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

Grey component replacement — where it appears

Substituting black ink for the grey that three chromatic inks have in common, which saves ink and picks one member of a family. The family it picks from is a genuine one: a four-ink press has three numbers to match and four controls, so the separation is underdetermined.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

The solids, by volume. CIELAB volume, all measured by the same cell count with the same extrapolation, and quoted against sRGB because that is the comparison the estimator supports. The four-colour press reaches 49 per cent of the sRGB solid's volume — and contains colours sRGB does not, which is why "half the gamut" is the wrong summary of a ratio of one half.

The fourth ink is not for colour

Black is in every process set and it extends the gamut by more than a third — and at every lightness above about forty it extends the chroma boundary by exactly nothing. What it buys is depth at the bottom, a neutral that holds its hue when the press drifts, and a way to put a full-strength dark on a sheet that could not carry three inks' worth of it.

applied · Delivery What separates the separations, and under which light. 11 separations of a single colour, differing only in how much of it is carried by black rather than by the three chromatic inks. Under D50 they agree to ΔE00 = 0.00 — the solver was asked for that and delivered it. Under illuminant A they spread to 6.75. They are metamers of one another, and the whole family is invisible to any instrument reading a single illuminant.

The separation is not unique

A four-ink press has one more control than a colour has numbers, so most colours can be printed several ways. The alternatives agree to a hundredth of a colour difference under the light the match was made in — and they are metamers of one another, so under a tungsten lamp the same eleven separations of one colour spread by nearly seven.

applied · Delivery A four-ink inverse table's error along the grey axis, 9 nodes, black from L* 60. The colour difference between a grey asked for and the grey printed, from L 92 to 14, for a 9-node inverse table whose separation strategy begins black generation at L 60 (dashed line). Three tables: filled exactly from the strategy, mean 0.073; refitted with each node sliding along the strategy, 0.050; refitted freely, 0.036. To the left of the dashed line the strategy-bound refit lies close to the filled table; to the right it lies close to the free refit. Every table is exact at its nodes, marked on the axis.

A strategy keeps the refit only where black prints

Refitting an inverse table's nodes against their own error halves it, and in a four-ink table the refit has to leave the separation strategy's choice of black alone. Held to the strategy, it keeps the whole of its gain wherever the strategy prints black and almost none of it where the strategy does not — and the share it keeps equals the share of the grey ramp printed with black, to within three hundredths. The start of black generation, predicted to be where the refit would fail, is where the table needs it least.

applied · Delivery

Named alongside it

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

SeparationHalftoneThe ICC profileInk limitProcess inksSpecificationChromaGamutIlluminant metamerismInterpolationInverse modelLeast-squares

All concepts