Lighting quality in interior spaces is not determined solely by horizontal illuminance at the workplace, but to a large extent by the spatial distribution of light, in particular by the luminance of ceilings and walls. Building on classical principles of lighting technology and visual perception, this article examines the influence of the ratio of indirect to direct lighting on the perception of room brightness, the spatial impression, and overall preference. To this end, two complementary studies were conducted: a visual assessment of realistic room simulations and a user study in a real meeting room with variable illuminance levels and systematically varied proportions of indirect and direct lighting. The results consistently show that perceived room brightness and user preference correlate much more strongly with the illumination of ceilings and walls than with the horizontally measured illuminance on the table, which was kept constant. A balanced ratio of indirect to direct light—typically in the range of approximately 35% to 65% indirect lighting—is preferred by users, whereas predominantly direct or nearly purely indirect lighting is associated with lower acceptance. The study clearly demonstrates that existing standards, which primarily focus on horizontal illuminance, neglect essential aspects of lighting quality. The findings highlight the need to systematically integrate light distribution, vertical illuminance, and spatial–psychological effects into lighting design, evaluation, and standardization in order to achieve visually comfortable, widely accepted, and spatially appropriate lighting solutions.
Khanh et al. (Thu,) studied this question.