The second pipe was exposed to global illuminance from the sun and sky.
The values of beam sunlight illuminance and irradiance, diffuse and global illuminance and irradiance, and zenith luminance are measured from equipment in a station located near the experimental site.
Global illuminance and global irradiance as well as total illuminance and total irradiance in each cardinal direction are measured by individual sensors.
The figure also shows plot of sky ratio, the ratio of diffuse to global illuminance.
g] = relative error in the measurement of the outdoor global illuminance
glob] = outdoor horizontal global illuminance (measured)
First, the outdoor diffuse and global illuminance measurements, which were measured by the weather station system and used in the simulation, were not completely synchronized with the measurements of the indoor illuminances (time delay or advance was on the order of one to three minutes).
Especially during dynamic days besides the seasonal/monthly and daily variability are occurring frequently shorter subhourly distinct changes in global illuminance levels.
Note, that at the same time the outdoor global illuminance in klux is valid independently, as indicated under Fig.