Why M2?

Every colour in this library is measured three ways, and one of them decides the codes. The choice was made in 2022 on an argument about where lighting was going. It has since become a clause in a published standard.

The choice, in one sentence

Codes are derived from M2 — the measurement with the ultraviolet cut — because the light the world is moving to has no ultraviolet in it.

The reasons, grouped

Lighting is losing its ultraviolet

LED sources emit essentially none. They are replacing everything, and fluorescent tube production is winding down as mercury is restricted. A brightener that needs ultraviolet to work is a brightener that stops working under the lamp the room will have in five years.

Adding ultraviolet to an LED to compensate would cost money and put a harmful emission into a room, so it will not happen.

M1 is hard to measure consistently

Simulating the ultraviolet content of D50 inside a handheld instrument is genuinely difficult. Two devices that agree closely on M2 can disagree considerably on M1, and there is no agreed procedure for comparing how well two instruments do it — inter-instrument agreement is quantified on M2.

Instruments are calibrated against ceramic tiles that contain no brighteners at all, which is to say the reference the whole measurement chain rests on is an M2 reference.

Brighteners do not stay put

They degrade. Paper loses its brightener over months and years and the white drifts back toward the yellowish base it always was. A definition anchored to how much a sheet fluoresced on the day it was made is a definition with a shelf life.

The industry already works this way

Most ICC profiles for large-format and digital printing are built on M2. Most paint and spot-colour libraries are M2. Most designers are handed M2 coordinates and never know it. Choosing M2 puts this library where the workflow already is.

And now it has a viewing condition to match

Until recently, measuring on M2 meant your booth and your instrument disagreed by design: ISO 3664:2009 defined only P1 and P2, both D50 including ultraviolet.

ISO 3664:2025 added P3 and P4 — the same D50 with the ultraviolet excluded. The pairing is now explicit: measure on M1, view under P1 or P2; measure on M2, view under P3 or P4. The library did not change; the ground under it did. See Measuring and Viewing.

What it costs, and it does cost something

The codes are derived from M2, so the coding convention describes the sample with the brighteners switched off. Look at the same colours under M1 and two things surprise people.

The neutrals stop being neutral. A sample that is exactly grey under M2 turns blueish under M1, because the paper beneath it is fluorescing and the ink is thin enough to let it through. If you are hunting for neutrals under M1, use the LAB view rather than the N family — the N codes are neutral by their M2 definition, which is the definition, not an accident.

Pale colours move most. Across the library, 55% of samples shift by more than one ΔE₀₀ between M2 and M1, and the ones that move furthest are all light and low in chroma. Measured, banded and listed: Optical Brighteners.

All three are published

M0, M1 and M2 are each measured for every sample and each ships with the library. Choosing M2 for the codes is a choice about the coding convention, not a decision to withhold anything — if your workflow is M1, switch the control and the whole library recomputes.