M2 Measurement

/ˌem tuː ˈmeʒərmənt/

General Definition

M2 Measurement defined by the ISO 13655 standard, refers to a method of color measurement by a spectrophotometer where the ultraviolet (UV) light content is completely excluded (UV-Excluded) from the instrument's light source. The purpose of excluding UV content in the M2 mode is to ensure that the effect of Fluorescence (caused by OBAs in papers) is entirely disregarded during the measurement. Consequently, the resulting L*a*b* values in the M2 mode reflect the measurement of pure pigment without the influence of optical brightening agents. In contrast, M1 Measurement utilizes a regulated UV component, making it the preferred method for measuring substrates that contain Optical Brightening Agents (OBAs).

Real-World Usage

Historically, M2 Measurement was a common mode for measuring density and spectral data, particularly when the variable effect of Fluorescence needed to be removed from the measurement output. However, in modern color management workflows (ICC Workflows), relying on M2 alone creates a serious challenge because its results do not align with the visual perception of the observer under standard lighting conditions (D50 Illuminant, which contains UV). This inconsistency is notably seen in software like Photoshop, which defaults to displaying digital spot color libraries (Pantone) using M2 values, while the final print quality control workflow should use M1 (UV-Included).

This conflict leads to high levels of uncertainty and reproducibility issues, as the difference in L*a*b* values for the same spot color between M2 and M1 can be significant (especially in the b* channel, which indicates blue/yellow). When measurement is performed on OBA-rich papers, the difference between M2 data (which ignores OBA) and M1 data (which includes it) can directly lead to ΔE deviations and disrupt color matching across different printing centers attempting to work to the same standard.

Consultant's Note

As a Color and Optimized Production Consultant, it is suggested that, the use of M2 Measurement as the default mode for determining spot colors or creating ICC profiles is largely obsolete due to its lack of compliance with visual perception under the standard D50 light. Given that all modern color management workflows (such as GMG and Esko) and the new generation of spectrophotometers fully support the M1 mode, insisting on M2 will only lead to errors and confusion. The only practical and accepted use of M2 today is in combination with M1; for example, to isolate or calculate the degree of Fluorescence in samples.

The major issue here is the inconsistency across platforms; while Pantone provides its libraries in M0, M1, and M2 modes, software like Photoshop must default to displaying M1 values for spot colors or allow the user to change the mode. Global organizations like Pantone have the responsibility to ensure the correct use of their digital standards to eliminate workflow errors. Until these corrections are made, users must interpret the L*a*b* values seen in Photoshop as M2 values and strictly use M1 values within their color management applications.

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