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

Degree of adaptation — where it appears

How completely an eye discounts the colour of the light it is under, entered in an appearance model as a number between zero and one. At one the adapting white is discounted exactly; below it a residue of the light's own colour survives into what is seen, which is where a measured colour and an appearance part company.

Named by 8 essays across 3 fields — each of them below, with the objects they name alongside it.

How much of the residual a partial correction removes. Between the diagonal gain and the exact matrix there is a line: apply the correction that would make a row exact, but only a fraction of it. The horizontal axis is that fraction and the vertical is the share of the row's residual it removes, for all fourteen census rows. The straight diagonal is where a correction worth exactly its fraction would fall, and in the published unit every curve lies on it to within 2.2 percentage points. The lower band of curves is the same interpolation measured in CAM16-UCS, which departs by up to 17 points — because its distance is a power of the Euclidean one and a power is not homogeneous along a ray, where every ordinary norm is. The straight line is therefore a property of the ruler rather than of the correction, and the exception is what says so.

A partial correction is worth its fraction

Between a diagonal gain and the exact matrix there is a line, and a bounded observer's natural hope is that the first part of it is worth a disproportionate share. It is not. On all fourteen changes of light, at every setting, the share of the residual removed matches the share of the correction applied to within 2.2 percentage points — which closes the last way the gap could have been cheap.

scene · Scene A gloss finish's loss of colour, read by the light and by a viewer in the room. Four changes against the matt room as the coloured walls are made glossier, from roughness 0.8 to 0.15: the chroma of the room's reflected light as a colorimeter reads it; the mean chroma of the six faces as CIECAM16 sees them adapted to the lamp and adapted to the room's own average light; and how far the faces sit from that average in the model's uniform space. At roughness 0.2 the light loses 8.3 per cent, the faces 8.6 per cent to the lamp-adapted viewer and 13.9 to the room-adapted one, and the spread 11.0 per cent against 11.2 read against the lamp.

A gloss room looks less colourful than it measures

A gloss finish takes 8.3 per cent of the chroma out of a green room's reflected light, and a viewer adapted to the room should discount a loss that affects everything alike. The appearance model says the opposite. Adaptation removes the colour the whole room shares, leaves the colour that differs from face to face, and the finish takes as large a share of that as of anything — so to a viewer standing in the room the faces lose 13.9 per cent of their chroma, not 8.6.

scene · Scene 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 What the choice of adopted white is worth, in a room lit by two lights. A room lit half by daylight and half by an incandescent lamp. The light on the surfaces is fixed; what varies is the white the model is told the observer has adapted to, running from the lamp on the left to the window on the right. The median of twelve surfaces moves 32.6 CAM16-UCS units if the lamp is adopted and 24.5 if the window is, against the room's own mixture in the middle. The model offers no way to choose, and its degree of adaptation is 0.94 at every point of the dial.

A room with two lights has no white

An appearance model takes one adapting white. A desk beside a window has two, and the mixture falling on the paper is not the same thing as the white the person reading it has adapted to. Mixing the lights is arithmetic. Choosing the white is not, and the choice is worth sixty units of appearance — most of which is a cast, and not all of which is.

brain · Appearance One lightness scale, drawn as a contour across the rooms. Three surrounds up the page and the background's luminance factor across it, on a square-root scale so that the model's own exponent base is linear in the axis. Each curve joins the rooms whose lightness exponent is the same, and every room on one curve returns the same lightness for every sample. The marked curve is the one through a television in a lit living room against a mid grey: it also passes through a print on a desk against a background of 2.8 and a projection in a dark room against 47.0. A curve that leaves the plot has no member in that surround, because each surround multiplies a base that runs only from 1.48 to 2.48.

Two rooms with one lightness scale

CIECAM16's surround and its background both reach lightness, and they reach it through one product. So the rooms fall into classes: a television in a lit living room against a mid grey returns exactly the lightness a print on a desk against a background of 2.8 does, for every sample, to the last bit of a double. It returns 0.76 of its chroma and 0.81 of its brightness. Two of the model's four viewing-condition parameters are one parameter, and only colour tells them apart.

brain · Appearance Every pair of departures, before adaptation and after it. The fifteen pairs of the six audited observer departures. Each row runs from the angle between that pair's two deviations with no adaptation to the angle with complete adaptation; an angle past ninety degrees is a pair pointing apart, which is where a pair can cost less together than the larger of the two costs alone. 3 pairs gain that behaviour as the eye adapts, 3 keep it, 5 lose it and 4 never have it. The pair followed here — the lens against the macular pigment — is in the smallest group that is not empty, and every result quoted from it generalises in the wrong direction.

Adaptation turns more pairs off than on

One pair of observer departures was followed across the degree of adaptation and found to cancel only in a bright enough room. The same calculation takes any two, and run over all fifteen pairs it says something the single pair does not: adaptation is a rotation rather than a mechanism for making departures oppose each other. Five pairs lose their cancellation as the eye adapts, three gain it, three keep it and four never have it — and the pair everybody quotes is one of the three it turns on.

difference · Metric Six departures, and three ways of adding them up. The six audited observer departures on 120 smooth reflectances, across the dial. Summing them assumes they all point the same way and is an overestimate everywhere; taking the largest alone assumes only one matters and is an underestimate everywhere. Quadrature — the usual way of combining contributions taken to be independent — assumes they are mutually perpendicular, and the measured combination crosses it at a degree of 0.8618. Below that the departures are on balance pointing together and quadrature is too small; above it they are on balance pointing apart and quadrature is too large. It is exactly right in one room.

Quadrature is exact in one room

An observer allowance is built by adding the departures in quadrature, which assumes they are mutually perpendicular. Over fifteen pairs their angles run from 18 degrees to 179 and hardly any are perpendicular. Measured against the real combination, quadrature is too small in a cinema by eight per cent and too large under the sky by fourteen, crossing at an adapting luminance of 22 candelas a square metre — and on individual surfaces it is out by a third in both directions in every room.

difference · Metric Where a surface's band sits decides the room it needs. Each of 168 surfaces at its own crossing — the degree of adaptation at which the part adaptation has yet to remove falls to the size of the part it will leave — against where that surface's absorption band sits. The line joins the median at each band centre and the rooms are marked across. A surface absorbing at 470 nm crosses at 0.883 and one absorbing at 670 nm at 0.972. Both of the filters this is about absorb in the blue, so a surface with a blue band is where the two differ in shape and has a large residual, while a surface with a red band is nearly invisible to both and its whole deviation is the shared yellowing of the observer's white.

The room a surface needs is written in its band

Whether two yellow filters cancel on a surface depends on the room, and each surface has its own crossing — the degree of adaptation at which the shared yellowing falls to the size of what is left underneath. Those crossings run from 0.68 to 0.99, and where a surface's absorption band sits accounts for almost all of the spread while how much light it returns accounts for almost none. The reds need a room brighter than a graphic-arts viewing booth, which is brighter than any room a sample is judged in.

difference · Metric

Named alongside it

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

Viewing conditionAdaptationCIECAM16Colour differenceIndividual variationChromatic adaptationLens yellowingMacular pigmentSpecificationToleranceAuditColour appearance

All concepts