Preferred Illuminance at Different Correlated Color Temperatures for Paper- and Tablet-Based Reading Tasks in an Office Environment

To understand the range of freedom that can be used for adjusting lighting levels with smart control algorithms in offices, we performed a laboratory study aimed to measure the preferred illuminance at different correlated color temperatures (CCTs) when performing reading tasks on paper and tablet. The experiment used a full-factorial three-by-two (i.e., 3 CCT × 2 task medium) within-subject design. Thirty-two participants could either decrease or increase the output of the ceiling luminaires to a preferred illuminance for each reading medium. In addition, three repetitions were included to understand how consistent participants were when indicating their preferred illuminance. The results show that both CCT and task medium have a significant effect on the preferred illuminance. We found a higher preferred illuminance for reading on paper than for reading on a tablet, and a higher illuminance for cool-white than for warm-white light. The range of spread in preferred illuminance was (averaged over all participants) about 250 lx, and larger than the difference in preferred illuminance between task medium and CCT.