ISO 13655 is a standard having to do with spectral measurement and colorimetric computation for graphic arts images, and objects that reflect, transmit, or self-illuminate.
The 2009 revision describes standards for colour evaluation including fluorescence under Illuminant D50 (daylight with a correlated colour temperature of 5000K).
In this standard, specific measurement conditions are defined having the labels: M0, M1, M2, and M3.
M0 delineates that the measurement was made using "illuminant A" which is the traditional unfiltered tungsten light. Most older measurement instruments would have this kind of light (ie: i1Pro, DTP41, DTP70) This also designates that there is no UV filtering and no polarizing in the process.
M1 labels the measurement light as conforming to CIE D50, that is - normal daylight. This would include UV light, and not be polarized. Instruments that have this capability are the Barbieri SpectroPad, the Konica Minolta FD-7 and FD-9, the X-Rite iSis2, the i1Pro2 and the eXact.
M2 describes any non-polarized light that has UV filtered out. This would include measurements made by a UV-cut i1Pro, a UV-cut DTP70, and an iSis in UV excluded mode.
M3 describes a measurement that is polarized and has UV filtered out.
ISO 13655:2009 has been superseded by ISO 13655:2017.