Are optical distortions used as a cue for material properties of thick transparent objects?

Fleming, Jäkel, and Maloney (2011) asked subjects to match perceived material properties of thick, clear transparent objects in photo-realistic scenes by adjusting the refractive index. They found approximate correspondence between standard and test objects but also large systematic deviations. Nevertheless, they concluded that estimated refraction is used as an indicator for material properties of light-transmitting objects and emphasized the role of object-induced background distortions in this process. This, however, seems not plausible, because the necessary information for inferring the refractive index from distortions-for example, the object's exact shape and thickness, its background distance, and the undistorted background-was not available in their situation. A more plausible alternative explanation is that the subjects did not match estimated refractive indices, but instead performed simple similarity matches based directly on image attributes related to background distortions or specular reflections. We tested this hypothesis in a similar matching experiment in which it was possible to predict the refractive index for a similarity match based on background distortions and for a similarity match based on specular reflections. Our subjects always chose a value between these two predictions. The specular reflection tends to be the dominant factor as soon as it becomes clearly noticeable. Our findings are compatible with the assumption that the subjects tried to find a compromise between two image-based similarity criteria. They do not seem to be consistent with the assumption that the matches are made on the basis of internal estimates of refractive indices.