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Illumination of Buildings using Light Pipes

The interior spaces of multi-story buildings often are not daylit, because of the distance from those spaces to the nearest exterior wall or roof, through which daylight must be admitted.

The development of efficient light pipes for transporting solar light over distances of many meters offers the possibility for providing high quality natural daylight to the interior, or core, spaces of multistory buildings. If these piped daylighting systems are designed into the buildings from the beginning, costs can be kept under control and the normal difficulties can be minimal.

For such systems to be effective, lighting relatively large areas, two essential requirements can be identified.

1. To place adequate quantities of solar flux into the light pipe, for subsequent distribution over useful areas of the building, one or more solar concentrators are needed. For adequate concentration this optical element will have a relatively narrow field of view and therefore needs to track the sun through the day. This tracking solar concentrating and collecting system is most often placed on the roof (or on an exterior wall) of the building.

2. Due to the flux concentration, the piping system must be able to transmit high solar flux levels captured and focused by the concentrator without damage to the light pipes.

Work has been done on both of these critical areas for many years and the literature is replete with a large variety of proposed solutions. Some fundamental problems remain, however, and these have so far kept these systems from seeing widespread acceptance by the building design community.

Optical Concentration

Conventional lens-type optical concentrators have been designed to produce high quality images of objects.

The goal in daylighting systems, however, is simply to deliver most of the concentrated flux into the system's light piping system rather than to produce a high quality image of the sun. Nonimaging systems can be developed using somewhat different design principles that come closer to the ideal. A search of the Internet on the topics "nonimaging concentrators" and "nonimaging optics" will reveal links to a significant number of information sources on this topic. Optical designers may be interested in the following textbooks on the subject:

1. Selected Papers on Nonimaging Optics , Roland Winston, Editor, MS 106, SPIE Press,
642 pages/101 papers, Published 1995.SPIE PRESS, P.O. Box 10, Bellingham, WA 98227-0010 USA. ISBN 0-8194-1799-8

2. High Collection Nonimaging Optics, W.T. Welford, R. Winston, Academic Press, 1989, 284 pages. ASIN: 0127428852

3. Nonimaging Optics: Maximum Efficiency Light Transfer V 21. 22 July 1999, Denver, Colorado, Roland Winston, ed. Society of Photo Optical and Instrumentation Engineers (SPIE) 236 pages. ISBN: 0819432679

For more than twenty-five years, the University of Chicago Nonimaging Optics/Solar Energy Group worked on developing novel nonimaging concentrator designs and optical elements that achieve performance thought to be impossible under the limitations of imaging optics. Led by Professor Roland Winston and Senior Scientist Joseph O'Gallagher, the group demonstrated solar concentrations more than 84,000 times the ambient intensity of sunlight. Such ultra-high flux levels exceed those found at the surface of the sun and can be used in a variety of applications other than solar illumination, which seldom needs such high levels of concentration. Simpler and less expensive optical designs are available which are more appropriate for solar lighting systems.

Minimizing losses in piping systems

It is possible to channel light down a hollow tube having a reflective metallic interior finish. Such metallic light guides date back to 1882 when scientist William Wheeler postulated that highly reflective mirrored pipes could be used to "pipe" light throughout a building from a central light source. This is an inventive idea, but unfortunately it has not been very practical because high reflectivity metallic surfaces were simply not reflective enough to deal with the large number of reflections that would occur in such a complex system. Even today, with the development of higher reflectivity coatings for metal surfaces, the dream of piping light throughout a building seems elusive. The reason is simply that the highest broadband reflectivity for coated metal is approximately 95%. After a conservative 50 reflections, the amount light available for emission is less than 8%. However, as the aspect ratio (ratio of length to diameter) drops, and the number of interactions is reduced through the culmination of input light, it is possible to create relatively efficient metallic light guides with longitudinal slits for light emission. This technique is used in a common solar lighting system called the tubular daylighting device (tubular skylights) which generally are not designed for concentration. See our section on skylights for more information on this type of product.  As new materials are developed having higher reflectivity, piped daylighting systems can become more cost-effective and practical.

With highly reflective surfaces, hollow tubes can transfer considerable quantities of flux. The optical phenomenon of total internal reflection is used in another type of light pipe, known as "fiber optics." Optical fibers are thin flexible cylinders of glass or other transparent material having the property that all rays striking their outer surface from within the cylinder are reflected without any loss—providing that the angle of incidence is greater than a value called the critical angle. Fiber optics has found widespread use in communications but is not suited for piping large quantities of solar flux over even the modest distances needed inside buildings. The reason is that the flux passing down the fiber is absorbed within the solid material of the fiber. Not only does this reduce the quantity of flux emerging from the fiber inside the building, it heats the fiber and when very high concentrated solar flux levels are involved, this can damage the fibers.

Lorne Whitehead at the University of British Columbia in Vancouver got around this problem by making large rectangular- and circular-cross-section hollow light pipes having bounding surfaces made of thin prisms in a special arrangement. The prismatic portions were placed on the outside of the light pipes and the interior surface was smooth and flat. Rays propagating down the hollow pipe strike the smooth surface and are partially reflected and refracted. The reflected rays continue down the light pipe. The refracted rays pass a short distance to the prismatic edges where they are totally internally reflected and then emerge again into the hollow interior section of the "prism light guide." The technology was patented and subsequently licensed to a large plastic products manufacturing company.

• More information on prism light guides can be found at http://www.physics.ubc.ca/ssp/.

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