Glossary → Measurement condition (M0, M1, M2, M3)
Measurement condition (M0, M1, M2, M3).
the illumination a spectrophotometer used, which has to be declared with the numbers it produced
ISO 13655 defines four illumination conditions for graphic-arts color measurement, and a measurement quoted without one of them is incomplete. M0 is the legacy source, close to CIE Illuminant A, with its relative ultraviolet content simply unspecified. M1 requires a match to CIE D50 across the full range including the UV, M2 is any source that excludes the UV component, and M3 adds polarization to suppress gloss. The choice only bites on substrates that fluoresce, which is most of them, and the ICC's guidance is specific: M1 is recommended when proofs and prints are to be assessed in a cabinet conforming to ISO 3664:2009, and M2 when the end-use viewing has little UV or is unknown. Mix conditions inside one workflow and the numbers stop being comparable: the profile was built on M2 data, the customer's spectrophotometer reports M1, and the same piece fails a tolerance nobody actually broke.