User Preferences in Office Lighting: A Case Study Comparing LED and T5 Lighting

Abstract

Lighting quality parameters were studied in an office lighting setting for three different luminaire types: 1) square LED panel luminaire (Sq_LED); 2) round LED downlight luminaire (Ro_LED); and 3) rectangular recessed T5 fluorescent lamp luminaire (Re_T5). The lighting conditions were evaluated with subjective assessments and horizontal desktop illuminance measurement in the middle of the task area. Re_T5 lighting was compared to Sq_LED lighting at 300, 600 and 1000 lx. Ro_LED lighting was studied at three different color temperatures: 3000, 4500 and 6000 K. Subjects responded to all nine conditions using identical questionnaires, which contained questions regarding glare, reading task ease, detail distinction from the wall, color naturalness and pleasantness, illuminance level sufficiency and pleasantness, general pleasantness of the lighting, naturalness of the shadows, and the appearance of the luminaires. Subjects also set three lighting conditions: 1) minimum illuminance for brief duration office work using Re_T5 lighting; 2) optimal illuminance for long-time office work using Re_T5 lighting; and 3) optimal illuminance and color temperature using Ro_LED lighting. The illuminance in the test room was preset at 300 lx before a subject entered the room and started these adjustment. The illuminance adjustment ranges were 0-1142 lx for the Re_T5 lighting and 0-738 lx for the Ro_LED lighting. In the CCT adjustment task CCT was preset to 3000, 4500, or 6000 K.Subjects evaluated 600 lx to be equally pleasant to 1000 lx and the reading task was evaluated to be equally easy at these two illuminance levels. However, 1000 lx caused slightly more glare. 300 lx was considered to be less pleasant and the reading and detail distinction from the wall was considered to be more difficult at 300 lx than at higher illuminance levels. At 600 lx the amount of light was considered to be more optimal than at 300 or 1000 lx. There were no significant differences in the user answers between the Re_T5 and Sq_LED lightings at equal illuminance levels. The Ro_LED lighting was considered less pleasant at 6000 K than at 3000 or 4500 K. However, CCT did not have a statistically significant effect on user answers to other questions. When the users adjusted illuminance the overall average preferred illuminance was 648 lx for Re_T5 lighting and 517 lx for the LED lighting; there were large variations in the preferred illuminances between subjects. The Re_T5 lighting had larger illuminance adjustment range than the Ro_LED lighting, which may have affected the preferred settings. The overall mean preferred CCT of the Ro_LED lighting was 4150 K. There were signs that preset CCT affected the user set CCT.