CIRCADIAN LIGHTING | HUMAN RESPONSE Absolute difference in subjective alertness scores plotted against the relative difference in EML Variation in average EML for the LED lighting with time of day Condition 2 v Condition 1 200 0 7 12 13 Time 19 Figure 3: Condition 2, March 2018 Mean EML 400 Absolute difference in subjective alertness Mean EML 400 200 Relative difference in EML All day Morning Afternoon Figure 5a: Condition 2 (the first dynamic LED lighting condition) v Condition 1 (the original fluorescent lighting) 0 7 12 13 Time Condition 3 v Condition 4 19 in Condition1, but this also happened for the small number of people who received less light in Condition 2. The increase in alertness did not depend significantly on how much extra light people had with the LEDs. All participants received more light in Condition 3 compared with Condition 4, and the increase in light level was much more uniform across participants, compared with the first conditions. However, the higher light levels in Condition 3 did not lead to higher scores for subjective alertness; only half of the participants felt more alert under the dynamic LED lighting (Condition 3). Participants also did computer tests to assess their reaction time and concentration. There were no statistically significant differences in either between the two conditions tested in each phase of the project. Around half of the participants wore an Actiwatch device to monitor their activity and sleep patterns; there were negligible differences in sleep under the first two conditions. On average, just more than half of the participants preferred dynamic lighting for their office, typically brighter in the morning and following the variation of natural light outdoors. Around one-third preferred the constant lighting. Discussion People felt more alert under the new lighting system, but there appeared to be no correlation with how much light they received or the way in which the system was operated. This contrasts with results from other studies, although these mainly used high illuminances or very blue light. Absolute difference in subjective alertness Figure 4: Condition 3, November 2018 Relative difference in EML All day Morning Afternoon Figure 5b: Condition 3 (the second dynamic LED lighting condition) v Condition 4 (constant LED lighting) There could be various explanations for this. The experiment had a relatively small sample size. People vary in their reactions to light and to their normal exposure to light; 18 of the 23 initial participants took at least one break to go out of the building during the day, so they would have been exposed to daylight then. Nevertheless, there were some valuable findings: Light levels vary across a space. The new LED lighting appeared more uniform and all the lamps were working, but there were still big differences in the amount of light reaching different peoples eyes. Under the 6,500K (late morning) setting, EML values ranged from 142 to 413. In multi-occupant spaces with conventional ceiling lighting, it is very difficult to achieve a standard dose of light for everyone. If people are facing the walls, or near a walkway, less light will reach their eyes than people with their backs to the wall facing a brightly lit space. A balanced visual environment is important. For Conditions 1 and 2, the occupants had white desks and complained about light being reflected from them. To avoid unwanted glare in Conditions 3 and 4, they were replaced in the summer with desks in a light-oak finish, which allowed higher illuminances to be used. Effective lighting control is critical. Dynamic lighting requires effective controls that work exactly as programmed. The dynamic lighting system used for BREs study had reliable controls, and was programmed to vary slowly between settings. Overall, the participants preferred the dynamic lighting, and elected to keep it at the end of the study. CJ For references, see the online version of this article at www.cibsejournal.com D R PAUL LITTLEFAIR and DR COSMIN TICLEANU are principal lighting consultants at BREs Fire and Building Technology Group 20 June 2019 www.cibsejournal.com CIBSE Jun19 pp18-20 BRE research Supp.indd 20 24/05/2019 15:19