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

Area coverage — where it appears

The fraction of a printed area a halftone dot pattern is meant to cover, which is the number a separation actually specifies. What the paper returns is not that fraction, because ink spreads mechanically and light scatters sideways under the sheet, so the effective coverage is always the larger number.

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

A halftone tint, its four regions, and what they average to. At 40% and 30% coverage the sheet is a mosaic of 4 fully-inked regions, not a mixture of anything. Their areas are the product of the coverages — Demichel's rule, which holds because the screens are rotated to make it hold — and the patch's reflectance is the area-weighted average of theirs, raised to the Yule–Nielsen exponent n = 1.8. Averaging the regions' CIELAB coordinates instead, which is what mixing means to almost everybody, lands ΔE00 = 0.76 away.

A halftone is not a mixture

Forty per cent cyan and thirty per cent magenta are not stirred together anywhere. They are laid down as dots, and the sheet is a mosaic of four fully-inked regions in proportions the two coverages fix — which is why the colour is an area average of four spectra rather than a blend of two, and why it lands nowhere near where mixing would put it.

applied · Delivery What the next thousand patches buy a printer profile. A profile is a table, exact at its nodes and interpolated everywhere else. Each mark is a grid: the horizontal axis is how many patches somebody had to print and measure, the vertical is the worst error found between the nodes. Going from 135 patches to 3645 — 27.0× the work — buys 8.4× the accuracy. The error falls with the square of the spacing and the count rises with its cube.

A profile is a fit between its nodes

A printer profile is a table, exact at every patch that was printed and interpolated everywhere else — and everywhere else is where every job lives. Going from a hundred and thirty-five patches to three and a half thousand is twenty-seven times the work for eight times the accuracy, and the exponents say that is as good as it gets.

applied · Delivery

Named alongside it

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

Process inksCalibrationDemichel's areasHalftoneHeld-out validationThe ICC profileIdentifiabilityIntegrationInterpolationMeasurement errorModel selectionThe Neugebauer primaries

All concepts