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

Phosphor — where it appears

A material that absorbs short-wave light and re-emits it at longer wavelengths, used to turn a discharge's ultraviolet or an LED's blue into visible light. Its emission may be a broad band or a cluster of narrow lines, and that shape decides most of how a lamp renders colour.

Named by 5 essays across 3 fields — each of them below, with the objects they name alongside it.

The lamps the four conditions shine, where they differ. The short-wave half of what each measurement condition puts on the sample, plotted to 560 nanometres because past that the three are indistinguishable in shape. M₁ is D50 with its ultraviolet; M₂ is the same lamp behind a cut filter at 400 nanometres, and at 360 it is 0.0 per cent of what M₁ delivers; M₀ is a tungsten lamp, which has some ultraviolet, has less than daylight, and is not specified at all by the standard — so two M₀ instruments need not agree with each other. M₃ is not plotted because its lamp is M₂'s; what makes it a fourth condition is a polariser.

The lamp that stopped emitting ultraviolet

A blue-pumped white LED has a die at 450 nanometres and emits nothing shorter. Between about 2005 and 2020 that lamp replaced almost every other indoor source, which removed the excitation supply from a great many rooms — so brightened materials stopped glowing indoors without a single one of them being reformulated.

imaging · Capture A pair is refused once it is split by about a grid step. Two 1.2 nm lines centred on 578 nm and split by 0.3 to 4 nm, tabulated through slits of one, two and three nanometres on grids of the same step, and the least miss of any single line fitted to the entries round the peak. Each slit's curve crosses half a per cent between three quarters of its own step and one full step: a one-nanometre table refuses a pair split by 0.9 nm, a two-nanometre table one split by 1.8, a three-nanometre table one split by 3.

A table can refuse a pair it cannot resolve

A narrow spectral table reads each of its peaks as one line and declares the line's width from the entries round it, and the assumption under that is that every peak is a line. Split the mercury tube's yellow into the real pair at 577 and 579 nm and the table notices: through one and two nanometre slits no single line fits the entries within half a per cent, so the table refuses to read a width there. Swept apart, a pair is refused once it is split by about the grid step and admitted when it is closer — always as one wider line, up to 3.3 times too wide. Through three nanometres the pair reads as a 3.2 nm line, and a 3 nm europium band beside it reads the same.

light · Light Inside its family a 620 nm channel reads the tube's error; across the census it does not. The halophosphate tube with its main band at 558, 568, 578, 588 and 598 nm, joined in order: the camera's error under each falls from 2.55 to 1.48 as the band moves up, and the 620 nm channel's departure rises from -0.47 to 0.09 with it. The census's other fifty-three lamps, dots, lie nowhere near that line: the same departure means a different error for an LED.

The tube a third channel misses has no family

Two narrow sensor channels read as one ratio estimate a camera's colour error under fifty-four lamps to ±0.141 ΔE₀₀, and their worst miss is the halophosphate tube, whose trouble lies in its long-wave phosphor. A third narrow channel in the long-wave gap was predicted to halve that miss near 610 nm. No channel does, at any centre or width: the best buys under a tenth of the whole error and leaves the tube's miss above three quarters of what it was. At 610 nm the camera already predicts the tube's share almost exactly. The tube's error follows where its main phosphor band sits, and a 620 or 640 nm channel does read that — inside the tube's own family. The census has one halophosphate tube, and a lamp left out of a census that holds nothing like it is a lamp no channel can be fitted to.

imaging · Capture A triphosphor tube costs a slow pigment what three emitters do; a halophosphate tube what daylight does. For each lamp, the share of the object-colour solid's directions a pigment model cannot reach within one per cent of the ideal — a slope limit, absorption bands no narrower than forty nanometres, and bands whose width is fixed in energy — with the count on each bar. Under the triphosphor tube the band model loses 26 of 154, beside the emitters' 23; under daylight 73 of 154, and under the halophosphate tube 71. The tube's coating alone loses fewer than the tube; its mercury lines, doubled, more.

A tube spares a pigment through its phosphors

Three models of a real pigment lose two and a half to three and a half times as many directions of the object-colour solid under daylight as under three narrow emitters. A fluorescent tube is both kinds of lamp — narrow mercury lines on phosphor bands — and the prediction was that its bands would light the gaps between its lines and put it two thirds of the way to daylight. A triphosphor tube sits with the emitters instead: 26 directions lost to forty-nanometre bands, against the emitters' 23 and daylight's 73. The credit is its phosphors'. Its coating alone, lines taken out, loses fewer still; its mercury lines cost directions whose edges sit in the gaps between the phosphors, and doubled they cost more. A halophosphate tube, the same lines on two broad bands, costs what daylight costs at the same rendering index.

limits · Limits A phosphor grid's camera error is a V about the locus. Forty-eight white LEDs varied in pump, converter share and red phosphor (circles, shaded by red phosphor) and the census's forty-four (squares), placed by how far each white sits from the Planckian locus and by the camera's mean colour error under it. Across the grid the error rises away from the locus on both sides — correlated 0.68 with the distance on the green side and -0.64 on the pink — and is lowest near it. Every one of the census's LEDs sits on the green side, from 0.007 to 0.034.

LEDs varied in their phosphors fall off every line

The merged smooth family estimates a camera's colour error from an ambient-light sensor's departures to ±0.127, and five new white LEDs, varied in their phosphors where the census's forty-four vary their pump, missed by ±0.196 because the family's line had learned almost nothing about the sensor's 640 nm channel. Rebuilt on forty-eight LEDs varied in pump, converter and red phosphor, the line learns it: its 640 nm weight goes from −0.13 to −0.74, most of the way to the −0.85 the new LEDs implied. The prediction was that the new LEDs would then land within ±0.13. They miss by ±0.229, worse than before, crossing to the other side of the line, and the census's own held-out error more than doubles, to ±0.294. No pool of LEDs restores it, and no line in the sensor's departures reads the grid's own LEDs better than ±0.29. Across the grid the camera's error is a V about the Planckian locus, rising away from it on the green side and on the pink — and every one of the census's LEDs sat on the green side.

imaging · Capture

Named alongside it

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

FluorescentSpectral power distributionHeld-out validationLeast-squaresModelling assumptionWhite balanceBandpassCamera rawColour renderingDeclared inputDuvFluorescence

All concepts