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

Lens density — where it appears

The optical density of the crystalline lens, which absorbs strongly at the short-wavelength end and increases through life. It is one of the two filters in front of the receptors, so it changes what the short-wave cone catches and leaves the other two nearly alone — which makes it a large part of why two observers differ.

Named by 5 essays across 2 fields — each of them below, with the objects they name alongside it.

A template that cannot place a point, fitted three ways. Three rows, one per set of stimuli the pigment template's cone matrix can be fitted over, each listing the three confusion points that matrix implies. The protanope's point wanders from (0.99, 0.20) to (0.76, 0.13) against a measured (0.75, 0.25), and the deuteranope's moves by 22.5 in chromaticity — further than the whole diagram is wide. A copunctal point is where two nearly parallel planes meet, so a template good to a few per cent, which is far more than enough to place a spectrum, is nowhere near enough to place this. It is why the population is built by moving the measured points rather than by deriving them.

A template cannot place a point

This collection's model of an eye is good to a few tenths of a per cent at predicting what a cone catches, which is far more than enough to place a spectrum. Asked where that eye's confusion points are, it puts the protanope's at (0.99, 0.20) against a measured (0.75, 0.25) and the deuteranope's anywhere from (1.1, −0.5) to (−18, 11) depending on which stimuli the fit was made over.

eye · Cones A population of receptor bases, in the plane the published ones live in. The two axes this collection scores an adaptation basis on — the residual across the illumination census on the horizontal, ellipse anisotropy on the vertical, lower better on both — with 200 extra points on it. Each is the basis a member of the population's own confusion points determine. The cloud is not a point: it runs from 1.22 to 2.24 ΔE00 horizontally, which is wider than the whole spread of the published transforms marked on it. The observer this site quotes sits inside the cloud and near one edge of it, and the sentence "the receptor basis costs seventy per cent" is a sentence about that one point rather than about the construction.

A trade between matrices, not people

Across the space of possible bases, adapting well and discriminating well pull in opposite directions — the two optima sit at the ends of an empty diagonal. Across a population of actual observers the same two costs move weakly together, at a correlation of +0.29. The trade-off is a property of the set of matrices somebody could choose, not of the eyes anybody has. One measurement inside the population does trade, and it is the macular pigment.

brain · Appearance More rods make a lens band's pull on the rod weight worse, unless the band is fitted. A sixty-year-old's lens with a band of a tenth of a density at 450 nm, fitted by the model with the rod signal a tenth to a half of each cone's peak: the shift in the fitted rod weight, in its own standard errors. With weight, lens age and macula fitted, one session's shift grows from +1.73 at a tenth to +2.53 at a third, and two sessions' more. With the band's amplitude fitted as a fourth number, one session stays under half a standard error at every share.

A dimmer session hides a lens band in the rods

A band of extra absorption in an older lens is read by a colour-matching session as lens age or as macular pigment, and it pulls the fitted rod weight with it. The rods see the lens and not the macula, so a dimmer session, where they carry a third of the signal instead of a tenth, was predicted to tell the band from the macula. It does the opposite. With the band at 450 nm left out of the fit, the rod weight's shift grows from 1.7 of its standard errors to 2.5, the misfit a session could notice shrinks by a third, and the macula and the band trade more tightly. The rods see the band — and the rod weight, which the fit is free to move, is exactly what absorbs it. What separates the band from the macula is fitting its amplitude as a fourth number, which works in daylight already.

eye · Cones How well a matching session places a lens band's centre. A sixty-year-old's lens with a band of a tenth of a density centred from 410 to 460 nm, fitted with the rod weight, lens age, macula, the band's amplitude and its centre all free: the centre's standard error, from one daylight-against-LED session and from designs adding a second session. One session places the centre to 11.5 nm at worst below 430 and within 5.7 above it; adding the LED against the three-emitter lamp brings every centre within 5.5.

A second lamp pair finds a lens band's centre

A band of extra absorption in an older lens is found by a colour-matching fit only if the fit has a parameter for it, and a band parameter at a fixed wavelength finds only a band near that wavelength. Freeing the band's centre as a fifth number asks whether the matches can place it. From one daylight-against-LED session they place a band above 430 nm to within six nanometres and one below it only to eight to twelve, where moving the band and ageing the lens change the matches together — and there they barely see the band at all. A second session, the LED against a three-emitter lamp, places every centre within five and a half nanometres. The fluorescent tube's line at 436 nm helps where it sits and nowhere else.

eye · Cones From which starts a fit finds a lens band, two sessions. For a band of a tenth of a density at 415, 430 and 445 nm in a sixty-year-old's lens, the number of twenty simulated experiments of two sessions each in which a fit of all five numbers, started with the band's centre at each of 400 to 470 nm, finds the centre within twice its standard error. The ticks at the left edge are the same count for a fit started at the true observer: 18, 18, 18 for the three bands.

A lost lens band goes to age or the macula

Two colour-matching sessions determine the centre of a band of extra absorption in an older lens to about five nanometres, if a fit can find it. Fitted in full from eight starts across 400 to 470 nm on twenty simulated experiments, two sessions find a band at 415, 430 or 445 nm from every start in fifteen to eighteen experiments of twenty, against eighteen for a fit started at the truth. The prediction was that a low band would be lost from high starts to the macula's flank. The start barely matters with two sessions, and a lost band does go somewhere — but a band at 415 or 430 goes to lens age, three to five years of it, and only a band at 445 goes to the macula. Every loss is a worse fit than the session's best, so eight starts find the band as often as the truth does. With one session the start decides.

eye · Cones

Named alongside it

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

Macular pigmentIdentifiabilityLeast-squaresIndividual variationModelling assumptionStandard errorBasisCone fundamentalsLens yellowingAnisotropyChromatic adaptationCondition number

All concepts