[skysurfer]

I mean particularly in transparency, not seeing.

* NELM: is very personal. Under the same sky one person sees until +6.2 and the other to +5.5.
* SQM: is brighness in magnitudes per square arc second. Unreliable. A "perfect" dark sky (which never happens) has a luminance of zero i.e. an SQM of -infinity as magnitude is logarithm of luminance. Real examples like a fully overcast sky of very hazy sky at a very dark location render a high SQM of more than 22 while no, resp. fewer stars are visible.

The best alternative is : Take a photo of a starfield around the zenith (always the same lens (or zoom factor in case of a compact camers), aperture, ISO setting, white balance and exposure time. Compare the photo with a star chart (Stellarium, Skysafari or a paper atlas) to get the actual limiting magnitude.

Does somebody have ideas on this ?