Probing light in real scenes using optical mixtures

In this research, a novel experimental setup was built to investigate human beings' sensitivity for lighting properties of a real scene using a gauge object or "probe". Light is a kind of medium that makes objects visible without being visible itself in empty space. The light field determines the appearance of objects and materials in it, and the appearance of objects is the main cue to the light field. Cuttle [2003] suggested to use probes (i.e., gauge objects) to reveal lighting characteristics: the sharpness of a light beam can be estimated from the shadow pattern of a peg on a disk and from the highlight pattern on a black shiny sphere, while the flow of light can be revealed from the shading pattern on a matte white sphere. In a workshop led by Madsen and Donn, white spheres were used as a light flow meter to visually assess spatial and form-giving characteristics of daylight [2006]. By putting white spheres into empty spaces of rendered artificial architecture, the judgments of the light flow improved. However, the shading pattern varied with different perspective views, which made the digital light-flow-meter perceptually ambiguous. Pont and Koenderink [2007] implemented the white sphere as their gauge object for estimating human beings' sensitivity to direction, diffuseness and intensity of illumination and found that human observers confuse illumination direction with illumination diffuseness (diffuseness-direction ambiguity). This explains the problem Madsen encountered and indicates that to implement the light-flow-meter as a quick and simple tool to visually assess the properties of light, alternative probes must be developed. Theoretically, the illuminance flow or texture contrast gradients over a rough sphere gives additional cues about its illumination [2004]. In this study we will address whether observers use such cues so that we can improve the probe by adding roughness. In addition, we will test how human beings perceive the global properties of the light field in a real scene instead of on photographs on a computer screen.