@MCG's answer mentions several methods to classify night sky quality or brightness, and goes on to say:

Additionally, you could always purchase a SQM (Sky quality meter) which is a small, portable battery powered device that you take out to your dark sky site, point it at the zenith, and the screen will display the darkness of the sky. Do beware that the SQM measures in 'magnitudes per square arcsecond' which means the higher the number, the darker the sky.

If I wanted to try to make a diy SQM, I'd need to understand if there was some established standards or practices associated with a sky quality measurement.

I might try to use a low dark current silicon photodiode in photoconductive or photovoltaic mode, low frequency couple it to a low noise op-amp, then do a cover-on measurement to zero out the leakage then do a over-off measurement to quantify the sky brightness.

  • What solid angle should it integrate over? Certainly more than one arcsecond, but would it be one square degree, or a 10 degree wide circle for example?

  • Silicon covers the visual spectrum and some near infrared as well. Depending on the protective materials over it, it might measure all the way out to a micron. Is that a problem, or is light pollution pretty much in the visible part of the spectrum (for obvious reasons) and I wouldn't need to hunt for an IR-blocking filter like the ones found on imagers for cameras.

  • With a single broadband (rather than spectrally resolved) measurement of photocurrent, I could choose a representative wavelength, or assume a flat 400-700 nm emission spectrum, or even make an educated guess about what the spectrum of noise pollution looks like, then use that along with an estimate of say 0.8 e-h pair per photon (roughly the shape of the sensitivity curve below) I could make some estimate on how much light was reaching the photodetector. But how might I convert that to magnitude? I understand the logarithmic scale, but right now I just start with 1300 W/m^2 is magnitude -27 (roughly the Sun's characteristic) as a reference. Is there anything better?

  • Anything else I haven't thought of?

silicon photodiode spectral response

above: Silicon photodetector typical spectral response, from here.

  • $\begingroup$ You could get an estimate by pointing a digital camera at the sky, set to say F-12, and see how long it thinks the exposure should be. $\endgroup$ – Wayfaring Stranger Aug 10 '18 at 15:13
  • $\begingroup$ @WayfaringStranger Do you really mean "f/12"? Have cameras gotten so good now that the exposure system can "see" the light difference between a rural sky and a truly dark sky? That's amazing! $\endgroup$ – uhoh Aug 12 '18 at 1:20
  • $\begingroup$ This might be better asked on EE.SE. I don't think you will be able to build this without the use of a microcontroller. The photodiode is a decent idea, but it may cost a bit to get one good enough. The op amp will be expensive too as you would have to get one that is extremely low noise/low offset and you would also have to find a way of calibrating out the rest of the offset, which would be the hardest part as you would have to use a known value to calibrate with. There would be quite a bit in the electronics of this. $\endgroup$ – MCG Aug 13 '18 at 14:00
  • $\begingroup$ Also, most photodiodes will be measuring darkness in the nA range, which makes the measurements even more difficult. Then there is temperature, tolerance of components etc. It would be much easier to just buy one as I can guarantee making a cheap DIY one will give you poor results, and be extremely unreliable unless you spend more money than it would cost to buy one (as it will have to be tested, and a one off build is always more expensive than a production line build) $\endgroup$ – MCG Aug 13 '18 at 14:05
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    $\begingroup$ People can feel free to use my link to post an answer! Unfortunately, that was really all I could find on the subject of the internal workings of an SQM. If I find any more in the future, I will turn it into a proper answer, or if someone else finds more info, they can use my link to add to it! $\endgroup$ – MCG Aug 15 '18 at 7:38

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