Evening office lighting – visual comfort vs. energy efficiency vs. performance?

During the study presented in this article, we compared two highly energy-efficient lighting scenarios for evening office lighting (i.e. electric lighting that is typically used for approxiamately 2 h in the evening). The first of these lighting scenarios (referred to as “Reference”-scenario, Lighting Power Density or LPD of 4.5 W/m2) has been successfully in use in many office rooms of the Solar Energy and Building Physics Laboratory’s experimental building, located on the campus of the Swiss Federal Institute of Technology in Lausanne, for several years. The second lighting scenario (referred to as “Test”-scenario, Lighting Power Density of 3.9 W/m2) is more energy-efficient, creates higher workplane illuminances but leads to an increased risk of discomfort glare. The aim of this study was to meticulously compare the two lighting scenarios in order to find a lighting solution for evening office lighting that offers an optimal trade-off between energy-efficiency, visual comfort and visual performance.For this purpose, objective visual performance tests (computer-based and paper-based) and subjective visual comfort assessments with 20 human subjects were carried out. The main hypothesis of our study was that the study participants would not perform worse under the more energy-efficient “Test” -scenario than under the “Reference”-scenario (which is extremely well accepted by the buidling’s occupants).We found that the two tested scenarios are comparable to usual lighting scenarios in other office rooms in terms of subjective visual comfort. The study participants preferred the “Test”-scenario to the “Reference”-scenario. Their performance in a paper-based task was significantly better under the “Test”-scenario than under the “Reference”-scenario. No significant differences in the performance during two computer-based tasks were found. We conclude that energy-efficient lighting with Lighting Power Densities of less than 5 W/m2 is already achievable in today’s office rooms without jeopardizing visual comfort and performance. Less powerfull electric lighting systems do not necessarily mean a decrease in visual comfort and/or performance; our results even show that better visual comfort and better visual performance can be achieved with less connected lighting power.