Exploring Preferred Correlated Color Temperature in Outdoor Environments Using a Smart Solid-State Light Engine

The existence of regions of correlated color temperature (CCT) and illuminance at which the illumination is considered "pleasing" (Kruithof hypothesis) has been tested in several outdoor environments using a smart tetrachromatic solid-state light engine with tunable CCT and high-fidelity color rendition. Subjects from a culturally homogeneous group were asked to find the most "pleasing" illumination conditions at the set illuminance levels of 5 lx and 50 lx by performing a CCT adjustment task within a wide range of 1850–10,000 K at a fixed initial condition of 1900 K. The selected intervals or preferred CCT were found to shift to higher values with increased illuminance almost independently of the content of the viewed scene with statistical significance, which qualitatively is in agreement with the Kruithof hypothesis. However, the intervals of CCT for pleasing illumination were found to be shifted to a broader and higher "warm white" range than those predicted by Kruithof. In addition, subjects were asked to find the most pleasing illumination conditions when the adjustment of CCT was accompanied by instantaneous dimming in order to maintain constant circadian irradiance. At low and high set circadian irradiance levels corresponding to 5 lx and 50 lx illuminance at the CCT of 1900 K, subjects preferred moderate dimming with lower CCTs and more noticeable dimming without the reduction of CCT, respectively.