Data & Analysis
Tubular Skylights:

These graphs show the contribution of the light tubes for the two conditions, with all other light sources blocked. The data shows the limited distribution of the light from the tubes. Only the north part of the room (the top of diagram) is lit at all by the tubes, reaching a high at 13 footcandles. On a cloudy day the tubes produce very low illuminance levels only providing 5 footcandles.