[R.sub.B], [R.sub.D], and [R.sub.G] are the beam, sky diffuse, and ground-reflected fraction of the global illuminance (or irradiance), respectively.
 reported global illuminance prediction RMSE of 3% and MBE of less than 1% for six American and European sites.
The selected models include the Perez model , average value model (110 lm/W for global illuminance and 120 lm/W for diffuse illuminance), and Littlefair model .
In general, the Perez model provides good performance among the models in global illuminance prediction especially in Lyon, France, although the [R.sup.2] values are not noticeably different in all the cities.
Perez model has been usually recognized as the least local-dependent luminous efficacy model but this is partly true when global illuminance is concerned.
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.
The two lines at the top are plots of transmitted global illuminance, and the two lines at the bottom are plots of beam illuminance, while the middle lines are for diffuse illuminance.
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.