Today, the challenges of sustainable development require revisiting all the parameters of architectural design. According to the International Energy Agency , in 2005, 19% of the electricity produced in the world was consumed for lighting. We know that the benefits of natural photons are much better than the blue light of the fluorescent lamps, or the bad chromatic rendering of the LEDs. Daylighting is an aspect which must not be overlooked to face the challenges of sustainable development. To this is added the human dimension of natural light, as a factor of well- being and social cohesion, when it is associated with the concept of "light-space." It is not only a matter of capturing more natural light , but also a matter of distributing by giving it a spatial dimension. By lack of knowledge and data, approaches allowing to simulate the lighting level of a building take only limited account of the spatial and human qualities of natural light. This research aims to articulate natural lighting and architectural design through the analysis of major examples of architecture. Therefore , libraries of the modernist Finnish architect Alvar Aalto, located along a north- south axis, from Germany to Finnish Lapland were studied in detail. These are characterized by high quality lighting scenes, despite the low sunlight under these latitudes. The subtle play of direct and indirect lighting installs some virtual boundaries, creating static and dynamic spaces, which are favorable places for activities and intergenerational meetings, under the light. For the first time, light levels have been measured extensively in a typology of Alvar Aalto's buildings, renowned for his mastery of space and light. This approach allows to consider the design of high performance lighting devices, both from a quantitative and qualitative point of view, and proves to be a relevant point of departure for the analysis of "light-spaces".