Assessment of Sky Diffuse Irradiance and Building Reflected Irradiance in Cast Shadows

Sky radiance field at the bottom of the atmosphere and building façades are invisible in the remote sensing images, but they are the two main light sources of ground surfaces in the shadows cast by buildings in urban areas. This work is interested in evaluating the impact of the anisotropic sky and the reflection of the building on the irradiance of shaded surfaces. The assessment is based on 3D radiative transfer simulations of urban scenes with different sky radiance distributions and different building façade reflectance. The results show that without taking into account anisotropic sky, the average error of sky irradiance estimation in cast shadows can reach 183.75% in a visible band centered at 550 nm. According to the geometry and reflectivity of the building façade, the contribution of the building reflection to the irradiance of the shaded surfaces varies from 0.95% to 84.23%.