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.