Re: Sky Quality Meter - Light Pollution Map Correlation

[Jan Hollan <jhollan@amper....muni.cz>]

> I am trying to correlate the Sky Quality Meter to the Light Pollution
> Map Colors.  The end result is an inexpensive method for determining a
> criteria of at what level a park should be considered in compliance for
> "bringing back the Milky Way".  We were looking at a goal of Sky
> Brightness not to exceed 30% natural sky brightness at zenith.  While
> perhaps simplified, I think this is a cost effective method for state
> parks to monitor light pollution.  I am estimating (Natural
> Background=250 S10 Units) it should be around 325.  With a reading of
> around 20.8 ~ 20.9 on the SQM.

Not exactly. 21.40 is a limiting acceptable SQM reading in such a case.

325 is the sum of 250 (natural) and 75 (pollution). So the "acceptable"
SQM readings are down to 21.40 (faintness of a one square second / 1 mag).
Reading of 20.8 would mean that the polluting component itself (added
light) corresponds to "325 S10 level", being larger than the natural
component!

I've recomputed the table to adhere to the chosen 250 S10 natural level a
bit better, and added the proper "public" unit of measurement, an SI one:
candelas per square metre (or millicandelas, in this case of night
environment). The corresponding quantity is called luminance.

In everynight work, SQM scale has still some advantage, being logarithmic
as our perception is. Thirty per cent more light translate easily to 0.3
mag lower readings. So instead of a natural sky of 21.70, you get 21.40 by
artificially adding 30 per cent of the natural amount of light.

Natural sky is often brighter than an equivalent of 250 stars of 10 mag
within one square degree, corresponding to a dream reading of 21.78. A
reading like 21.60 may represent still a calm natural sky, esp. during
times of large solar activity. However, even then a reading of 21.40 is
the proper limit to consider a park not-so-much polluted: to see the
splendour of the heavens well even in such times of large natural airglow,
the man-made addition should be rather 20 per cent than 30 per cent.

> TABLE II EQUIVALENT MEASURES OF THE SKY BRIGHTNESS

 slightly reformatted and supplemented (hundredths of mag are given
just there, where they are below 9 or over 1):

    	Light Pollution	             Sum of Pollution and Natural Light

		Luminance   Equivalent Magnitude of     Luminance
    S10 units  / 1 mcd/m2     1 arcsec2        1 deg2   / 1 mcd/m2
      (i.e., No Nat. Background)     (i.e., with 250 S10 Background)
       0	0	  --		21.78    4.0    0.21
       20	0.017	  24.5		21.7     3.9    0.227
       50	0.042	  23.5		21.58    3.8    0.252
       100	0.084	  22.8		21.4     3.63   0.294
       200	0.168	  22.0		21.14    3.36   0.378
       320	0.268	  21.5		20.88    3.1    0.479
       500	0.42	  21.0		20.6     2.8    0.63

Last but not least: a reading of 21.40 or better (21.50, ...) should be
obtained from a place where no trees, building, rocks are around, ideally
at the top of the hill. With a couple of obstacles screening the sky close
to horizon, which is always brighter than that in zenith, less light is
captured by the SQM. Even a rather polluted sky might produce a reading
like 21.7 in such unfavourable circumstances.

cheers,
 jenik