Beginning in the 1970s, Guy Rottier began focusing on solar urban planning. Along with the heliophysicist Maurice Touchais, he was one of the first to analyze the impacts of the poor distribution of sunlight on buildings. Together, they developed the “lumiducs” concept, composed of tubes whose inner walls were reflective, which when linked to a mirror sensor made it possible to bring sunlight into deep and blind volumes, with the added benefit of reducing pollution. In this type of urban organization, streets no longer delimit buildings. The city becomes a continuous system bringing together collective spaces as neighborhoods of light: “Differentiated from current solar research, this approach mainly aims to utilize the properties of sunlight as a "source of light and life,” and not to transform it into other energies. Our agglomerations have undifferentiated functions, and their quantity of sunlight is insufficient for their respective needs. Indeed, in today’s cities, 25% of volumes need sunlight (dwellings, schools, hospitals, etc.), 25% of volumes require intermittent sunlight (offices, patios, etc.), 50% of volumes do not require sunlight (cinemas, supermarkets, garages, etc.). Better distribution of sunlight across these volumes is a priority, and the new idea consisted of utilizing "lumiducs" to transport and distribute sunlight where it is needed. This involves four new concepts: that sunlight is transportable; that ideas about orientation are outdated; that sunlight can be equitably distributed and that traditional urban planning and architecture are pollutants, because they inhibit the proper distribution of sunlight. Therefore, it is possible to design an agglomeration where sunlight does not depend on the built volume of the urban fabric. The city becomes an “inhabitable green hill,” and its depths are lit with "lumiducs": ECOPOLIS” (Guy Rottier).