Colour Science & Perception
what the eye settles, and what it cannot
Concept

Confusion line — where it appears

A line in a chromaticity diagram along which one class of dichromat cannot tell colours apart. The lines for a given class are not parallel and all converge on a single point, because the colours being confused differ by a multiple of one fixed direction in tristimulus space.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

One observer's matching functions, in three of the bases the matches leave free. The three colour-matching functions after a change of basis 0.00 of the way from Hunt–Pointer–Estévez towards the set built from the dichromat confusion points. Every one of these triples predicts exactly the same matches as every other, because a match is an equality and a matrix applied to both sides of an equality changes nothing. What moves is where the peaks are and whether the curves go negative — these ones do not, and going negative is what the 1931 committee constructed XYZ to avoid.

The matches do not name the cones

Colour matching is the whole empirical basis of colorimetry, and it fixes the observer's three curves only up to a nonsingular 3×3 — nine numbers that no match, in any quantity, to any precision, can see. One particular choice of those nine is used throughout here, and it was made for a different purpose.

eye · Cones Where a dichromat's confusions converge. Every pair of colours a protanope cannot tell apart lies on one of these lines, and all the lines meet at a single point — at (0.7465, 0.2535) for this class. The point is the chromaticity of the missing cone's own response direction, which is why it need not lie inside the diagram or correspond to any light at all. Two of the three do not. The three points between them carry six numbers, and six is two thirds of what the matching data leave undetermined.

A confusion point is a missing pigment

The nine numbers colour matching leaves free are fixed by three points on a chromaticity diagram, each of them the place where everything one class of dichromat cannot tell apart converges. Two of the three lie outside the diagram entirely, which is not a defect — a direction in tristimulus space need not correspond to a light.

eye · Cones

Named alongside it

The objects these essays reach for when they reach for this one.

Colour vision deficiencyCone fundamentalsCopunctal pointDegrees of freedomDichromatic modelIdentifiabilityColour-matching functionsDeuteranopiaDichromacyMetamerismProtanopiaStandard observer

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