The experimental illuminance data consist of two data sets. The first data set consists of mean
hourly values of global illuminance measured on a horizontal surface and on vertical surfaces
facing north, east, south and west during the period June 1994–July 1995. The second data set
includes 15 min mean hourly values of global illuminance measured on a horizontal surface and
on 16 surfaces of different orientations and slopes during the period January 2000–June 2000.