Physically-based simulation of twilight phenomena

We present a physically-based approach to compute the colors of the sky during the twilight period before sunrise and after sunset. The simulation is based on the theory of light scattering by small particles. A realistic atmosphere model is assumed, consisting of air molecules, aerosols, and water. Air density, aerosols, and relative humidity vary with altitude. In addition, the aerosol component varies in composition and particle-size distribution. This allows us to realistically simulate twilight phenomena for a wide range of different climate conditions. Besides considering multiple Rayleigh and Mie scattering, we take into account wavelength-dependent refraction of direct sunlight as well as the shadow of the Earth. Incorporating several optimizations into the radiative transfer simulation, a photo-realistic hemispherical twilight sky is computed in less than two hours on a conventional PC. The resulting radiometric data is useful, for instance, for high-dynamic range environment mapping, outdoor global illumination calculations, mesopic vision research and optical aerosol load probing.