The distribution of daylight with a solar tube: a monitoring campaign in Italy

Solar tube systems capture daylight and channel it into interior spaces, but their performances vary in relation to the year period, the number of sunny days and the geographical latitude. The presented research tests solar tube impact on indoor lighting during not favourable conditions at an intermediate latitude (Milan, 45°N) room. A monitoring of the luminance and illuminance is conducted from October to February on days of Clear or Overcast Sky (CS or OS, respectively). While horizontal illuminance (EH) is monitored continuously, measurements of vertical illuminance (EV) and of surface Luminance (L) from HDR images are taken in the room centre at a specific time. Starting from illuminance data daylight metrics DA and Uniformity Ratio (UR) are calculated while mean luminance and Contrast Ratio (CR) related to sectors/elements of the room (ceiling, walls, HVAC cabinet, floor) are evaluated. As a result, 12 days of monitoring and 96 HDR luminance images are collected. The main findings are: 1) EV values are low but sensible to the height of the measurements during CS days; 2) EH is significantly affected by the distance from the solar tube, during CS days leading high DA variations and low UR values; 3) L is 30% lower in OS days than CS days; 4) CR is high (>10) only in 7% of cases in OS days and up to 28% during CS days. In conclusion, the study demonstrates a slight but tangible impact on lighting environment even in low lighting year period that can be further investigated.