Few hours ago I measured the night sky quality at my home roof at Shatin. Here is the brief result.
Date/time: 2008 June 20 10~12 pm, ambient temperature 29 C.
Need patience to wait for cloudless during moment of measurement.
------- By “less precise method” -------
Canon 350D set to AV mode, ISO 800, spot metering.
Lens: Canon 100 mm set to F2.8, focus at infinity and pointing to zenith.
Camera exposure reading: F2.8, 6 sec
The equivalent exposure at ISO 100 is F2.8 48 sec
Since 2^ EV = the square of F number / shutter speed in sec,
so the calculated EV for ISO100 = -2.6
Sky luminance (天空光度), assuming ISO100
= 2^ (EV-3) = 2^-5.6 = 0.021 cd per sq m (candela per sq meter)
------- By Sky Quality Meter -------
Meter reading = 16.5 mag per square arcsec (averaged from 3 consecutive measurements)
Sky luminance = 10.8 x 10^4 * 10^ (-0.4 * 16.5) = 0.027 cd per sq m.
The above two methods look “reasonably consistent”. But I am not too sure my formulae were applied correctly or not. Will check again later.
Anyway I think the absolute value of sky luminance is not important. The importance is the CHANGE of meter reading (sky brightness) before and after light pollution.
Alan |