One reflectance, two illuminants, two colours
Drawn at its defaults, in spectra and illuminants. It takes no options at all, so every essay calling it gets this exact drawing.
Called by 4 essays
the blast radius of changing it
Constancy is the default
A sheet of paper looks white in daylight and white under a tungsten lamp, although the light reaching the eye differs enormously. The visual system is solving one equation with two unknowns, and it solves it by assumption.
What light isThe illuminant is half the answer
An object has a reflectance, not a colour. The colour appears when a specified light falls on it, which is why two surfaces can match in a shop and clash outside, and why every serious matching standard names the light.
What a scene doesThe colour is in the thickness
Transmittance is exponential in path length and the observer is linear, so doubling the depth of an absorbing medium squares the transmittance rather than halving the colour. A translucent object therefore has no one colour — its thin edge and its thick middle are different spectra of the same substance, and the hue moves between them.
What a scene doesA mean has a set under it
Every adaptation number this collection publishes is an average over a hundred and twenty-five surfaces that were written down once, in one file, with no argument for how many there should be or how saturated. The average runs from exactly zero to twice itself across them, and the set has never been varied.