INTRODUCTION

Traditional vertical windows can provide adequate daylight within about six metres of a window but, since daylight levels decrease asymptotically with distance from a window, a disproportionate amount of solar radiation must be introduced into the front of the room to achieve small increases in daylight at the back. While this will contribute to energy savings by offsetting electric lighting, the corresponding increase in cooling due to solar heat gain, and/or heating due to structural heat loss, can negate these savings. Atria, rooflights and roof monitors may light areas remote from vertical windows but are of little use in lighting deep core areas. A number of systems exist to redirect daylight into areas of buildings that cannot be lit by conventional glazing. One major generic group known as "beam daylighting" redirects sunlight by adding reflective or refracting elements to conventional windows. This work is concerned with the second major group of redirecting devices known as tubular daylight guidance systems.

ubular daylight guidance systems are linear devices that channel daylight into the core of a building. They consist of a light transport section with, at the outer end, some device for capturing natural light and, at the inner end, a means of distribution of light within the interior. The light capture device may be located at roof level of a building enabling light from the zenithal region of the sky to be gathered. Alternatively, light may be gathered from a device mounted on the building façade. Zenithal openings allow intensive use of daylight but may cause glare or overheating due to penetration of direct solar radiation especially during summer. For a horizontal aperture the quantity of solar flux entering through a façade mounted collector depends on façade orientation and season and these systems are more likely to be influenced by external obstruction than zenithal systems. Collectors may be either mechanical devices that actively focus and direct daylight (usually sunlight), or be passive devices that accept sunlight and skylight from part or whole sky hemisphere. The transport element is usually a tube lined with highly reflective or prismatic material or may contain lenses or other devices to redirect the light. Light is distributed in an interior by output components, commonly diffusers made of opal or prismatic material.

The major emphasis of this Report is on passive zenithal systems. These are by far the most commercially successful types of tubular daylight guidance, being manufactured and installed in large numbers in numerous countries. The design related material in this Report relates to passive zenithal systems only. The Report includes a contextual review of the technology of all generic types of daylight guidance system and includes case studies. The sections on performance indices, photometry of components and systems, design methods, cost and benefits, human factors and architectural issues relate to passive zenithal systems.