Reflected sunlight in urban canyons. Towards a new approach

In urban canyons reflected sunlight can play an important role in the illumination of buildings, particularly in orientations and during times of the day where sunlight is not incident on the windows. Facing buildings provide considerable obstruction to daylight by reducing the skylight contribution and sometimes blocking the access to sunlight. However, reflected sunlight is an important contribution to the internal illuminance of a room and should not be underestimated. Furthermore, obstructions and ground can redirect the light to other interior surfaces rather than the horizontal plane and promote a better uniformity of light inside the space. Real data collected in Lisbon showed a linear relationship between the global horizontal solar illuminance, Egh, and the total vertical illuminance, Etv, at the building facade, when the facade is not receiving direct sunlight. This relationship can be described by an equation Etv = k * Egh + C where the slope k depends on the reflectance of the obstruction, the canyon geometry and the height of the point under investigation on the faade. The constant C is mainly the contribution of the diffuse sky illuminance to building daylight and is more significant at higher floors. Simulations undertaken with RADIANCE as well as analytical calculations confirmed that an approximate linear relationship existed except under specific conditions occurring mainly in the summer when the street or ground was fully sunlit. This paper presents a simplified method for estimating the illuminance that reaches an external point on the faade under clear skies distributions taking into consideration the reflected sunlight from the obstruction and the ground, as well as the diffuse illuminance from the sky.