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

Contrast ratio — where it appears

The ratio between the brightest and darkest a display can produce, quoted from a measurement made in the dark. In a lit room the reflected ambient sets the practical black, so the number a specification gives is not the number a reader gets.

Named by 6 essays across 3 fields — each of them below, with the objects they name alongside it.

Brightness against chroma, at exactly constant luminance. Seven stimuli of identical luminance and rising chroma at hue 25. The model's brightness moves by 2.2 per cent across the whole sweep, and at some hues it moves the other way. The Helmholtz–Kohlrausch effect — measured repeatedly, by several methods — is that a saturated colour looks as bright as a neutral of 1.3 to 2 times its luminance, shown as the band. The gap is the model's, and nothing here closes it: the term that would is not in CIECAM16 and is not invented for the occasion.

Brightness is not luminance

Photometry is additive because the CIE defined it that way. Brightness is not, and a saturated colour looks as bright as a neutral of one and a half times its luminance — an effect this site's appearance model moves by two per cent, in a direction that depends on the hue.

brain · Appearance How much colour a display delivers, against how bright the room is. The appearance solid of the whole code cube, with the room's reflected light added to every code value and the surround ratio computed from the room's own white against the display's. A 300 cd/m² panel delivers most at 200 lux, and the curve falls on both sides: darker costs the surround, brighter costs the black. At the 32 lux the softproofing standards specify, the solid is 89 per cent of its best — which is not a criticism of the standard, since it is written for matching a screen to a print viewing booth rather than for delivering the most colour.

A display in a room is a smaller display

Two results here turn the room's light in opposite directions and neither knows about the other. A room has one light level and does both at once — so there is a brightness at which a display delivers the most colour, it is not at either end, and it scales with the panel.

limits · Limits 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 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 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

Named alongside it

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

Viewing conditionAbsolute luminanceΔEHigh dynamic rangeToleranceDeclared inputDynamic rangeVeiling glareCIECAM16Colour appearanceLightnessPQ, the perceptual quantiser

All concepts