Cues to the Illuminant
Illuminant Cues in Surface Color Perception:
Joong Nam Yang1 Laurence T. Maloney1,2
1Department of Psychology
2Center for Neural Science
Draft: May 9, 2000: Please do not quote.
Running head: Cues to the Illuminant
Keywords: Color, color perception, surface color, color constancy, illuminant cue
Joong Nam Yang, Ph.D.
Visual Sciences Center
University of Chicago
939 E. 57th Street
Chicago, IL 60637
Tel: +1 773 702-0393 Fax: +1 773 702-4442
Many recent computational models of surface color perception require information about illumination in scenes. The models differ primarily in the physical process each makes use of as a cue to the illuminant. We evaluated whether the human visual system makes use of any of three of the following candidate illuminant cues: 1. specular highlight, 2. full surface specularity (Lee, 1986; D’Zmura & Lennie, 1986), and 3. uniform background.
Observers viewed simulated scenes binocularly in a computer-controlled Wheatstone stereoscope. All simulated scenes contained a uniform background plane perpendicular to the observer’s line of sight and a small number of specular, colored spheres resting on the uniform background. Scenes were rendered under either standard illuminant D65 or standard illuminant A. Observers adjusted the color of a small, simulated test patch to appear achromatic.
In a series of experiments we perturbed the illuminant color signaled by each candidate cue and looked for an influence of the changed cue on achromatic settings. We found that the specular highlight cue had a significant influence, but that the influence was asymmetric: greater when the base illuminant, CIE standard Illuminant A, was perturbed in the direction of Illuminant D65 than vice versa. Neither the full surface specularity cue nor the background cue had any observable influence. The lack of influence of the background cue is likely due to the placement of the test patch in front of the background rather than, as is typical, embedded in the background.
[I]n our observations with the sense of vision, we always start out by forming a judgment about the colors of bodies, eliminating the differences of illumination by which a body is revealed to us.
-- von Helmholtz (1896/1962, p. 287).
In this remarkable sentence, von Helmholtz proposes a theory of surface color perception: bodies have intrinsic surface colors, and, while the initial visual information available to biological systems confounds light and surface, the visual system manages to arrive at surface color estimates that are invariant under changes in illumination, that depend only on the intrinsic properties of surfaces. Now, over a century later, we might want to qualify every part of the statement above. First of all, the degree of surface color constancy that we experience depends on viewing conditions: under some circumstances we have essentially no color constancy (Helson & Judd, 1936) and under others we show a remarkable, nearly perfect, degree of constancy (Brainard, Brunt, & Spiegle, 1997; Brainard, 1998). Von Helmholtz’s assertion can only apply to the latter sort of viewing conditions.
In addition, a mathematical analysis of how surfaces and light interact and how spectral information is encoded in the retina leads to the provisional conclusion that von Helmholtz posed an impossible task for biological vision. The color signal that comes to the eye has two components, illuminant and surface reflectance, and the data available to the visual system are simply the excitations of photoreceptors at each location xy in the retina:
Here, is used to denote the surface spectral reflectance function of a surface patch imaged on retinal location