Chapter 2 · Optional lab
The visual lab
Realistic illustrations of the eye and retina, an interactive model of receptive fields and clickable maps of the visual pathways. No scores or assessments – explore for as long as it helps.
Colour in changing light
What happens to colour when the light or the surroundings change?
The same surface can meet different light and different surroundings. The cup, the colour patches and an arrangement of coloured surfaces offer three ways into this interplay.
Colour sample on the cup
Both scenes are now shown in exactly the same neutral light (setting 0). The sample squares are identical. Change the illumination to see what happens to the same surfaces.
Both target patches have the same fixed colour and stand on identical neutral backgrounds. Choose a warm or cool surround, or change the lightness, to change only the context.
Target patch: rgb(150,150,148) · exactly the same value in both patches, in every state.
Look at the two small squares. They have exactly the same screen colour, but different surroundings. Switch between blue–green and yellow–red neighbours and compare how the right-hand square looks. Then hide the surroundings.
Against blue–green neighbours the sample may look warmer; against yellow–red neighbours it may look cooler. The surroundings change colour. The sample itself changes only when you adjust the illumination.
The two samples have exactly the same physical screen colour in every state. Adjusting the illumination changes all surfaces together. Hiding the surroundings does not change the samples.
Here we explore how surroundings influence colour appearance. Colour constancy, by contrast, means that a surface can retain roughly the same perceived colour when the illumination changes. The phenomena are related, but this comparison does not by itself show how colour constancy arises.
The strength of the effect varies between observers and displays. The illumination is a simplified screen illustration, and the colours are not individually brightness-matched. The images do not reproduce Land’s original experiments.
Everything is shown on an ordinary screen. How strongly you experience the changes varies between observers and displays; the lab does not measure your perception and gives no “correct” answer.
How the colour model works
The illumination experiment uses a simple screen model: each surface colour is decoded to linear light, multiplied by three channel gains for the chosen light and encoded back. The same surfaces and the same gains apply throughout the scene, including the white reference card. The gains are chosen for clarity — they are not LMS signals, wavelength spectra, kelvin values or measured light.
In the surround experiment the target patches keep exactly the same fixed colour value throughout, with no filters, transparency or overlays. Only the field around them changes. Simultaneous colour contrast is related to colour constancy but is not in itself proof of, or a complete explanation of, colour constancy.
Sources
- Gordon, J. & Shapley, R. (2006). Brightness contrast inhibits color induction: evidence for a new kind of color theory. Spatial Vision, 19(2–4), 133–146. DOI: 10.1163/156856806776923498.
Experiments with small achromatic targets against chromatic surrounds: colour induction depended on boundaries and was strongest at minimal brightness contrast. Our screen colours are not a calibrated reproduction or individually brightness-matched.
- Land, E. H. & McCann, J. J. (1971). Lightness and Retinex Theory. JOSA, 61(1), 1–11. DOI: 10.1364/JOSA.61.000001.
Historical Retinex theory of lightness and comparisons across a scene. Not a complete established brain algorithm; the illustration does not reproduce the original experiments.
- Linnell, K. J. & Foster, D. H. (2002). Scene articulation: dependence of illuminant estimates on number of surfaces. Perception, 31(2), 151–159. DOI: 10.1068/p03sp.
Human experiments on illuminant discrimination and estimation with different numbers of surfaces. More surfaces helped under certain conditions; not evidence that every extra contrast improves perceived colour constancy.
- Stockman, A., Sharpe, L. T. & Fach, C. (1999). The spectral sensitivity of the human short-wavelength sensitive cones derived from thresholds and color matches. Vision Research, 39(17), 2901–2927. DOI: 10.1016/S0042-6989(98)00225-9.
Human S-cone estimates from thresholds and colour matches; supports the S-cone sensitivity curve, not the lab’s RGB model.
- Stockman, A. & Sharpe, L. T. (2000). The spectral sensitivities of the middle- and long-wavelength-sensitive cones derived from measurements in observers of known genotype. Vision Research, 40(13), 1711–1737. DOI: 10.1016/S0042-6989(00)00021-3.
Measurements of M- and L-cones in observers of known genotype; supports overlapping sensitivity of these classes, not the lab’s RGB model.
- Conway, B. R. (2001). Spatial structure of cone inputs to color cells in alert macaque primary visual cortex (V-1). Journal of Neuroscience, 21(8), 2768–2783. DOI: 10.1523/JNEUROSCI.21-08-02768.2001.
Single-unit experiments in alert macaque V1: spatial and chromatic comparisons; not proof of a complete human perceptual model.
- Brainard, D. H., Brunt, W. A. & Speigle, J. M. (1997). Color constancy in the nearly natural image. 1. Asymmetric matches. JOSA A, 14(9), 2091–2110. DOI: 10.1364/JOSAA.14.002091.
Human psychophysics in controlled room illumination: partial colour constancy in asymmetric matching.
- Kraft, J. M. & Brainard, D. H. (1999). Mechanisms of color constancy under nearly natural viewing. PNAS, 96(1), 307–312. DOI: 10.1073/pnas.96.1.307.
Human experiments varying illumination and surface collection: local surround, spatial mean or brightest region alone is insufficient; does not rule out adaptation contributing.
Illustrations and models for learning; not measurements.