What is the difference between M0, M1, M2 and M3?

They are measurement illumination conditions defined in ISO 13655, distinguished by UV content. M0 uses illuminant A with undefined UV. M1 simulates D50 including a correct UV component. M2 is UV-cut, excluding UV from the illumination. M3 adds a polarizing filter to M2's UV-restricted illumination.

Which measurement mode is required for G7 and GRACoL 2013?

M1. It is the required measurement mode for submitting G7 data to Idealliance for the GRACoL 2013 print specification, because its D50 illumination with defined UV content correlates with standard D50 viewing conditions.

Which measurement mode should be used for optical brightening agents?

M1, when you want the brightener's effect captured as a viewer would see it under D50 lighting. M2 (UV cut) is used instead when the paper fluoresces but you want that effect excluded from the data.

What measurement mode is best for UV inks?

M2 is generally preferred for UV inks, because excluding ultraviolet from the illumination removes fluorescence effects that would otherwise distort the color reading.

M0, M1, M2, M3 Measurement Modes Explained

When measuring color from printed materials with a spectrophotometer, choosing the right measurement mode is crucial to getting accurate, meaningful, and shareable results. Most instruments offer several modes, designated M0, M1, M2, and M3.

Here's the single most important thing to understand up front: these modes are defined by the UV content of the instrument's illumination — not by gloss, not by finish, and not by which color space you're computing in. They exist because optical brightening agents (OBAs) in paper fluoresce under UV light, and without a defined UV condition, two instruments will read the same sheet differently.

These conditions are specified in ISO 13655.

Quick Reference: The Four Modes

Mode	Illumination	Use it for
M0	Illuminant A (tungsten), undefined UV	Legacy workflows; substrates and inks that don't fluoresce
M1	D50, UV included	Substrates with OBAs. Required for G7 / GRACoL 2013. The modern default.
M2	UV cut (UV excluded)	Removing OBA fluorescence from the data; UV inks
M3	UV cut + polarizing filter	Minimizing surface reflections — wet press sheets, metallics, textiles, films, plastics

M0 — Legacy / Illuminant A

M0 is the legacy mode. It uses a light source approximating CIE Illuminant A (historically a gas-filled tungsten lamp) with no defined or stable UV content.

Best for: substrates with no optical brightening agents, printed with conventional non-UV inks. Since a non-OBA substrate doesn't fluoresce, M0 captures its color without interference.

The catch: because M0's UV content is undefined, two M0 instruments from different manufacturers can read the same fluorescing sheet differently. ISO 13655 specifically does not recommend M0 when the sheet exhibits fluorescence and you need to exchange data between facilities. From a purely technical standpoint, M0 is now considered obsolete for most graphic arts work.

M1 — D50, UV Included

M1 uses a light source simulating illuminant D50 with a correctly specified UV component. It was defined to reduce the variation between instruments caused by fluorescence — whether from optical brighteners in the paper or from fluorescing colorants.

Best for: substrates containing OBAs, which is most modern paper. Because M1's UV content is matched to D50, brighteners "glow" during measurement roughly the way they will in a D50 viewing booth — so your numbers actually correspond to what people will see.

M1 is the required measurement mode for submitting G7 data to Idealliance for the GRACoL 2013 print specification. If you take one thing from this article: for most modern print work, M1 is the answer.

ISO 13655 defines two routes to M1 compliance. Part 1 is a full D50 spectral match, valid where either substrate or colorants may fluoresce. Part 2 addresses the case where the substrate fluoresces and its brightener response needs capturing, but you're confident the imaging colorants do not fluoresce. (If you're unsure, ask your ink manufacturer.)

M2 — UV Cut

M2 was standardized to define what UV exclusion means in a measuring instrument — variously called UV-cut, No-UV, or UV-filtered. The illumination contains essentially no energy below 400 nm.

Best for: situations where the paper fluoresces but you want that effect removed from the data. It simulates a viewing environment with no UV content — a museum, for instance.

M2 is generally preferred for measuring UV inks, because eliminating UV from the illumination removes fluorescence effects that would otherwise distort the reading.

M3 — Polarization

M3 takes M2's UV-restricted illumination and adds a polarizing filter (a crossed pair, in the emitted and reflected light).

Best for: minimizing first-surface reflections. That makes it valuable for wet press sheets, metallic inks, and materials with a glossy, transparent, or heavily textured surface — textiles, films, backlit and transparent media, and plastics.

What polarization does: it reduces or removes the specular reflections coming off the surface, so the measurement reflects the color of the material rather than the shine on top of it. This is a surface-reflection control — it does not address metamerism, which is a different problem entirely.

Which Mode Should You Use?

In principle the decision is clean:

M0 — when neither the substrate nor the colorants fluoresce.
M1 — when the substrate, the colorants, or both may fluoresce. This is the modern default, and the requirement for G7 / GRACoL 2013.
M2 — when the paper fluoresces but you want that effect kept out of the data, and for UV inks.
M3 — special cases where surface reflections must be minimized.

The one rule that matters most in practice: don't mix modes. A proof verified in M1 and a press sheet read in M0 are not comparable numbers, no matter how confidently either party quotes them.

Spectral curves of Pantone Reflex Blue measured in M0, M1, M2 and M3 measurement modes, showing how the curves differ by mode — Pantone Reflex Blue measured in all four modes. Note the difference in the spectral curves — same ink, same sheet, four different answers.

A Note on Viewing Conditions

It's equally important to consider your viewing conditions, and whether UV is present in them. UV light "activates" optical brighteners. Compared against a substrate with no OBAs, that can produce a very noticeable color difference — and it's why ISO 13655 (measurement) and ISO 3664 (viewing) have to be considered together. Your instrument and your light booth need to agree.

Schedule a Consultation

Not sure which mode your workflow should be standardized on — or whether your instrument and light booth agree? That's exactly the kind of thing we sort out.
billo@aldertech.com — 720-933-4413