16 for bright urban skies to 22 for the darkest skies on Earth.

## Limits and considerations

SQM response can be influenced by ambient temperature variations, so it is important to verify these effects. Since SQMs are not waterproof, they must be protected from moisture using a housing, which is generally provided by the manufacturer. This housing protects the device but also traps heat generated during operation, which is minimal for USB models (SQM-LU) but more significant for Ethernet models (SQM-LE).
In urban environments, SQMs frequently record large variations in radiance due to the presence or absence of clouds. Radiance measurements taken by SQM-LU devices are stable within the temperature range of −15 °C to 35 °C, with variations smaller than the 10% systematic uncertainty stated by the manufacturer.

## Citizen science

SQM measurements can be submitted to a database on the manufacturer's website and to the citizen science project GLOBE at Night.