Light, Spectrum, Observer

A colour is not a property of an object. It is what happens when a particular light meets a particular surface and a particular pair of eyes looks at the result. Take any one of the three away and there is no colour to talk about — which is why a colour number always carries all three with it, whether or not anyone wrote them down.

Three things, and a number at the end

What it is	Described by	Belongs to
The light	How much energy it emits at each wavelength	the room
The surface	What fraction of each wavelength it gives back	the object
The observer	How sensitive the eye is to each wavelength	the person

Multiply the three together, wavelength by wavelength, add up the result, and out comes a colour in numbers. Every Lab value you have ever seen was produced that way. The arithmetic is not the interesting part — the interesting part is that only one of the three belongs to the object.

The spectrum: what the object actually is

Point a spectrophotometer at a printed patch and it reports a curve: at 380 nm this much comes back, at 390 nm this much, and so on to 730 nm. ChromaSpot records 36 such readings for every sample, which is the whole of what the surface does to light.

That curve does not change when you carry the sample into another room. It is a property of the ink and the paper, in the way that weight is a property of an object and not of the scale. Everything else on this site is computed from it.

Why 380 to 730 nanometres. Below 380 is ultraviolet, which the eye cannot see but which brighteners in paper turn into visible blue — see Optical Brighteners. Above 730 is infrared, which nothing in printing cares about. The 36 readings between are where colour happens.

The light: why the same object measures differently

A spectrum of a light source is the same kind of curve, read the other way: how much it emits at each wavelength. Daylight is broad and fairly even. An incandescent lamp is heavily weighted to the red end. A fluorescent tube has sharp spikes where its phosphors happen to emit and near-nothing between them.

Feed a different lamp into the multiplication and a different number comes out. Not because the surface changed — because you asked a different question. The eleven standard lights the library offers are named descriptions of common rooms, and how far each one moves a colour is measured on the Colour Inconstancy page.

The observer: whose eyes

The last curve is the one people forget. The standard observer is a description of average human colour vision, built in the 1930s from a small number of people matching colours in a laboratory. It exists so that two laboratories computing the same measurement get the same answer.

There are two of them in common use, and they disagree.

Observer	Made for	Used in
CIE 1931, 2°	A small patch, held at arm’s length	Printing and graphic arts — and this library’s codes
CIE 1964, 10°	A large field, filling much of your view	Interiors, automotive, textiles, plastics

The 2° observer covers about the area of a thumbnail at arm’s length. Anything bigger — a wall, a car door, a bolt of cloth — falls on parts of the retina with a different mix of cones, and the 10° observer describes that instead.

Swapping the observer from 2° to 10° moves the median colour in this library by 1.98 ΔE₀₀ — further than the step to its nearest neighbour in the book. Same light. Same sample. Same instrument.

That is not a rounding difference; it is a different answer to a different question. Which is why every ΔE worth quoting names its observer, and why a figure measured at 2° cannot be compared with one measured at 10°.

And the observer is an average

Real eyes scatter around the standard. Two people at the same booth, looking at the same pair of samples under the same lamp, can honestly disagree about whether they match — and neither of them is wrong. Colour science has a name for that — observer metamerism — and it decides which colour matches are robust and which are balanced on a knife edge.

What this means for a specification

A colour written down as three Lab numbers has silently answered all three questions and told you none of the answers. A colour written down as a spectrum has answered only the one that belongs to the object, and left the other two open for whoever needs them.