Colour appearance of perceived illuminants and surfaces

Abstract

The colour appearance of a patch of light is only partly determined by the light itself. A patch's perceived colour can also be influenced by cognitive processes. Cognitive effects on colour appearance were examined by modifying an observer's perceptual representation of surface-ambiguous and textured patches of light. Observers were instructed to view a 1° square test patch as either an illuminant source or as an object reflecting light from its surface. The test patch was presented on a high-resolution Nanao 9080i colour monitor driven by a 32 bit microprocessor (T1 34020 GSP) specialized for graphics operations. The luminance of the phosphors was photometrically calibrated and linearized. The five test conditions were a(n): (1) isolated homogeneous test field (ambiguous test); (2) isolated test containing a random-dot speckled pattern (textured test surface); (3) homogeneous test with homogeneous surround (ambiguous test and surround); (4) test containing a similar texture surrounded by a homogeneous background (textured test surface with ambiguous surround); (5) textured test with a textured surround (textured test and surround surface). Changes in colour appearance were assessed using a red/green hue cancellation technique based on opponency mechanisms in the human visual pathway. The test was an admixture of "red" (Xd = 625 nm) and "green" (kd = 535 nm) light. The chromaticity of the backgroimds were "red" (Xd = 625 nm) and "green" (Xd = 535 nm). The test patch was varied across five luminances (0.92 to 19.9 cd/m^). Observers monocularly viewed the CRT with their left eyes, and were required to adjust the radiance of the "green" component so that the test appeared neither reddish nor greenish. Results showed that all but one observer viewed the CRT images independently of instruction set. The observer showed a greenness (redness) shift in test colour appearance for surface (aperture) mode perception when viewing a homogeneous test with a 625 nm surround. The remaining observers showed either no shifts or shifts in colour appearance that were in the same chromatic direction as the surround. The data can best be described by neural assimilation processes.