Generator
A spectrum, weighted three ways, and the three numbers left over
The illuminant D65 above; below, the same spectrum multiplied by each matching function. The area under each product is one coordinate of XYZ. Everything else about the spectrum — its shape, its structure, all its remaining degrees of freedom — is discarded here.
Drawn at its defaults, in spectra and illuminants. It takes no options at all, so every essay calling it gets this exact drawing.
Called by 2 essays
the blast radius of changing it
What the eye does
Three numbers
A spectrum has as many degrees of freedom as anyone cares to give it. The eye reports three. Everything colour science can do, and every way it fails, follows from that one collapse.
Matching and measuringThe eye weights where the light is not
A brightened sheet returns a quarter more light than arrives at 430 nanometres, and it is three tenths of one per cent brighter for it. The luminous efficiency function is 0.017 there against 1.0 in the middle of the band, so the whole effect lands in the blue-yellow axis — brighter than white is a colour claim wearing a brightness word.