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

Absolute luminance — where it appears

The actual amount of light in candelas per square metre, as distinct from a ratio to a white. Appearance depends on it and matching does not, which is why an appearance model takes it as an argument and a colour space does not.

Named by 17 essays across 3 fields — each of them below, with the objects they name alongside it.

Colourfulness rises with light level; apparent contrast does not. Predicted colourfulness M for one stimulus across four decades of adapting luminance, and the exponent of the lightness curve over the same range. M rises by a factor of 2.24 — the Hunt effect, which the model does predict. The lightness exponent changes by -2.2%, and in the wrong direction — the Stevens effect, which it does not.

Brighter looks more colourful

Colourfulness rises with light level and the model predicts it. Apparent contrast is supposed to rise too, and the model does not — which turns out to be right, because the thing that actually carries contrast is the surround.

brain · Appearance Three ways to spend a thousand code values. Normalised code value against luminance, for PQ, HLG and a conventional gamma curve, all covering 0.001 to 10000 cd/m². PQ spends 51% of its range below 100 cd/m² — roughly where a picture lives — and the gamma curve spends 15%, leaving the rest for highlights. A PQ code is the only one of the three that names a luminance rather than a fraction of whatever the display can manage.

How bright is white

An sRGB value says a pixel is some fraction of whatever the display can manage. A PQ value says it is two hundred candelas. The change is the largest in display encoding since gamma, and what it removed was the honest admission that nobody knew.

limits · Limits The appearance model's three rooms, read as three moments. CIECAM16's degree of adaptation is a function of the surround and the adapting luminance and of nothing else — the model has no time in it. Solving for the moment at which an observer who will adapt completely has got that far turns each of the three tabulated surrounds into a reading on a clock. At 100 candelas per square metre they are 106, 49, 21 seconds. They are presented as three rooms. They are also one observer, in one room, at three times in the first two minutes.

A viewing condition is a moment

CIECAM16's degree of adaptation is a function of the surround and the adapting luminance and of nothing else, because the model has no time in it. Solving for when an observer who will adapt completely has got that far turns the standard's three surrounds into three clock readings — 107, 50 and 21 seconds — and the two readings are distinguishable by waiting.

brain · Appearance The angle between the two filters against how completely the eye has adapted. The angle, in the local metric, between what an older lens does to a reading and what a denser macular pigment does, on 120 smooth reflectances, as the degree of adaptation runs from nought to one. The median angle is 8 degrees unadapted and 156 at complete adaptation, and almost all of the turn happens in the last tenth: it passes a right angle at a degree of 0.928. The marks are the degrees CIECAM16 gives five rooms — an overcast sky 1.00, an office 0.94, a lit living room 0.86, a dim room 0.75, a cinema 0.66 — so only the outdoor one is at the end of the dial.

Two filters cancel only in a bright enough room

An older lens and a denser macular pigment cancel each other once an eye has adapted — and that result belongs to the end of a dial nobody stands at. Read at the degree of adaptation CIECAM16 gives an ordinary room, the two barely cancel; in a living room they add, and in a cinema they cost six times what they cost under the sky. The room has to be about as bright as an office before the cancelling begins at all.

difference · Metric The same pairs, held at one colour difference, read in a unit that knows the room. 23 pairs of reflectances built to sit at exactly ΔE₀₀ 1.000 under D65, read in CAM16-UCS as the adapting luminance runs from a third of a candela a square metre to ten thousand, in an average surround. ΔE₀₀ has no argument for the room, so in that formula every pair stays at 1.000 all the way across — the flat line. In the model's unit the same pairs rise from a median of 0.76 to 1.30, and they do not rise together: at the bright end they run from 1.11 to 1.65.

A tolerance has no light level

Twenty-three pairs built at exactly one colour difference stay at exactly one in every room, because the formula has no argument for the room. Read in the unit that does have one, the same pairs are 0.72 in a cinema, 1.04 in an office and 1.30 in direct sun — and inside any one room they spread by half again, so no single conversion between the two units exists at all.

difference · Metric How far CIECAM16 moves a monochromatic hue when the light is brightened thirtyfold. A monochromatic stimulus at a relative luminance of 2 and of 60, read through CIECAM16 in one room, and the difference between its two hue angles. The model was never fitted to this effect and has it anyway: the shift is positive at the short end, negative through the greens, positive again in the yellows and reds, and crosses zero at 459, 495, 502, 570 nanometres. The marks are the invariant wavelengths the literature reports — 474, 506, 571.

