Measuring and Viewing: M-conditions and ISO 3664:2025

How a colour is measured and how it is looked at are two halves of one decision, and until recently only one half was standardised for light without ultraviolet. That changed in July 2025. ChromaSpot’s choice of M2 now has a viewing condition to match it, by name, in a published standard.

The measurement half: M0, M1, M2

A spectrophotometer has to light the sample before it can measure it, and the lamp it uses is part of the answer. ISO 13655 gives the options names.

Condition	The instrument’s lamp	What it tells you
M0	Incandescent, unfiltered — illuminant A	The oldest convention. Some UV, but not a defined amount.
M1	D50 with its ultraviolet present	What the sample looks like when brighteners are working.
M2	D50 with the ultraviolet cut	What the sample looks like with the brighteners switched off.

ChromaSpot publishes all three, measured, for every sample. The codes are derived from M2 — and Optical Brighteners is where the difference between them is measured rather than described. Why M2? gives the full reasoning.

The viewing half: ISO 3664

The other standard describes the booth rather than the instrument. For years it defined two conditions for looking at prints, P1 and P2, both D50 including ultraviolet — a critical condition and a lower-level one for appraisal.

That was a problem for anyone measuring on M2. The measurement said one thing, the only standardised booth said another, and the mismatch had no name.

What the 2025 edition changed

ISO 3664:2025 — the fourth edition, published July 2025, replacing the 2009 one — keeps P1 and P2 and adds P3 and P4: the same D50, with the ultraviolet excluded.

	Critical	Appraisal	Pairs with
D50, UV included	P1	P2	M1 — and M0
D50, UV excluded	P3	P4	M2

P1 and P2 share one spectrum and differ only in light level; so do P3 and P4. There is no new reference illuminant — it is still CIE D50. What is new is that the same D50 now exists in two ultraviolet regimes, each with a name, and that the standard carries a normative way of testing whether a lamp really excludes the UV it claims to.

Measure on M2, view under P3 or P4. Measure on M1, view under P1 or P2. A mismatched pair is now something you can point at, not just something that feels wrong.

Why this matters to a library. ChromaSpot chose M2 in 2022 for reasons that were, at the time, arguments about where lighting was heading — LEDs emit no ultraviolet, brighteners fade, and fluorescence has never been comparable between instruments. Those arguments are now a clause in a published standard. The library did not change; the ground under it did.

Two other things the 2025 edition did

Monitor and soft-proofing conditions left. They are covered by ISO 12646 and ISO 14861 instead, which is where anyone comparing a screen to a print should now look.

A colour-fidelity requirement arrived. The 2025 edition expects a viewing lamp to be judged on how faithfully it renders colour, not only on where its white point sits. That is a real change for LED booths, which can land exactly on D50 and still render badly, because a chromaticity coordinate says nothing about the shape of the spectrum that produced it.

What to ask a supplier

Three questions settle most disputes before they start.

Ask	Because
Which measurement condition — M0, M1 or M2?	Without it a ΔE is not comparable to anything.
Which viewing condition — P1, P2, P3 or P4?	And is it the half that pairs with the measurement.
What is the OBA index of the substrate?	It decides how much the two halves can possibly disagree.

The standard itself. ISO 3664:2025, Graphic technology and photography — Viewing conditions, from ISO. This page describes what it changed; it is not a substitute for it, and anyone specifying or building a viewing booth needs the document.