The benefits of daylight through windows

Abstract

The use of daylight as the primary light source in buildings is of interest to those concerned with energy conservation because it is assumed to minimize the use of electricity for lighting. However, it is difficult to justify the cost of extensive daylighting on the basis of energy savings alone. Rather, to justify the widespread use of daylight in buildings it is necessary to demonstrate that such use has a beneficial financial impact for the organization owning and/or occupying the building. This literature review considers the impact of daylight on human performance and workplace productivity; human health; and financial return on investment. These impacts of daylight are reviewed for buildings that are used for work and for which daylighting has been extensively studied, namely offices, schools, hospitals, and retail stores. Daylight in housing is not considered. This literature review examines the benefits and problems of both daylight, as light, and windows, as the most commonly used method to deliver daylight. From this literature review, a research agenda is developed. The following conclusions are drawn from the literature review: 1. Physically, daylight is just another source of electromagnetic radiation in the visible range. Electric light sources can be constructed to closely match a spectrum of daylight, but none have been made that mimic the variation in light spectrum that occurs with daylight at different times, in different seasons, and under different weather conditions. 2. Physiologically, daylight is an effective stimulant to the human visual system and the human circadian system. 3. Psychologically, daylight and a view are much desired. 2 4. The performance of tasks limited by visibility is determined by the stimuli the task presents to the visual system and the operating state of that system. Daylight is not inherently better than electric light in determining either of these factors. However, daylight does have a greater probability of maximizing visual performance than most forms of electric lighting because it tends to be delivered in large amounts with a spectrum that ensures excellent color rendering. 5. There can be no guarantee that daylight will always be successful in maximizing visual performance. Daylight can cause visual discomfort through glare and distraction, and it can diminish the stimuli the task presents to the visual system by producing veiling reflections or by shadows. The effectiveness of daylight for visual performance will depend on how it is delivered. The same conclusion applies to electric lighting 6. People will take action to reduce or eliminate daylight if it causes discomfort or increases task difficulty. 7. The performance of both visual and non-visual tasks will be affected by disruption of the human circadian system. A disrupted circadian system will also create long-term health problems. Exposure to bright light during the day and little or no light at night will accurately entrain the circadian system. Daylighting is an attractive way to deliver bright light during the day. 8. Different lighting conditions can change the mood of occupants of a building. However, there is no simple recipe for what lighting conditions produce the most positive mood. Windows are strongly favored in work places for the daylight they deliver and the view out they provide, as long as they do not cause visual or thermal discomfort or a loss of privacy. Whether windows will produce an improvement in mood seems to depend on what the individual’s preferences and expectations are. For people who prefer daylight but who have become accustomed to little daylight, moving into a well daylighted space can be expected to lead to an improvement in mood that will diminish over time as new expectations are established. For people who prefer daylight and who are accustomed to a lot of daylight, moving into a space with little daylight is likely to lead to a deterioration in mood that will recover over time. 9. The understanding of how mood influences productivity is weak. Different studies have emphasized worker happiness, well-being, and job satisfaction as predictors of productivity while others have suggested that productivity is itself a generator of feelings of happiness, well-being, and job satisfaction. The basic problem for daylighting is that mood is subject to so many influences that unless the lighting is really uncomfortable, its influence is likely to be overshadowed by many other factors. 10. Exposure to daylight can have both positive and negative effects on health. 3