We present a method for measurement and reconstruction of second order approximations of light fields in closed spaces. We visualized their structure using light tubes and rendered objects at several points along a tube.

The structure of light fields of natural scenes is highly complex due to high frequencies in the radiance distribution function. However it is the low-order properties of light that determine the appearance of common matte materials. We describe the local light field in terms of spherical harmonics and analyze the qualitative properties and physical meaning of the low-order components. We take a first step in the further development of Gershun's classical work on the light field by extending his description beyond the 3D vector field, toward a more complete description of the illumination using tensors. We show that the three first components, namely, the monopole (density of light), the dipole (light vector), and the quadrupole (squash tensor) suffice to describe a wide range of qualitatively different light fields. In this paper we address a related issue, namely, the spatial properties of light fields within natural scenes. We want to find out to what extent local light fields change from point to point and how different orders behave. We found experimentally that the low-order components of the light field are rather constant over the scenes whereas high-order components are not. Using very simple models, we found a strong relationship between the low-order components and the geometrical layouts of the scenes.

Light fields [Gershun 1939] of natural scenes are highly complex and vary within a scene from point to point. However, in many applications complex lighting can be successfully replaced by its low order approximation [Ramamoorthi and Hanrahan 2001]. The purpose of this research was to investigate the spatial behavior of light fields in natural scenes. We describe the light fields in terms of spherical harmonics and analyze their qualitative properties. We show that low order approximations of natural light fields vary smoothly and systematically, in accordance with very simple models. This finding has important implications for graphics, visualization, architectural design and scene perception research.