Ground albedo impacts on higher-order scattering spectral radiances of night sky

Abstract

The contribution from ground reflected light to the night sky radiance is of increased interest because of constant modernization of street lighting systems that now efficiently eliminate light emissions above the horizontal plane, so the appreciable fraction of artificial light directed upwards is from ground reflection. Diffuse light of a night sky shows a positive correlation with ground albedo (α), but it seems there is no linear trend between α and the night sky brightness (NSB), at least not for all sky elements, and, the mechanism of this relationship becomes even more complicated due to multiple scattering effects. The extent to which the ground reflectance influences the higher-order scattering radiance of night sky, and, the factors that initiate the crossover from single- to multiple-scattering dominance of the sky glow has been virtually unexplored until now.We demonstrate here that albedo-induced effects in NSB exhibit an angular dependence, with amplitudes enhanced towards shorter wavelengths. For low values of ground reflectance and at short distances from a light source, the second-scattering radiance is found to be only a few percent of the first-order scattering radiance. However, the ratio of a higher- to the first-order scattering radiance gradually increases near horizon, specifically at the side opposite to the azimuthal position of the light source. Also the NSB in blue band has decreasing gradation tendency when increasing the altitude above sea level. The findings in this paper are significant in a proper incorporation of higher-order scattering in modeling the NSB under elevated reflectance conditions, and may be critical for saving computational time.