Short answer
Every printed colour sits on the paper, so the paper's own colour shifts all of them and sets the lightest white. Many papers contain optical brightening agents (OBAs) that absorb ultraviolet and re-emit blue, making them look whiter — by an amount that depends on how much UV is in the light. ISO 13655 measurement condition M1 exists so that instruments see that fluorescence as it appears under D50 viewing light.
Process inks are transparent, so the paper shows through every tint and even through solids. A cream paper warms every colour; a blue-white paper cools them. The paper also sets the white point of the whole reproduction: nothing on the sheet can be lighter. That is why characterisation data always start with paper white, and why ISO 12647-2 groups papers into classes with their own aims — premium coated at about L* 95 with a slightly blue b*, down to uncoated and newsprint papers that are darker and warmer. Printing a condition's separations on a different paper moves every colour by roughly the difference between the papers.
Optical brightening agents are fluorescent compounds added to paper. They absorb ultraviolet light, invisible to us, and re-emit it as visible blue, which cancels the natural slight yellowness of cellulose and makes paper look 'whiter than white'. The effect needs UV to be present: under daylight or a UV-containing booth light the paper glows bluish, under a UV-free LED or behind glass it looks plainer and warmer. Colours printed on it shift with it, most noticeably pale tints and neutrals. ISO 15397 classes papers by fluorescence (as the change in brightness with and without UV) from faint to high.
Older instruments lit samples with tungsten lamps that contain very little UV, so they barely excited brighteners: a highly brightened paper measured much yellower than it looked in a D50 booth. Different instruments also carried different, undefined amounts of UV and disagreed on the same sheet. ISO 13655:2009 introduced four measurement conditions: M0 (the legacy tungsten-like light, UV undefined), M1 (illumination matching D50, including its UV, so fluorescence is measured as seen), M2 (UV excluded) and M3 (M2 plus polarising filters for wet ink). The 2013 offset standard and the FOGRA51 and FOGRA52 data use M1.
The move to M1 is visible in the data: FOGRA39's paper white is b* −2, measured before M1, while FOGRA51's is b* −6 and the heavily brightened uncoated FOGRA52 paper is b* −10. A proof on unbrightened paper cannot glow like a brightened production sheet, so ISO 12647-7:2016 asks proof papers to be in the same fluorescence class as the production paper. And the viewing light has to contain the right amount of UV — a UV-free LED booth makes brightened press sheets look warmer than the proof they were matched to. Research comparing ten instruments found M1-compliant devices agree with each other far better on brightened papers than legacy ones.
Why: The production paper is brightened and the proof paper is not; daylight's UV makes only one of them glow.
Fix: Proof on paper of the same fluorescence class and judge under a booth light with appropriate UV.
Why: One measures M0 (little, undefined UV), the other M1.
Fix: Set both to M1 for current printing conditions and record the mode with every measurement.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
StandardStrong evidence
ISO 13655 defines measurement condition M1 as specimen illumination whose colorimetry closely matches CIE illuminant D50, M0 as illumination closely matching illuminant A, M2 as illumination with no substantial power below 400 nm, and M3 as M2 with crossed polarisers.
FactStrong evidence
The published paper-white aim is L* 95, a* 0, b* −2 in FOGRA39 (pre-M1), L* 95, a* 1.5, b* −6 in FOGRA51 (moderate fluorescence, M1) and L* 93.5, a* 2.5, b* −10 in FOGRA52 (high fluorescence, M1).
Source: FOGRA39 characterization data (coated offset, ISO 12647-2:2004/Amd 1); FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013); FOGRA52 characterization data (wood-free uncoated offset, ISO 12647-2:2013)
StandardStrong evidence
ISO 12647-7:2016 says the proof substrate should be in the same fluorescence category as the production paper, using the faint, low, moderate and high categories of ISO 15397.
Source: ISO 12647-7:2016 … — Part 7: Proofing processes working directly from digital data
FactModerate evidence
A comparison of ten commercial spectrophotometers found much better agreement between instruments measuring in ISO 13655 M1 mode than between legacy M0 hand-held instruments, particularly on media containing optical brighteners.
Caveat: One study of ten instruments; agreement will vary with the specific models.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.