Giving it the light of day

Day lighting offers many important benefits. First, there are a variety of psychological benefits associated with human needs for connections with the outdoors. We evolved mostly under tree canopies and on open plains, almost totally immersed in the natural environment. It is not surprising, then, that we have strong needs for continuous interaction with that environment.

The second most important benefit is for interior illumination. Day lighting was necessary before inexpensive, smokeless electric lighting came along, so our architectural design traditions have provided us with extensive knowledge and many ancient strategies for introducing natural light into structures. Finally, using natural light to provide illumination saves energy by diminishing the use of electricity for electric lighting while lowering cooling loads.

Technology

It is difficult to beat a conventional window for the many benefits it provides. Proper design, of course, is critical in order to avoid problems of glare and overheating, or draftiness on cold winter nights. However, unless windows extend upward a significant distance, or are turned into roof monitors or clerestories, their benefits decrease rapidly with distance from exterior walls.


In attempts to provide good illumination deeper into a living or working space, scientists have designed and constructed various systems that combine glazing with reflectors (see Figure 1). As the sun moves through the sky, however, passive versions of this strategy have difficulty working well all day long. When the window is south facing, active tracking can improve performance. Conventional roof skylights are another widely used passive day lighting strategy. The principal problems with this approach are that clear skylights transmit direct beam daylight that is hardly impeded. This can result in localized overheating and glare from the very bright sunbeams directly or from the light reflected from surfaces they strike inside the room.

The use of diffusing glazing can soften the glare somewhat, but this kind of glazing rejects much of the incident light, due to reflection and scattering,resulting in a ceiling that alternates between quite bright areas and quite dark areas. The use of deep wells below the skylight glazing can help ameliorate glare problems but the deep wells further reduce light transmission.

Tubular skylights composed of clear glazing on top, a highly reflective light pipe from roof to ceiling, and a diffusing glazing at the ceiling height can solve many of these problems,but not the hotand-cold spot issue. Further, both kinds of skylight result in substantially reduced light levels when the sun is low in the sky—during mornings and late afternoons.

Some reflecting and refracting elements have been added to the top domes of tubular day lighting devices in an attempt to catch and use low sun rays,but the effectiveness of these approaches has proved poor so far. When placed on east and west sides of the light tubes, the reflecting and refracting devices generally block about as much light as they capture.

Many active day lighting systems developed in the past have failed because the systems were overly complex, made of high-cost components, lacked durability, or because low energy costs have reduced people’s incentive to save electric light energy.

A number of recent developments offer renewed opportunities for reliable active day lighting designs of broad applicability and effectiveness. For example, owing to developments in modern electronics and the emergence of inexpensive computer-controlled mechanisms for the robotics industry, active control mechanisms can be simple, reliable, and relatively inexpensive. In addition, several new,high-reflectivity optical materials are available at modest prices, mainly for  advanced luminaire design in electric lighting applications,but equally applicable for day lighting.

Furthermore, advances over the last couple of decades in our understanding of non-imaging optical systems for illumination have opened the way for controlled beam day lighting systems that can be both cost effective and glare free. Finally, as world oil production peaks and then declines, while large developing countries demand more petroleum for their growing industrial base and transportation uses, energy prices will soar over the coming decade and over-dependence on coal will exact high environmental costs.

Thus, opting for a well planned day lighting system will be cost-effective and the environmental benefits will be lasting.

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