The light field

The spectral power distribution of the light emitted by the Sun is almost constant. The variations in daylight (Figure 13.1) that we experience over the course of a day and with changes in seasons are due to the interaction of sunlight with the Earth's atmosphere (Henderson, 1977). The resulting spectral distribution of daylight across the sky is typically spatially inhomogeneous and constantly changing (Lee and Hernández-Andrés, 2005a,b). The light arriving at each small patch of surface in the scene depends in general on the patch's location and orientation in the scene. Furthermore, objects in the scene create shadows or act as secondary light sources, adding further complexity to the light field (Gershun, 1936/1939; see also Adelson and Bergen, 1991) that describes the spectral power distribution of the light arriving from every direction at every point in the scene. The light field captures what a radiospectrophotometer placed at each point in the scene, pointing in all possible directions, would record (Figure 13.2).

When the light field is inhomogeneous, the light absorbed and reradiated by a matte smooth surface patch can vary markedly with the orientation or location of the patch in the scene.

In Figure 13.3, for example, we illustrate the wide range of the light emitted by identical rectangular achromatic matte surfaces at many orientations, illuminated by a distant neutral, collimated light source.