Two operators can measure the same sheet and read different numbers, simply because they used a different measurement condition. Here is what M0, M1, M2 and M3 mean, and when to use each, so your DeltaE results actually compare.
Why measurement conditions matter
A spectrophotometer reading depends on the light it uses and how it handles UV and gloss. The M conditions standardize that, so a measurement is reproducible between instruments, operators and sites. Mixing conditions is one of the most common reasons two readings disagree.
The four conditions
- M0: legacy unfiltered illumination (close to incandescent). Still common, but sensitive to paper optical brighteners.
- M1: defined D50 daylight with a specified UV content. The modern reference, because it handles brighteners consistently.
- M2: UV-cut. Excludes UV so optical brighteners do not fluoresce, useful for isolating the ink from the paper effect.
- M3: polarization. Reduces the gloss and wet-versus-dry density difference, often used at the press.
Optical brighteners and UV
Many papers contain optical brightening agents that glow under UV, shifting the measured white and the colors on it. Because M0, M1 and M2 treat UV differently, the same sheet reads differently under each, which is exactly why the condition has to be stated.
Which to use
For new work, standardize on M1: it is the current ISO-aligned reference and travels best between partners. Match whatever condition your customer or standard specifies, and make sure every instrument in the loop is set the same way.