Detectability and Acceptability of Illuminance Reduction for Load Shedding

Abstract

Reducing the peak demand for lighting energy through load shedding can be an effective method of load management. However, before such a load-shed technique is applied to the real world, it is important to understand occupants' requirements for the change in illuminance. Two laboratory studies were conducted to better understand occupants' sensitivity and acceptance to dimming-memory study and dimming curve study. The memory study addressed whether one can memorize an initial illuminance level and how long this memory is sustained. The dimming curve study investigated the effects of dimming function on detectability and acceptability of illuminance reduction. Both experiments used a windowless private office illuminated by three dimmable direct/indirect pendant luminaires. After adapting to an initial illuminance of 500 lx, subjects closed their eyes in the memory study while the lighting was dimmed to various illuminance levels. Subjects conducted a paper task or no task (free gaze) in the dimming curve study. The dimming speed and the dimming function varied in the memory study and the dimming curve study respectively. Subjects indicated whether the lighting level had increased, decreased, or remained unchanged and whether the illuminance change was acceptable. Within the range of the experimental conditions, the memory study and the dimming curve study, concluded: 1. Regardless of task, 50 percent of the subjects could detect the change in illuminance after about a 15 percent reduction from the initial illuminance. 2. Regardless of task, 50 and 80 percent of the subjects accepted illuminance reductions up to about 40 and 20 percent respectively. 3. Dimming curvature has no effect on detectability and acceptability of illuminance reduction at slow rates of change.