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Some time ago I was experimenting with
the use of some low leakage photovoltaic solar cells, to harvest some energy from the
diffuse lighting inside a room. The plan was to use a small array of solar cells
(the type often used on solar powered calculators) to keep a super capacitor charged,
drawing power from the light within an inside room. Our human eyes are a poor gage
of the intensity of the light within a room, so a more accurate instrument was needed to
determine how much power was available. I blew the dust off an old 1-milliamp
meter and attached a one centimeter square PIN photodiode to it. This allowed me
to quickly measure how much current a solar cell array could generate anywhere there was
light. I was surprised by just how much useful power there was, especially when a
room was brightly illuminated by sunlight reflecting off walls and ceilings.
Often, the current exceeded 200 microamps per square centimeter. Of course in direct
sunlight, the current exceeded 8 milliamps for a 1cm x 1cm solar cell.
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