Introduction: In previous work, we demonstrated that observers, making achromatic settings for a lambertian surface patch at different orientations, systematically discount the relative contributions of a yellow punctate light and a blue diffuse light as patch orientation changes (Boyaci et al. JOV, 4, 664-679, 2004). We concluded that the observer's visual system effectively estimated a representation of the spatial and chromatic distribution of the illuminant. In this study, we investigate whether the visual system can represent and discount more complex spatial and chromatic distributions of illumination.

Methods: The stimuli were computer-rendered 3D scenes, containing a rectangular test patch at the center. Observers viewed stimuli in a stereoscope. Scenes were illuminated by a composition of a diffuse blue and two yellow punctate sources placed symmetrically about the observer's line of site and either 90 degrees apart or 160 degrees apart. A blue diffuse and single yellow punctate source condition was included as a control. The orientation of the test patch was randomly varied among 9 orientations from –60 deg to 60 deg. On each trial the observer adjusted the color of the test patch until it was perceived to be achromatic. We analyzed the amount of relative blue in the observers' achromatic setting as a function of test patch orientation. Six naive observers repeated each orientation x light condition 20 times. We fit a generalization of the equivalent lighting model (ELM) developed in Boyaci et.al. to predict settings at each test patch orientation for an ideal observer with imperfect knowledge of the spatial and chromatic distribution of illuminants.

Results: Observers systematically discounted the relative contribution of diffuse and punctate light sources at the various test patch orientations for all illuminants. We conclude that the visual system effectively represents complex lighting models that include multiple punctate sources.