Six functions of wavelength, and the six different places they stop
Drawn at its defaults, in the band below 380 nanometres.
It reads 3 options and
no others: show, stockKey, which.
Called by 8 essays
the blast radius of changing it
The tables do not stop together
This collection integrates from 380 to 780 nanometres, and decided once, in writing, that the range could not honestly be widened. The argument was correct at the long end and wrong at the short one — the CIE publishes the daylight basis from 300 nanometres, and publishes it from there for exactly the reason it matters.
What the eye doesThe eye stops at the lens
Neither standard observer is tabulated below 360 nanometres, and the reason is not that the photopigments stop absorbing there. It is that the light never arrives — the cornea and the crystalline lens take it — so the short-wave limit of human colour vision is a piece of optics, it moves by a factor of twenty across a lifetime, and it can be surgically removed.
What a scene doesTwo sheets that match until the window
Ordinary metamerism is two reflectances that agree under one light and not another, and it can always be undone by putting the first light back. A dyed sheet and a brightened one have the same reflectance everywhere an eye can see, agree exactly under any lamp with no ultraviolet, and separate by ten units under daylight — and no change of light puts them back together.
What a scene doesThe window is part of the light
A viewing condition here has always been a spectrum and a geometry. For anything fluorescent it needs a third thing — the transmittance of whatever the daylight came through — because ordinary window glass, a laminated windscreen and a museum filter remove three quite different parts of the band a brightener eats, and the sheet is a different colour behind each.
What a camera doesA camera cannot record the excitation
Two lamps of nearly the same chromaticity, one with ultraviolet and one with none, put a brightened sheet twelve units apart after a perfect white balance. Nothing in the camera measured the difference — the filter stack removed the band before the sensor saw it — so the correction that would fix the picture needs a quantity the file does not contain.
What a camera doesThe 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.
What light isThe ultraviolet is half the product
A fluorescent sample's departure from the reflectance model is exactly proportional to the ultraviolet the lamp carries — to twelve figures, over a range that includes zero. It is the only one of the round's four departures that really is a product of two magnitudes, and the reason is a rule about how molecules relax rather than anything about arithmetic.
What light isThe grid is a range, not an index
Three of the four departures in this round restore an argument the model dropped. The fourth does not — the 380-to-780-nanometre grid is the range of the one argument the model kept, chosen in the first weeks and never revisited. It costs 6.70 ΔE₀₀ on a coated printing paper, it is the cheapest of the four to fix, and it is the one still unfixed.