1. Introduction
Any light device, whether natural or artificial, affects our hormones, body temperature, cognitive activities and mood. In particular, this influence is determined by: the illumination level, the light spectrum, the colour temperature and the variable (or static) nature of the source.
The excessive use of electric lighting can lead to undesirable effects from the point of view of both the inhabitants of a building (Heschong Mahone Group, 1999) and an
energy conservation perspective.
A review of people’s reaction to indoor environments suggests that daylight is desired as it provides the condition for good sight and guarantees good colour discrimination
and colour rendering, if opportunely controlled.
However, daylight can cause glare which interferes with comfortable sight, depending on how the daylight enters the rooms. Thus, a careful design of the daylight components
(i.e. windows, light-duct, atria, lightshelf, skylight, louvres, etc.) is necessary to guarantee a good contribution from the daylight and a uniform distribution of indoor illumination avoiding the direct solar radiation.
In recent years, many offices or commercial buildings have paid particular attention to daylight design. If daylight is available when it is wanted, the natural light can
make an important contribution in terms of visual comfort and the rational use of energy, particularly if it is appropriately integrated with electric light.
Several prototypes have been developed and experience has been gained in collecting and transporting daylight. In this field it is possible to draw a distinction between a heliostat, a day lighting system and a sun lighting system.
A heliostat is a movable system which tracks the sun as it crosses the sky, thus constantly reflecting the direct sunlight to any desired location. For example, the Himawarii system consists of a movable heliostat that concentrates the direct sunlight and a flexible fibre cable to guide the light indoors; however, this system has a low efficiency (<5%). The Heliobus is a pilot system installed in a school building in St Gallen (Switzerland). This system is realized with a static heliostat located on the roof, and with a vertical prismatic light guide (Aizenberg and Korobko, 1997).
The pipe extends over three floors and also reaches the underground floor.
A sun lighting system is an individual solution for capturing, guiding and distributing sunlight indoors to places where it is always needed. It is composed of Fresnel lens
and optical fibres organised to transport light. This subject has been studied by Wilson et al. (2002a,b).
In a day lighting system there is no movable heliostat and so the direct sunlight cannot be captured all the time; it provides diffuse daylight which cannot be transmitted efficiently over long distances. In the case of the sunshine the direct sunlight can enter the system and it provides a lot of brightness inside the room. An example of this type of system is the simple light guide (mirrored tubes) for the
skylights. It collects the light through a transparent exterior dome and brings it to the interior through a reflecting metal pipe and a diffuser that are located at ceiling height in the space. The efficiency of the fixed light-pipes is reduced by the absorption of the walls of the pipe. The light is repeatedly reflected as it goes through the pipe. The light loss is proportional to the length-to-width ratio of the pipe (the more times the light is reflected within the pipe, the more the light is absorbed). If the light-pipe is properly sized, it can provide daylight to interior spaces without the associated heat gain and glare problems of skylights. In most
applications the light-pipes are used to integrate the artificial lighting and it could be useful to know their contribution to the lighting level in the room where they are located.
Consequently, this device that can be compared to daylight capturing and transporting systems, is very simple. It channels daylight from a hole in the roof, down to a ceiling-mounted diffuser which looks like a conventional ceiling-mounted light fixture. The light is transmitted through a cylindrical tube with a highly reflective interior
surface.
The design of these systems is almost straightforward, though there are som significant differences among the various products on the market. Many of them have acrylic
domes that are mounted on or above the roof. A few of them use polycarbonate for the domes: although it has about 10% lower light transmission than the acrylic, it is
much stronger. Several products include a prismatic or collimation lens beneath the dome, which improves the capture and downward channelling of low-angle sunlight
during the morning or the afternoon.
The light-pipe system also requires a modelling analysis to predict its performance. Several models have been developed. For example Zhang and Muneer (2005) have proposed
a model for an opal diffuser light-pipe. Their model was able to predict the illuminance levels within the quoted limits. However, it did not accommodate a different
diffuser material. Jenkins and Muneer (2003) have proposed a model to investigate the effectiveness of the light-pipes. It predicts the light levels resulting from pipes
of given dimensions. They have also proposed methods to calculate the illuminance resulting from a given luminous flux in overcast skies.
The aim of this paper is to show the results of monitoring a light-pipe located inside a laboratory in Ancona (in Italy). The system has been monitored with a luxmeter to
evaluate the distribution of illuminance on a working-plane from January 2005 to June 2006. The indoor and outdoor illuminance and the internal/external illuminance ratio are
shown in the graphs.
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