[SQM] SQM unit vs luminance meter

James Ronback jim_ronback at dccnet.com
Sun Feb 27 08:02:51 UTC 2011

Hi Dan,

NASA reports that roughly two-thirds of the world's population can no
longer look upwards at night and see the Milky Way.
http://science.nasa.gov/science-news/science-at-nasa/2001/ast01nov_1/ .

When examining a new development it may be onerous to predict what
impact it may have in increasing local light pollution, light trespass,
glare and deleterious impacts on the environment. There are all sorts
of guidelines on how to mitigate these undesirable effects. So to be
able to control light pollution you need to measure and monitor it, and
establish criteria to control and mitigate it.

To compare the different luminance meters the key questions are:
1) For the luminance meter
          a) what is the angular response of the device  measuring
       ( http://unihedron.com/projects/darksky/images/fovcurves.jpg ) and
         b) its sensitivity in the visual spectrum?

But in addition.  you should also consider:
2) Do the proposed light fixtures have the IDA Dark Sky seal of
approval and are their light levels, spectrum and on/ off periods dark
sky friendly?

3) Will the development achieve LEED 3 credit that meets five separate
sets of criteria?
http://apus-green-building.com/tag/leed/page/3/ .

    * ANSI/ASHRAE/IESNA Standard 90.1 for exterior lighting power
    * Standard 90.1 exterior lighting controls.
    * Interior lighting.
    * Light trespass.
    * Horizontal light cut off.

The human eye is very sensitive instruction. A dark adapted eye using
its rods, as opposed to cones, can see a candle 30 kilometers away.
Thus you need an instrument with comparable sensitivity.

Typical luminance meters used for photography usually have a very wide
angle of response through a bulbous diffuser and are not sensitive
enough to accurately measure above the horizontal plane the night sky
glow or the quality of the starlight overhead at different levels of
light pollution. They are geared to measure much brighter levels of
luminance and even when it registers near zero for the ambient
luminance, the brightness of an offending distant source causing light
trespass may still be a nuisance and disruptive to one's enjoyment of
the night sky.

A spot photometer could be used provided it is sensitive enough. It may
have too narrow an angular response, e.g., 1 degree, so it may require
many more measurements in different directions overhead to average out
bright and dim stars so that the results are similar to what can
measure with an SQM  whose Full Width Half Maximum (FWHM) is  ~20°. The
sensitivity to a point source ~19° off-axis is a factor of 10 lower
than on-axis.

To compare two measuring devices, one can convert the luminance values
from visual mags/arcsecond² to cd/m² .
[value in cd/m^2 ] 	= 	10.8×10^4 × 10^(-0.4*[value in mag/arcsec^2 ])

23.00 mag/arcsec² 	= 	6.814×10^-5 cd/m²

15.00 mag/arcsec² 	= 	1.080×10^-1 cd/m²


I just measured the night sky tonight with my SQM at 10 pm and it
registered 15 mags/arcsecond². This due to having the Delta, BC,
residential and street lights bouncing off the snow onto an overcast
sky. So it is important to make the overhead  measurements on a clear
dark moonless nights, with and without snow on the ground.

This suggests any measuring device should be able to measure night sky
brightness/ luminance to less than 6.8 x !0^-5 candelas per square
meter. I strongly suggest that using inexpensive SQMs allows you and
the lighting engineer to establish a set of baseline measurements to
compare the results with many other villages, towns, cities and dark
sky reserves around the world. These comparisons would make a more
convincing argument to the regional council and indicate how good a
dark sky friendly and energy saving community can be.

If you do not initially have a luminance meter you can effectively
measure the night sky using following the "Globe at Night" charts. They
also collect SQM measurements around the world.


cal-SPOT 401 Calibrated Spot Photometer

See also:

Yours under beautiful dark night skies,

Jim Ronback, P. Eng. (retired)

On 2010-11-30 2:01 AM, Dan Taylor wrote:
> Hello,
> I have been asked by a local conservation group, to present an argument for 
> lighting restrictions at a proposed development, that will go before a 
> regional municipal board.
> At issue is the sky conditions at the site. In short, developer proponents 
> have hired a lighting engineer who has used a luminance meter to form the 
> basis of his position.
> Are there any documents available that explain the differences and 
> similarities between a SQM unit and a luminance meter and why a SQM unit is 
> better suited to measure sky brightness?
> Dan
> _______________________________________________
> sqm mailing list
> sqm at unihedron.com
> http://unihedron.com/cgi-bin/mailman/listinfo/sqm
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