The model has a hue shift it was never given

A monochromatic light changes hue as it is brightened, except at three wavelengths that do not move — an effect measured since the nineteenth century and not among the things CIECAM16 was fitted to. The model has it anyway: brightening a stimulus thirtyfold moves its hue angle, and the places where the movement crosses zero land at 459, 495, 502 and 570 nanometres against the reported 474, 506 and 571. The sizes are another matter.

brain · Appearance Twenty-three pairs at one ΔE₀₀, read in two units that know the light level. Twenty-three pairs of surface colours, each exactly one ΔE₀₀ apart, on a display whose white runs from 1.5 to 10,000 cd/m² across, with a background at a fifth of the white. ΔE₀₀ has no argument for the light and stays at one. The median ΔEITP rises from 1.01 to 2.75 and flattens near the top, and the median CAM16-UCS distance from 0.76 to 1.20.

Two units with a light level disagree about lightness

ΔE₀₀ has no argument for how bright a display is. Two colour differences do: CAM16-UCS takes the room's adapting luminance, and ΔEITP — the difference defined for high-dynamic-range television — takes the stimulus's own absolute luminance. Twenty-three pairs at exactly one ΔE₀₀ grow in both as the display brightens, 2.1 times in ΔEITP and 1.5 in CAM16-UCS from a 5 to a 5,000 cd/m² white. But in ΔEITP the lightness part grows fastest, 2.7 times, and in CAM16-UCS it does not grow at all.

difference · Metric How each unit balances lightness against chroma, as a display brightens. Twelve pairs built to differ in lightness alone and twelve built to differ in chroma alone, each at exactly one ΔE₀₀, read in both units at nine display levels. Up is the median lightness pair's reading divided by the median chroma pair's, so a falling curve means chroma differences becoming relatively more visible. Both fall. ΔEITP's median runs from 0.91 at a 1.5-candela white to 0.81 at 10,000, a fall of 11 per cent; CAM16-UCS's from 1.13 to 0.87, a fall of 23 per cent. Neither turns: both units say chroma differences gain on lightness differences as a display brightens, and CAM16-UCS says it twice as strongly.

The units part by hue, not by light level

Two colour differences with a light level in them were compared on a display running from 5 to 5,000 cd/m², and a prediction was drawn from how each divided a difference between lightness and chroma. Built into pairs that differ in lightness alone and in chroma alone, both units say the same thing about light level: as a display brightens, chroma differences gain on lightness differences — ΔEITP by a tenth, CAM16-UCS by a quarter. Where they part is hue. The appearance model moves every colour's balance by the same factor; ΔEITP moves the violets, reds and cyan-blues the other way, and at nine of twelve base colours the two units disagree about the direction.

difference · Metric What a lit room takes, and where it takes it. On a display whose white is 1,000 cd/m², how much of the ΔEITP a one-unit lightness step is given survives a veiling luminance reflected off the screen, against where on the lightness scale the step sits. At L 2 — a deep shadow — one candela of reflected light removes 22 per cent of the difference and three candelas remove 46. At L 90 the same veils remove nothing measurable. The shadows the unit counts most are the ones a room removes first.

The shadows a unit counts are the ones a room removes

ΔEITP's growth with display brightness is largest in the dark greys, and dark greys are where a lit room's light reflected off the screen sits. One candela a square metre of veiling luminance removes 22 per cent of the difference the unit gives a step at the bottom of the scale on a 1,000-candela display and nothing measurable at the top. The same veil raises the growth the unit reports across display levels from a factor of six to a factor of seventeen, because it destroys a dim display's shadows first.

difference · Metric The appearance model's lightness-to-chroma balance, as the room takes a share of the adaptation. The median lightness pair's reading over the median chroma pair's, for twelve base colours, against the display's white, in a room of 20 cd/m². Solid: CAM16-UCS, with the viewer taking all, three quarters, half, a quarter and none of the adaptation from the display — darker lines take more from the display. Dashed: ΔEITP, which no room enters. With the display alone the model's balance falls 23 per cent; with half from the room, 13; with none from the display it does not move.

