In order to have the correct value of the average day light factor the measurement should be made at cloudy conditions, so to realize the perfect anisotropy and uniformity of the sky; in alternative it is necessary to shadow the detector so to measure only the distributed component of the sun light.

The measurements have been performed at noon when the sun was at 0 azimuth on the horizon by means of luxmeters.

According the rules the measurements inside the building should be performed at 0.90 m above the floor, corresponding to our working plane height.

Figure 3. Points location for the illumination measurements Outside the measurements has been achieved on an horizontal plane near the building facing the whole sky dome and avoiding the direct sun component.

The measurements have been referred to an area Ap and the average lighting has been measured as the average weight on the total area.

The DF is defined as :

DF = Em / Ee = (XEi x Ap) / (Ee x At)

Ei indoor point of lighting measurement [lux] Ee Outdoor measurement lighting [lux]

Ap Area of measurement [m2]

At Total area [m2]

We can notice that the DF value, though halved after the insertion of PV modules, from 17.7 % to 8.9 %, is well beyond the minimum suggested, 3-5 %. It can be concluded that with the modules we have reached a greater control on the entering light. At the same moment the overheating and the glaring effects are largely reduced.