1 Geometrical interpretation of sky light in architecture projects

Natural light comes from a main source, which is sunshine, and from a secondary source, sky vault, illuminated by the sun, but the latter produces a very different light: lighter, cooler (its spectrum moves towards blue colours) and diffuse (it does not project shadows). When the sky turns overcast, this light becomes hegemonic and, depending on the type of clouds, uniform or full of changing patterns. In the North of Europe, low sun and cloudy sky are usual, and diffuse light has been a recurrent theme for painters – from the Dutch school till the impressionists – and an evident appeal for architecture – zenithal light of Alvar Aalto, for example. Nevertheless, it is in the Southern countries where sky light should offer a determinant design criterion for the project of architecture. The Northern solar radiation is always welcomed, indeed, and simple curtains can eliminate the visual trouble of lower and more penetrating sunbeams. On the other hand, protection from summer heat becomes necessary in the Mediterranean climate and sun radiation is a whole year enemy in the tropical regions. Thus, once the sunbeam is maintained out of Architecture, buildings only dispose, for their illumination, of casual reflections on the ground, on vegetation or on others buildings, and of sky light, on top. This diffuse light offers important advantages with regard to the artificial one: it is free, its variation is generally appreciated, and, moreover, it has an excellent luminous efficiency (until 150 lumens for any Watt of solar energy). That is, for the same quantity of light it heats much less than the incandescent lamps, and twice less than fluorescents (efficiency is about 75 lm/W) [1]. Here, we will talk about blue sky, once the sun is masked, and about how we can aid the designer to open the project to it, by means of Descriptive Geometry, through its clear representation.