A lit room brings the units' medians together

As a display brightens, CAM16-UCS says chroma differences gain a quarter on lightness differences and ΔEITP says a tenth. All of the appearance model's movement comes from what the viewer is adapted to, and every calculation had the viewer adapted to the display alone. Give the room its share of the adaptation and the model's movement shrinks at every step: with a 20 cd/m² room supplying two thirds of it, the two units' medians fall by the same amount. What does not shrink is their disagreement about direction. The model still moves every colour the same way, ΔEITP still moves violets, reds and cyan-blues the other way, and in a lit room that becomes the whole of what separates them.

difference · Metric How wrong a declared veil makes the unit, for four true veils. On a 100 cd/m² display, the worst error over grey steps from L 2 to L 90 — the size of the natural logarithm of the declared reading over the true one — against the veil declared, for rooms putting 0.1, 0.3, 1 and 3 cd/m² on the screen. Each curve reaches nought at its own true veil and rises on both sides. The flat stretch at the left is declaring almost nothing, which is declaring none: 0.22 for a true veil of 0.1, 0.54 for a true veil of 0.3, 1.14 for a true veil of 1, 1.89 for a true veil of 3.

A guessed veil halves the error

A colour difference that takes a display's absolute luminance leaves out the light a room reflects off the screen, and on an ordinary display in an ordinary room that makes it wrong about the darkest greys by a factor of three. Giving the unit the veil as a declared argument fixes that when the veil is known. The worry was that it never would be — that a guessed argument is no better than none. It is better: any declared veil up to about twice the true one beats declaring none, and one middling guess for every room halves the worst error. What a guess cannot do is reach ten per cent; that needs the veil known within a sixth, which is what a luminance meter aimed at a black screen gives.

difference · Metric The hue disagreement with the quantiser, and without it. For twelve base colours of CIELAB lightness 50 and chroma 40, the factor by which ΔEITP's reading of a lightness pair over its reading of a chroma pair changes from a 1.5 to a 10,000 candela white. With the perceptual quantiser, it runs from ×0.75 at the yellow-greens to ×1.23 at the violets. With the quantiser replaced by a pure power law and nothing else changed, it is ×1 at every hue.

The hue disagreement is the quantiser's

As a display brightens, the appearance model shifts every colour's balance between lightness and chroma by one factor, and ΔEITP shifts violets towards lightness and yellow-greens towards chroma. The proposed source was the perceptual quantiser acting on three cone-like signals at unequal levels. Replacing the quantiser with a pure power law and changing nothing else removes the disagreement exactly: every colour moves by a factor of one. With the quantiser, how far a colour moves follows the spread of its three signals, and which way follows where its S signal sits — above the other two, towards lightness; below, towards chroma.

difference · Metric Three panels in three rooms: the veil alone, and the best single floor. On a 1,000 cd/m² display, for each panel and room, the worst error over five-per-cent grey steps from 0.005 cd/m² upwards of declaring only the room's veil (upper bar) and of the best single declared floor (lower bar). For an LCD without dimming and for an OLED the best floor is the display's black plus the veil, and it is exact. For the LCD it is also necessary: the veil alone misreads by 2.8 in a dark room. For a local-dimming LCD no single floor is exact, and in a dark room the best errs by 0.96.

A dimming panel has no single black

ΔEITP reads a display's shadows correctly only when it is told the floor under every pixel, and that floor has two parts: the room's light reflected off the screen and the display's own black. They fold into one declared number exactly, and on a bright LCD the display's part is the one that matters — leaving it out misreads the deepest shadows seventeenfold. A local-dimming panel breaks the folding, because its black changes with the picture: in a dark room its floors vary twenty-six times across one scene, and no single declaration comes within a factor of two and a half. A lit room squeezes them back into one number.

difference · Metric A local-dimming panel's backlight behind a small highlight on black. The backlight of each of a panel's 32 by 18 dimming zones behind a small highlight on black, on a logarithmic scale from one per cent to full, after a diffuser that spreads each zone's light with a standard deviation of 1 zone. The zones driven by the picture are the darkest or the brightest; the diffuser's halo fills 3.5 per cent of the screen with levels in between, and each zone's floor follows its backlight.

