Pitch Black Meter
or Internet keywords:
Dark Sky Meter
Light Pollution Meter
Sky Quality Meter
The
PBM ( Pitch Black Meter ) is designed to
put numbers on the sky quality, e.g. the darkness. You aim the PBM to a selected part of
the sky and take a reading by comparing the sky with a local lightsource (LED). You adjust the current through the LED in
order to make it as bright as the surrounding sky. This value is what I let represent the
sky quality. A green glassfilter equivalizes the colourtemperature of the sky and the
local lightsource. The principle is like a balance. In one bowl you put the lightintensity of the sky and in the other you put the lightintensity of the local LED. The only thing that matters is your ability to judge when the intensities are equal. No problem at all, with a normal eye. It doesn't matter how keensighted you are, or your subjective feeling of a black night. The intrinsic properties of the PBM eliminate all personal influences. The DVM is a common digital multimeter able to measure 0 - 200 microamps. The power supply is a homebrewed one, delivering the current proper (DC) to the LED (Light Emitting Diode). In the view end, you have an option to add a 90 degree prism
(or mirror) to avoid strain in your neck whilst measuring the sky around zenith. One design feature not resolved as yet, is a calibration-procedure to make values of different PBMs possible to compare. This device is NOT supposed to produce any "scientific" data. Its sole purpose is to play around with, during fieldtrips doing amateurastronomy at different sites.
The bar chart above shows the "pitch-blackness" at an observing site some 35 km east of Stockholm. Visually the sky was good during the night. This gadget has been presented in the Februari 2001 issue of Sky & Telescope.
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