Perceptual Coherence : Hearing and Seeing

The Perception of Quality: Visual Color 321
In another type of asymmetric matching procedure, the test patch is
achromatic, somewhere between white and black seen under normal daylight.
Since achromatic colors (i.e., the grays) reflect all spectral wavelengths
equally (termed spectrally nonselective), their reflected light will
match the spectral properties of the illuminant. If the illuminant has more
energy in the reds, then the achromatic patch will look reddish. The matching
task requires the observer to use the adjustable projection colorimeter
to add spectral energy to make the test patch look gray (again). The
observer must adjust the colorimeter so that the reflected light from the
achromatic patch, now based on the sum of the ambient and adjustable colorimeter
spectral energies, would equal the reflectance of gray under normal
daylight illumination (Brainard, 1998). To do so, the observer must use
the reflected light from the standard patch and background to estimate the
ambient illumination in order to adjust the colorimeter.
A good starting point is experiments by Kraft and Brainard (1999) that
made use of a highly realistic visual scene and that attempted to tease apart
the contributions of various cues that have been hypothesized to account
for color constancy. Observers looked into a test chamber that contained a
flat panel composed of 24 different colored squares, a tube wrapped in tinfoil
that could produce specular reflection and thereby illuminate other
parts of the scene by means of interreflections, and three-dimensional cube
and pyramid shapes constructed from gray cardboard that could provide
shadow cues (see figure 7.8). One wall of the chamber was covered with
the same cardboard as used to construct the cube and pyramid. The dark
gray test patch hung on the back wall. In all of the experiments, the observer’s
task was to make the test patch look gray by manipulating the red,
green, and blue beams of a hidden projector.
The purpose of the experiment was to determine the relative importance
of the various cues postulated to account for constancy. Prior to the actual
experiment, Kraft and Brainard (1999) determined the maximum degree of
constancy achievable in the “rich” test chamber and the minimum degree of
constancy achievable when all cues were removed. The authors first maximized
the cues for constancy. The experimental chamber was lined with the
same gray background for the neutral illuminant and for the orange-red test
illuminant. These combinations made the test patch look dark gray for the
neutral illumination, but orange-red for the second illumination. In this
case, observers easily adjusted the test patch to look gray under the orangered
illuminant. The constancy index was 0.83, a remarkably high value.
Kraft and Brainard then minimized the cues by removing all the objects
(only the test patch remained) and comparing one condition in which the
background was gray with a neutral illumination to a second condition
in which the background was blue with an orange-red illumination (i.e.,
the patch still looked orange-red). Here the local illumination from the