Chromatic Anchoring of Lightness

This study introduces a new, fundamental determinant of colour appearance: chromatic anchoring. We tested whether observers use the chromaticity of colours to anchor the colour gamut and estimate lightness (chromatic anchoring of lightness). A colour patch was shown on an achromatic background. Without knowledge about the brightness settings of the monitor, the observer cannot know not whether that achromatic background corresponds to the maximum of the monitor (white) or to a lower brightness (grey); but they can infer the lightness based on the chromaticity of the colour patch. 400 naïve observers, who had not seen the experimental set-up before were tested in 16 independent measurements. Results show that observers judge lightness based on the chromaticity of other colours that act as chromatic anchors. This implies that they make inferences about the shape of the colour gamut under given viewing conditions. These measurements do not involve surface colours, implying that lightness judgments and gamut inference do not depend on whether colours are surfaces or lights. In combination with brightness anchoring, chromatic anchoring makes it possible to establish colour constancy independent of surface colours, illumination estimation, and in the absence of any white-point or other achromatic cues.