Colour Science & Perception
what the eye settles, and what it cannot
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The thread: Matching is not appearance — page 7

CIE XYZ predicts when two lights will match under identical viewing conditions. It was never a model of how anything looks, and most of the confusion in applied colour comes from using it as one.
Where each display's boundary and the spectrum fail, as the adapting white warms. For radiators from 6500 K down to 2700 K, the temperatures at which the two-matrix front end sends some point of each display's boundary, or some spectral light, negative: shaded where it fails. sRGB and Display P3 first fail at 2871 K, just above illuminant A; BT.2020 at 3626 K. P3 and BT.2020 also fail at their red corners under whites cooler than 5700 K, and no spectral light fails between 5700 and 4700 K. The dashed line is the 3500 K the prediction named. What the brain does

Two matrices hold to almost tungsten, and the blue turns long before

An appearance model that adapts in CIECAM16's space and compresses in Hunt–Pointer–Estévez gets the blue's hue turn right under daylight and sends the display's blue to a negative signal under illuminant A. The prediction was that the failure reaches sRGB's blue near 3500 K, a warm-white LED, and that the blue's turn holds its daylight value down to about 4000 K. Walked down the radiator and daylight loci, sRGB's boundary holds to 2872 K — fifteen kelvin short of illuminant A — while BT.2020's fails at 3626. There is a neutral window, 4700 to 5700 K, where no colour at all goes negative. But the blue's turn does not wait for the failure: it is five degrees above its daylight value by 4500 K and nearly three times it before anything breaks, because it follows the blue's long-wave signal down towards nought.

 What adaptation cannot remove from an orange tint, with and without a thin dot edge. For a reader fully adapted to brightened office paper, the orange tint's hue turn against its solid beyond what a non-fluorescent sheet of the same colour leaves, at coverages from five to sixty per cent, with a uniform ink film and with rims of three, six and twelve micrometres. The residue is largest at the palest tint for every rim, and every wider rim raises it: at a fifth coverage from 6.8° to 7.9°. What the brain does

A thin dot edge lets the glow back in

A pale tint printed on brightened paper keeps part of its hue turn however fully a reader adapts to the sheet — ten degrees for a fifth-coverage orange where a non-fluorescent sheet of the same colour leaves three — because the ink's vehicle puts out the brightener's glow under the solid and the tint's bare paper keeps it. That census gave every dot a uniform film. A real dot thins at its edge, a thin film lets ultraviolet through, and at the pale end of a ramp most of a dot is edge. The prediction was that the edge would restore the glow under the tint's dots, shrink the residue by a third at a fifth coverage and move the worst of it to the mid-tones. The edge does restore the glow — nine times as much under a small dot with a twelve-micrometre rim. And the residue grows: 6.8 degrees to 7.9 at a fifth coverage, 7.2 to 9.0 at a twentieth. The residue is the tint glowing where the solid does not, and more glow under the tint's dots is more of the difference. The palest tint keeps the most, with or without an edge.

 What a viewer adapting to the warm room sees of the tint. The largest tint anywhere on the screen as a viewer sees it in CIECAM16 — the CAM16-UCS colour distance between a grey on the declared grey floor and on the true warm one — for the three pictures in a 2,700 K room putting a tenth of a candela on the screen, as the viewer adapts from the display's white to the room's. In the moon picture it grows, 2.75, 3.35, 3.92, 4.42; in the bright window picture it shrinks. Difference and uniformity

Adapting to a warm room deepens the shadows' tint

A warm room's light reflected off a local-dimming screen tints the shadows of every dark picture when the screen's black is declared in grey — by up to 17.7 ΔEITP — and ΔEITP has no viewer in it. A viewer who has sat in a 2,700 K room all evening is adapted partly to the room, and adaptation to a warm white discounts warm light. The prediction was that a viewer adapted a third of the way to the room would see under half the tint, and one adapted two thirds of the way would see the darkest greys turn blue. Read through CIECAM16, the opposite happens in a dark picture. The tint the viewer sees grows at every step of adaptation: in the moon picture's darkest grey from 2.7 CAM16-UCS units for a viewer adapted to the display to 3.3, 3.9 and 4.4. The darkest greys never turn blue; they stay orange and fade. What turns blue is the mid-greys, whose light is the display's own and now looks cool. In the bright window picture the tint shrinks, and declaring the floor in the room's colour still removes it for every viewer.

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