Sky Quality Measurement

Sky Quality Measurement

SQM logo

One of the greatest legacies we could leave after the 2009 IYA (International Year of Astronomy) is greater public awareness of the plight of the nocturnal environment. Understanding light pollution and the growing list of affected items will most certainly lead to increased public sensitivity, reduced energy use and habitate protection. Understanding the extent of the problem was one of the key programs the RASC initiated to support this goal.

The RASC purchased 10 SQM-L units from Unihedron Inc. and these were loaned to Centres that applied for them. The SQM-L, when operated in controlled conditions, has the potential to be highly useful research tool by documenting the amount and extent of sky glow across a region.

Night time image of earth light pollutionThe now-famous "Earth at Night" poster, produced with data through defense meteorological satellite (DMSP) imagery, is a graphic display of the global extent of light pollution. The subsequent release of data documents the growth of LP over time with resolution of a roughly a kilometer.


The ground-based measurements by the SQM-Ls add scientific and aesthetic value because thSQM metere SQM-L measurements generate real data on the light pollution "we see".

Centres can begin their own SQM-L Project by requesting a unit from the Light Pollution Abatement Committee. They are obliged to Use their sky glow readings in public presentations, where appropriate to demonstrate the extent of the LP Problem, and garner grassroots support for public LPA policies. After use you shall return the unit to the RASC or to the next user as instructed. The potential for groundbreaking understanding of light pollution consequences cannot be understated. Your active participation is vital to the success of the RASC SQM Project!

Measurement Method

Every measuring device, whether a thermometer, speedometer, or tape measure, has inherent sources of error. The same is true of the SQM unit. But the variable conditions of the sky compound the variability of the SQM reading.

The manufacturer, Unihedron, states this error is ± 5 %. For casual readings this is just fine. A simple click of the device will dispense a reading that will approximate light pollution conditions above that site.

For scientific measurements however, it is important to reduce the probability of these errors for reliable results. The easiest means for use to do this is by taking multiple samples from the same site during the same session. Haze and other aerosols affect transparency and most importantly skew SQM readings. Thus caution should be exercised when selecting an observing session.

When and Where to Take Readings:

  1. Use regularly, within the lending period.
  2. Take readings from as many sites as possible with the goal of developing a grid of readings in your region.
  3. During each session, take one final reading from a control site should be taken. The control site may be as simple as out your back door. This will ensure the reliability of the unit during the session.
  4. Take readings of clear, moonless sky
  5. It is best to avoid pointing towards the Milky Way, especially in rural dark locations.
  6. No buildings or vegetation should be within the field of view (±10 degrees, wider if using the older wide-field SQM model)
  7. Follow the SQM-L Manual.

The geographic location of your reading trial must be pinned to the dataset. Without this info your data can not be combined with the data from the other units on loan. A GPS reading along side the SQM-L would be ideal. (query online map sites that can generate latitude and longitude coordinates). Please report in the following format: ±degrees:minutes:seconds. The SQM also records temperature. Please see Unihedron for more on why the temperature reading is important.

A preferred spreadsheet form is available for data submissions. Note: there are places for ten SQM readings, a temperature reading, and one for latitude/longitude. Please consider taking at least five readings per observing site.

The preferred method to record your measurements is to submit them to the Globe at Night website.

Last modified: 
Wednesday, March 17, 2021 - 6:23pm