Interaction of daylight and electric light on subjective light appraisals in office environments

Background To improve human daytime functioning and provide healthy lighting regimes in office environments, better insights are required in how to provide light conditions that cater optimally to both non-image forming processes and visual performance and comfort. Controlled lab studies are suggesting that we should offer more light during daytime to users for healthy circadian entrainment and higher state alertness, as brighter light in the evening/night may impede healthy circadian entrainment. However, providing pleasant and bright light conditions has proved to be quite challenging. The challenge is even bigger when we consider that most office workers work in environments where there is also a contribution of daylight, resulting in light patterns that change substantially over the day and over seasons. Objective A field study was conducted in a real office environment, with the objective to explore the relationship between subjective light appraisals and objective light conditions. To allow for meaningful variations in both daylight and electric light, longitudinal measurements were performed across the day, across seasons, and under both standard and bright electric lighting. Method The field study was performed twice, over three weeks in summer and three in winter, in a Dutch office environment. The study was approved by the local ethical committee. In summer 12 office workers (1 female, mean age 45, SD = 10.06, range 25 to 55) participated; in winter 11 (1 female, mean age 43, SD = 10.59, range 25 to 55). Each of the three weeks had a unique electric lighting regime. In the first week, participants received extra bright light (average electrical desktop luminance 110 cd/m2) in the morning, standard lighting (average electrical desktop luminance 15 cd/m2) in the afternoon, in the second week only standard lighting, and in the third week they received extra bright light in the afternoon. Ecological momentary assessment was employed, combining frequent experience sampling (ES, self-report) with continuous measurements using ambulant and office-bound sensors. The ES questionnaires probed subjective light appraisals, alertness, well-being and thermal comfort, 10 times a day. Light loggers provided estimates of personal light exposure. Additionally, the lit environment was continuously measured using three image-based luminance distribution measurement devices, referred to as Bee-Eyes, attached to the ceiling with a sampling interval of ten minutes. Based on these luminance measurements relevant light performance indicators such as the desktop luminance, luminance ratio between electric light and daylight, and luminance contrasts were extracted. Results The measurements resulted in a rich dataset consisting of both subjective and objective data captured in a real office environment with seasonal and daily variations in daylight under varying levels of electric light. Statistical analyses will relate objective measures such as desktop luminance, luminance ratio between electric light and daylight, and luminance contrast in the periphery to subjective appraisals of pleasantness, brightness, and color. Discussion This field study can provide relevant insights for designing light interventions applicable to other office situations. The combination of both objective and subjective measures allows for a deeper understanding of the relationship between actual light exposure and the potential interaction between NIF and IF effects