Photon flow: A three-dimensional expression of the light field using volume photon mapping

This paper proposes a new method to depict the light field based on the volume photon mapping algorithm. In the context of the light field simulation, a participating medium serves to deposit the photons, but does not disturb their propagation. The photons are therefore neither scattered nor absorbed in order to preserve their energy and trajectory within the environment, thus providing an unbiased luminance distribution. A visualisation of the photon distribution enables an intuitive interpretation of the light propagation that helps designers to understand the basic light field in the space. In addition to visualisation, the magnitude of the simulated physical light field can be numerically evaluated from the volume photon map distribution using, for example, cubic and scalar illuminance. This can further inform the designer on the light density distribution in the space, since the latter directly correlates with the density of the photons, and therefore the scalar illuminance. The accuracy of the proposed method was ascertained by comparing it with the original RADIANCE. Furthermore, its advantage in visualisation was demonstrated using a complex case study involving strong indirect lighting, reinforced by a comparison of the simulation and measurement in the actual space. In addition, photon mapping was found to evaluate illuminance in multiple grid points much faster than RADIANCE Classic, notably due to the complex ambient lighting from specular reflections. The implementation of the specialised volume photon mapping software is now part of the RADIANCE software and is available as a lighting research tool for the community.