Used units
The surface brightness of the night sky, denoted as Sa, measured by the SQM is given in the commonly used astronomical units of magnitudes per square arc second (mag/arcsec2). It is a derivative of the magnitude scale (mag, magnitude, m) defining the visual impact of the star's brightness as a point light source. Magnitude scale is a logarithmic, relative and reverse one, in which a star of magnitude 0 is 100 times brighter than a star of magnitude 5. The mag/arcsec2 scale determines the surface brightness of diffuse astronomical objects such as nebulae, galaxies, comets, or just a background sky. The unit of surface brightness in the SI system is candela per square meter (cd/m2). The relationship between these quantities is described by the following formula:
Because of the very low brightness of the night sky the commonly used unit is milicandela per square meter (mcd/m2). Due to the reference to other publications regarding this problem as well as readings from SQMs, we use mainly the scale [mag/arcsec2], giving also the appropriate values in the scale [mcd/m2].