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

Photon noise — where it appears

The square root of a count, which is what a receptor reports and what sets the light level below which a difference cannot be seen. It is the floor no better sensor or better algorithm removes, because it is a property of the light rather than of the measurement.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Photons caught by one cone in one integration time, under D65. Each curve is one cone class, counting isomerisations during a 0.1 s integration through a pupil that closes as the light rises. At 1000 cd/m² a long-wavelength cone catches about 11522 and at 0.001 it catches 0.12 — which is where the square root of the count stops being a small correction and starts being the signal. The rate works out at 30 isomerisations per second per troland, inside the published band of 5–50.

Colour goes first in the dark

A cone reports a count, and a count carries the square root of itself as noise. Counting the photons says where colour vision stops — a chromatic difference runs out four hundred times sooner than a lightness difference of the same size — and says just as clearly that in daylight the eye is nowhere near that limit.

eye · Cones The colourfulness the sRGB cube reaches, against the light in the room. The display is the same display throughout and the signal is the same signal. What moves is the adapting luminance, which enters the appearance model and nothing else. The furthest colourfulness the cube reaches falls from 131 to 57, a factor of 2.29. A gamut in CIELAB cannot show this at all, because CIELAB has no light level in it — which is why every gamut percentage in circulation is quoted without one.

The gamut shrinks in the dark

Every gamut number here is a property of a device and of nothing else, which is why the same display has the same gamut in a studio and in a cinema. It does not. Turn the room down four decades and the solid loses a fifth of its reach and two thirds of its volume, with the signal unchanged.

limits · Limits

Named alongside it

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

AdaptationCIECAM16Colour appearanceColourfulnessDisplay gamutGamutIndividual variationJust-noticeable differenceLuminanceObject-colour solidOptical densityRetinal illuminance

All concepts