A comparison of physical and visual light fields structures

Abstract

Human observers are sensitive to light properties such as the intensity, direction and diffuseness. We study how well they are able to estimate the light field (the structure of the net flux transport) in rooms, by empirical measurement and comparison of the physical and visual light fields. We made regularized measurements of the physical light field in a room under three light conditions with a custom-made cubic illuminance meter based on six Konica Minolta T-10MA sensors. Next, we photographed the room and conducted a psychophysical experiment, in which the observer’s task was to change the lighting (direction, intensity and ambient components) of a “probe” in order to make it appear like it belonged to the pictured scene. The probe, a white Lambertian sphere on a black monopod (defining the location of the probe in the scene), was superimposed on predetermined locations in the image. As a separate measure of the visual light field, the participants indicated the perceived (“subjective”) light source position on the extended scene pictures and described it in words. For each light condition we made reconstructions of light fields consisting of light vectors in a grid: physical from measurements, visual from observers’ settings on a probe, and simplified models of light fields with all vectors pointing to the subjective light source positions. Participants’ interpretations varied remarkably in position and number of the subjective light sources. Many of them were far from veridical. We made pairwise comparisons between light field reconstructions. For the most prominent case, with a visible light source, the average angular difference between visual and physical light vectors was 57 degrees, whereas between visual and simplified model for subjective light sources it was 29 degrees. The results suggest that, overall, the visual light field’s structure is simplified with respect to the physical one.