The Impact of Daylight in the Built Environment

While not an electric light source, daylight is an important layer to consider in a lighting design in any space that has daylight apertures such as windows and skylights. In its simplest definition, daylighting is the use of daylight as the primary source of illumination in a space. Daylighting is also related to providing access to a view, optimizing quality of the visual environment, and reducing whole building energy consumption. When designing a lighting system, one must consider where daylight is allowed to enter the space and in what intensity, how it is controlled, and how the electric lighting and daylighting will be integrated.

Access to a view

An important concept related to daylighting is providing a view. While view windows can be potential sources of glare (that can be addressed with good design and technologies such as micro-blinds), a majority of people want them.

In windowless offices, people tend to experience stress and feelings of being cooped up, feelings that can be alleviated by providing access to a view. Jay Appleton, in The Experience of Landscape (1975), theorized that humans evolved a preference for environments that are safe locations from which they can view and survey the surrounding environment (the prospect-refuge theory). Occasionally viewing distant outside objects relaxes the eye muscles and eases discomfort. People also tend to prefer a connection with nature and a sense of time. In short, they want a view.

In a 2003 Heschong Mahone Group study of 100 workers in a call center, workers with the best possible view (and all other things being equal) processed calls 6-12 percent faster. In a related study of 200 office workers, those with ample view performed 10-25 percent better on a variety of cognitive tests versus workers with no view. Those workers with no view self-reported greater fatigue throughout the workweek. This research suggests that a view can be as valuable as having daylight in a space.

Benefits of daylight

Daylighting can impact people and spaces by providing sensory availability, connection to nature, time/weather information, full-spectrum light, modeling and an indirect component of light producing wall- and ceiling-washing effects, which can provide a more pleasant and comfortable visual environment. Many of these benefits boil down to simple mental stimulation due to moderate changes in the environment, so long as these changes are meaningful and patterned, which research indicates is beneficial to workers in monotonous, uniform office environments.

The impact can be dramatic, as indicated by numerous studies over the past 50 years. Various Heschong Mahone studies, for example, discovered an increase in sales as high as 40 percent in retail stores with skylights versus those without any daylighting, and a 21 percent improvement in learning rates (one study) and 7-18 percent higher test scores (another study) in school classrooms with daylighting.

Integration with electric lighting

Daylighting should be controlled to avoid unwanted glare and heat gain. To integrate daylight with the electric lighting, the luminaires should emit light on the same surfaces and in the same direction as the daylight apertures. If daylight is placed on high walls and ceilings, then the electric lighting system should place light on these surfaces as well. In deeper spaces where daylight does not penetrate to the rear areas, consider wall washing on the rear wall to prevent excessive contrasts. To prevent excessive contrasts between daylight apertures and surrounding wall or ceiling, consider lighter finishes and placing light on those surfaces.

Besides distribution, the light source itself should be specified with daylight in mind: Diffuse sources such as fluorescent complement daylight’s diffuse characteristics. Consider cooler (4000K to 6000K) light sources in spaces occupied mainly during the day; if cooler light sources are unwelcome, consider a neutral (3500K to 4100K) source.

Additionally, if daylight is a primary source of illumination, consider adding lighting controls that automatically dim or turn OFF the lights in response to daylight contribution to task light levels. Switching is generally considered more suitable for spaces where users do not perform critical tasks, such as lobbies and atria, while dimming is generally considered more suitable for spaces where they do, such as offices and classrooms. When daylight harvesting control will be employed, the lighting system ideally will be designed so that circuiting aligns with daylight patterns, and enable occupants to adjust or override the automatic control.