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

Acuity — where it appears

The finest spatial detail an observer can resolve, usually quoted as a limiting spatial frequency. The achromatic limit is near fifty cycles per degree and the chromatic ones are several times coarser, so an edge can be sharp in lightness and blurred in colour at the same distance.

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

Contrast sensitivity, three channels, normalised to each channel's peak. Spatial frequency in cycles per degree against relative sensitivity. The luminance channel runs to 50 cycles per degree, red–green to 12 and blue–yellow to 8, each by the same criterion of 5 per cent of that channel's own peak. Only the luminance channel dips at low frequency; the two chromatic ones are low-pass, so a chromatic edge of any size at all is seen at its full contrast.

How fine a colour edge can be

The eye resolves a lightness pattern to about fifty cycles per degree and a red–green one to twelve. Every colour difference here is quoted as though a patch had no size, and the same difference is plainly visible at one scale and gone at another.

eye · Cones What is read at each distance from the edge of a lit region. Three materials under a half-plane of light, with the boundary at the centre of the horizontal axis and the lit side on the right. The vertical axis is the radiance leaving the surface as a share of what it leaves far inside the lit region. On the unlit side the sample is emitting light while receiving none, so the ratio the model calls a reflectance has a zero denominator there. The distance over which the curve runs from a tenth to nine tenths is 0.21 millimetres on coated paper and 5.1 on pale marble — which is the width of the neighbourhood a point's colour is decided by.

Two blurs before the eye's own

A translucent object arrives at the eye already blurred, by a kernel that is a property of the material rather than of the optics. On every material in this collection's table that blur is coarser at ordinary reading distance than the finest detail the eye can resolve — so the softness of marble or skin or wax is not a failure of vision, it is the object.

eye · Cones

Named alongside it

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

Contrast sensitivitySpatial frequencyViewing distanceChromaDiffusion lengthEdge integrationImage differenceIndividual variationLuminanceMarginalisationOpponent processingPoint spread function

All concepts