**2. Traditional light pipes**

The lack of daylight in buildings is often a cause of large amount of electric energy consumption. This problem is particularly present in the underground buildings or in industrial or commercial large plant area edifices in which electric light is used all the time in the spaces occupied. In recent years many technological devices, called light pipes, sky lights or tubular sky lights, have been proposed in order to offer a solution to the problem. They are able to collect natural light with a mobile or fixed collector and redirect it into the interior spaces away from the collection point.

Light pipes with mobile collectors can be equipped by heliostats that are solar tracking devices able to rotate with respect to one or two axis, so optimizing the direct solar radiation inlet into the tube.

Fixed collectors are cheaper than mobile collectors. They are usually constituted by a transparent polycarbonate dome with the function of collecting natural light from the sun and the sky and redirect it into the tube. In many cases they also have Fresnel lenses able to concentrate direct solar radiations coming from various directions in the direction parallel to the pipe.

The entrance point is in most cases located on the roof-top of the building or alternatively on one of the external walls. A free collection of light is needed for optimizing the efficiency of the device so that any obstruction of solar radiations from adjacent structures may be avoided.

The tube is internally covered by a highly reflecting film along its length from the collector to the diffuser for long-distance transport of natural light in the underground areas or spaces without any direct interface to outdoor. The larger the diameter of the collector, the longer the distance that can be covered by light with minimum dissipation of energy.

Light pipes on the market have diameters of the collector varying between 0,25 to 1 m. The bigger light pipes are usually adopted in commercial or industrial applications. The smaller ones are used for residential installations and they are typically 1-5 m long.

Natural light is more efficiently transported with straight pipes, but curved or angled tubes can be adopted if necessary. Light transmission losses are reduced by the highly reflecting film applied on the internal surface of the tube. Reflectivity ranges from 0,98 to 0,995.

In recent years many authors carried out numerical and experimental analysis with the aim to determine the performances of light pipes (Carter 2002, Zhang et al. 2002, Jenkins et al. 2003) and theoretical or empirical calculation methods were set up by various researchers for the prediction of the illuminance distribution inside a room equipped by light pipes (Jenkis & Muneer 2004, Jenkis & Zhang 2004), but standard design methods have not yet been developed. The difficulty in standardizing the calculation methods is mainly connected with the influence on internal illuminance distribution of the temporary weather variations that commonly occur during a day, a season or a year, and it is very difficult to describe the real climatic situation with a simplified model based on standard conditions that are necessary for a correct design of the system.

And finally in a room of a building equipped by light pipes, a polycarbonate light diffuser is applied to the ceiling in order to spread light into the room.

The authors published in Baroncini et al. (2006) the results of an experimental comparison between two types of commercial diffusers installed on the same light pipe for residential applications, and demonstrated that the geometry adopted in the diffuser can be modified for spatial distribution of light.

If compared to traditional skylights, the light pipes offer better performances in term of summer heat gain reduction, due to a less visual contact with outside. In addition the availability of natural light in hypogeal environments improves the occupants' well-being, avoiding over illumination effects. A significant energy saving contribution is offered by light pipes installations, by reducing the electric energy consumption.

Light pipes are often characterized by considerable dimensions, their installation in the centre of a passage room is unacceptable, being very bulky systems. When a light pipe is installed to transport natural light from the rooftop of the building to an underground room, the passage spaces are occupied by a very voluminous system which constitutes an undesirable stumbling block and makes it inconvenient to install it in the centre of the room.
