Seminars of the Department of Psychology

With regards to achromatic surfaces (the so-called grey scale), humans are able to easily perceive the lightness of an object. However, the intensity of the light (luminance) coming into our eyes from a physically grey surface depends both on the object’s reflectance and its illuminance, which are both unknown variables, hence reversed optics cannot be the solution for lightness perception. It has long been a research object in vision science what information is utilized by the visual system to achieve precise lightness perception. Previous works have discussed the effect of several factors on lightness perception, such as contrast, depth, or spatial frequency. The present study found a new factor of lightness perception: the surface’s orientation. Experiment 1 found that a surface in a 2-D image appears darker when the image is rotated so that the surface faces upward. This result means that lightness of the same image can be altered by the change of its rotation. That was confirmed again by Experiment 2, where surface images were presented stereoscopically. It was hypothesized that the effect of surface orientation stems from humans’ prior assumption that illumination comes always from above. In Experiment 3, perceived lightness was measured using a surface image with 12 orientation conditions, and the result was in agreement with the hypothesis. Experiment 4 tested if the illumination assumption used for lightness perception is the same as that used for 3-D shape perception, and results suggest that they are independent. The present study revealed a new general factor of lightness perception.