During this time of COVID-19, I sat back in casual clothes and enjoyed attending FTA’s Virtual FORUM 2020.
There were great topics that ranged from doctor blades, color management and expanded gamut (EG), to GRACoL and brand management. But it didn’t matter what the topic was—During what seemed like every Q&A session, inevitably somebody asked about measurement mode—“Should I use M0 or M1?” What are these measurement modes? Which one should I use? Why?
Let me be very clear—measurement mode M0 is bad for you.
This whole issue is to do with spectrophotometers and relates to fluorescence and optical brighteners. Fluorescence is the process where invisible ultraviolet radiation (<400-nm.) is absorbed and converted to visible light and emitted in the 400-nm. to 500-nm. “blue” wavelength region.
Chemicals called optical brightening agents (OBAs) are commonly included within the coating layer of many types of paper and media. When these papers are viewed under a source that contains ultraviolet (UV) radiation, the UV energy is absorbed by the OBA chemicals and emitted in the 400-nm. to 500-nm. part of the spectrum, creating a boost in the sample’s spectral response.
In other words, fluorescence turns invisible ultraviolet light into visible blue light. Fluorescence is a desirable effect as it makes the paper appear bluer and brighter, thus counteracting the yellow hue naturally found in wood fiber papers.
One challenge with measuring instruments has been that, when dealing with samples containing OBAs, there is often a variance in reported measurement between different instruments. In legacy (M0) instruments, the UV component of the illuminant was not specified and was often distinctive in non-identical instruments, which caused two devices to give a different reading for the same sample.
If you have, for example, an M0 instrument such as an inline spectrophotometer within the press, and a trusted X-Rite 530 handheld device at press side, these M0 instruments can read differently for the same sample. The inline spectrophotometer may measure OK and the operator runs the job, but it is very expensive (and embarrassing) when a customer at press side uses a handheld device and rejects it!
Because instruments can have different amounts of UV in their internal lamps, two separate M0 instruments can read the same sample and report different values.
Instrument manufacturers can be forgiven their trespasses as the UV component in the measuring instrument light source was not clearly specified, which meant each manufacturer could do, and does, its own thing!
The situation was pretty chaotic, so vendors developed their own fixes. X-Rite, for example, created XRGA following its merger with GretagMacbeth. Optical Brightener Compensation (OBC) was a fix for the optical brightener problem. XRGA and OBC were awkward and irritating “patches,” and are indicative of two related challenges in spectrophotometry—inter-model differences and measuring color in materials with OBAs. Both problems have been elegantly addressed by ISO 13655.
ISO 13655 & ISO 3664
This chaotic situation has been corrected by an international standard, ISO 13655:2009 – Graphic technology – Spectral measurement and colorimetric computation for graphic arts images. ISO 13655 describes four measurement modes—M0, M1, M2 and M3—which clearly define the UV component in a measuring instrument. That means instruments from different manufacturers now all read the same!
The new ISO 13655-defined instrument measurement modes are:
- M0: Legacy mode (any illumination source, tungsten lamp commonly assumed)
- M1: D50, UV-included mode (this is the recommended mode for color measurement)
- M2: UV-excluded mode (removes all UV light from the measurement system, below 400-nm.)
- M3: Polarizing mode (for measurement of wet press sheets or metallic inks)
M0 is known as the “legacy mode” and is a catch-all mode that represents older measuring instruments and older, established workflows and numbers. But the M0 mode is very dangerous, especially if interchanging data with others and seeking external validation and verification. In terms of their UV content, M0 instruments can be anything. I repeat, in M0 instruments, anything goes: a little UV, a lot of UV, no UV—all are welcome, creating the potential for a huge variation in measurements. Is this how you want to measure and manage color for your customer’s brand?
M1 is the “UV-included” mode, or we may describe this as “full spectrum” or normal lighting. A major improvement is that in this mode, the relative amount of UV and visible wavelengths is now fully specified and in fact the measurement condition should approximate CIE Illuminant, D50. The M1 mode is therefore also known as the “D50” mode.
This clarification in measuring instruments, ISO 13655, is accompanied by a similar clarification in the standard for viewing booths, ISO 3664:2009 – Graphic technology and photography – Viewing conditions. ISO 3664 basically states that we should have D50 (M1) in the light booth as well. Via these two standards ISO 13655 and 3664, the amount of UV illumination is clarified in both the measuring instrument and the viewing booth. By implementing these two ISO standards, for the first time, the instrument-reported values agree with what is observed visually in a viewing booth. What you measure is also what you see.