Two floors trade the screen for the halo

A local-dimming panel in a dark room has no single black: its floor varies twenty-six times across a scene, and one declared floor misreads ΔEITP's grey steps by a factor of 2.6. The proposal was two floors, one for dimmed zones and one for lit ones, on the reasoning that a picture's zones cluster at the two ends. They do cluster, and two floors still do not work. Placed for the worst zone they halve the error and misread almost the whole screen a little; placed on the two clusters they read most of the screen exactly and misread the halo between as badly as one floor. How many floors a panel needs is set by its diffuser, not by the picture — and the only declaration that works everywhere is the backlight map.

difference · Metric How far each unit moves a colour's balance, at the most saturated colour BT.2020 allows. For every fifteen degrees of hue at lightness 25, 50 and 75, the most saturated base colour whose one-unit lightness and chroma pairs stay inside a BT.2020 display, and the factor by which each unit's ratio of the two pairs changes from a 1.5 to a 10,000 candela display. CAM16-UCS moves every one by ×0.76 to ×0.89. ΔEITP moves them from ×0.54, a yellow-green at hue 90, to ×1.64, a red at hue 30 — with the red-purples beside it and the dark blues well below.

At the gamut's edge the reds move as far as the violets

As a display brightens, ΔEITP shifts a saturated colour's balance between lightness and chroma by an amount set by its three quantised signals, and CAM16-UCS shifts every colour alike. The largest shifts found so far were a yellow-green at ×0.63 and a dark blue at ×1.62, both at the census's edge — and the blue turns out to be a colour no BT.2020 display can show. Walked out to the display's real boundary, the yellow-greens go lower, to ×0.54, and the dark blues never pass ×1.35, because BT.2020's blue edge is at low chroma. The top of the range goes to the reds and red-purples at ×1.6: at the edge the L-minus-M term grows to two thirds of the S term. Pairing the two extremes still needs a third of the observers the violet experiment needs.

difference · Metric How much of the halo the eye's own glare hides. For three pictures behind three diffusers, the share of a local-dimming panel's zones in which declaring each cluster's own floor misreads a five-per-cent grey step by more than ten per cent — as the panel delivers the steps, and with the veil the eye's own scattered light adds from every bright zone. The eye hides the whole misreading in six cases of nine, all three behind the window; what it leaves is behind the widest diffuser for the small highlight, and behind the two wider ones for the subtitles.

The eye's own glare hides most of the halo

A local-dimming panel's black can be declared as two floors, and the pair misreads only the diffuser's halo round the bright content — by up to a factor of 2.7. The halo sits next to the light, and light scattered inside the viewer's eye puts a floor of its own there. Added under every grey step, the eye's veil hides the whole misreading in six of nine cases: every window, and the highlight and subtitles behind the narrowest diffuser. It leaves a sixth of the misreading's share for a small highlight behind the widest diffuser, and a quarter for subtitles, all of it in the halo's inner half — not its outer edge, as predicted. The panel dims by a zone's brightest pixel and the eye veils by its average, so a line of text lights the backlight fully and the eye a quarter. The viewing distance cancels.

difference · Metric Where a red window's veil leaves the halo tinted: σ 2. The 32 by 18 zones behind a window filling a fifth of a dark room drawn in red, with a diffuser of spread 2: the zones where the cluster declaration's misread floor, seen through the eye's red veil, tints a grey by more than one ΔEITP — 18.9 per cent of the screen, at worst 4.5 — and those still misread in lightness by more than ten per cent, 4.3 per cent.

A coloured highlight tints the halo it cannot hide

A local-dimming panel lights a zone for its brightest channel, so a pure red or blue highlight drives the backlight as hard as a white one and sends a fifth or a fourteenth of the light into the viewer's eye. The prediction was that a saturated highlight's veil would hide the halo's misread greys as a white one does, and add a colour. It hides far less. Behind the middle diffuser a white highlight hides the whole misreading; drawn red, blue, green or even yellow — nine tenths of white's luminance — it hides none of it. The yellow fails because a veil hides a grey step only in the cone signals it reaches, and yellow light reaches a fifth of the short-wave one. And where the veil does fall on a misread floor, it colours the error: the darkest greys are tinted by up to fifteen ΔEITP, and behind a red window, where the veil hides the lightness almost entirely, a fifth of the screen is tinted by more than a unit.

difference · Metric

Named alongside it

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

ΔEHigh dynamic rangeToleranceViewing conditionPQ, the perceptual quantiserCIECAM16LightnessChromaSurroundVeiling glareCIEDE2000Colour difference

All concepts