OBA, M1, M2, and M3 — and the Things That Could Go Wrong (Part 1)

Editor’s Note: A few years ago, writer and color management expert Kevin O’Connor — (1954 – 2019) — delved into the complicated issues regarding optical brighteners in press sheets. Because optical brightening agents (OBAs) and the M standards are still confusing to many, we bring his article back to light, with updates on the technology and how it has fared over the past three years.

We remember and thank Kevin O’Connor for this in-depth article written shortly before his death in 2019. The original version was published in Rods and Cones’ magazine Out of Chaos. It has been edited to reflect current practices and thinking. 

Credit: Sappi

Many contemporary printing substrates contain OBAs (Optical Brightening Agents), which make them appear brighter and whiter. These are very popular, and are used to make less expensive papers look better than they would otherwise. The OBAs work when ultraviolet light (UV) strikes the paper. However, the fluorescence of the OBAs can create significant challenges for color management, fooling devices into reading the blue end of the visible spectrum incorrectly. In addition, different light sources used to view these papers will cause the paper to appear differently, depending on whether the light source emits the UV to which the brighteners respond.

Older color standards assumed that a standard D50 light source guaranteed viewing consistency, but in practice, OBAs could skew measured color to create serious mismatches. To ensure consistency, new standards addressing OBAs have been defined by the International Organization for Standardization (ISO) to work with modern measurement technologies. These new technologies allow for using light sources that include UV light in the devices’ illumination, not using UV as part of measuring, or combining both.

New technologies and new standards

New illumination sources — including UV-emitting LEDs — can be built into color measurement instruments to help measure correctly and consistently. To ensure consistency, a new series of measurement conditions was defined as part of ISO 13655-2009: Spectral Measurement and Colorimetric Computation for Graphic Arts Images. These are referred to as the “M” series because they’re numbered M0, M1, M2, and M3.

The graph above demonstrates the reason for choosing a measurement standard. Papers with OBAs can be measured multiple ways, giving different results for the same substrate and ink combinations. Unless a particular method can be matched at multiple locations, the variations between methods would be unacceptable. Agreeing on which defined standard to use becomes essential, both to raise color-matching quality at any location, and to raise consistency between multiple locations.

Four standard measurement conditions meet differing needs

The ISO document referenced earlier (ISO 13655-2009) defines four conditions, numbered zero through 3. They are meant to be used under specific conditions (or in absence of conditions), and in specific environments.

M0: What most of the world is still using

Until the M standards were released, almost all instruments for measuring color used a gas-filled tungsten bulb as a light source, designed to match a color temperature standard called Illuminant A, defined by a standards body called the CIE (Commission Internationale de l'Eclairage /International Commission on Illumination).

The M0 measuring condition was included in the M Standards to describe the condition almost everyone used before they were defined and published — and what most people are still using with their legacy instruments.

In M0, the standard specifies that:

1. Instruments must use the previous standard light source, designed to provide illumination matching CIE Standard Illuminant A, within a tight tolerance to the color temperature of 2856 K + 100 K.
2. Measurements can be made with any spectrophotometer, old or new, including those that measure at one angle only. Single angle measurement can deliver differing results from the same sample, depending on which way the sample is turned relative to the measuring device.
3. If exchanging between multiple locations, measurements should not be made with M0 when M1 instruments are available. However, if all locations are using M0 measurements made with the same model of instruments — which are properly serviced and maintained — and there is adequate data for process control to give consistent results, the standard says M0 can be used.

Unfortunately, the M0 measuring standard does not specify the amount of UV the light source must or must not emit; the starting point for the UV will vary by manufacturer. Also, the light source’s UV output in a particular instrument can change over time, but because M0 doesn’t specify a particular UV behavior, the instrument will remain within specifications.

This was the only light source earlier instruments used, and legacy instruments all around the world are still being used to measure color every day with Illuminant A.

New instruments retain this lighting option, while offering other options to meet some or all of the other M standards.

Who should use M0 and why?

1. Anyone getting good results and pleasing clients with current tools can stick with what they have if this meets their customers’ needs.
2. Printers that have instruments reading M0 have no choice until they acquire a new instrument. Meanwhile, existing instruments should be checked and recertified regularly.
3. Anyone who only prints on papers without brighteners, with inks that have no brighteners, can continue to work with M0. No brighteners, no problem.
4. New bulbs must be filtered so no ultraviolet light excites any brighteners present, and all people who have approval rights on proofs must view proofs in the same non-UV lighting. Filtering accessories are available from various lighting manufacturers to match their specific products.

What are the disadvantages of using M0?

1. The amount of UV emitted by the instrument used to read color can vary with different instruments, delivering varying results from OBA-brightened papers. This makes it more difficult to deliver consistency.
2. It’s often more difficult to match output from multiple locations, a challenge for preserving brand identity and integrity.
3. It doesn’t account for the presence of paper brighteners. It will not be easy to make in-plant matches of proof and press sheet if either has OBAs.
4. A match made in UV-filtered lighting won’t hold up once the job is viewed in some other lighting sources. But a match made in lighting with UV components won’t hold up in some other lighting environments, such as halogen-lit conference rooms.

M1: What much of the world is encouraged to use

It takes a sure hand, a steady eye, and a clear head to implement an M1 workflow. It was developed to be a more precise way to measure color, specifically to address the presence of ultraviolet in print workflows. It attempts to solve the problems of mismatched proofs and press sheets or other final output arising from non-coordinated management of ultraviolet light sources and optical brighteners in substrates and inks.

M1 (Part 1)

In Part 1, the M1 standard specifies that the illuminant used in devices reading color samples should match Illuminant D50 within a tight tolerance. This illuminant has a clearly defined ultraviolet component that the instrument must deliver. Due to this tight matching to D50, these devices can be used to measure fluorescence from brighteners in ink, substrate, or both.

M1 (Part 2)

Part 2 of the M1 standard specifies that it’s designed to address fluorescence in papers, but not in ink or toners. The reason for this exclusion has to do with the complexity of designing a handheld instrument that can deliver true D50. Instead, manufacturers use approaches that calculate OBA response as closely as possible. The important point is that it’s for paper only — and many manufacturers list their devices as doing M1, but they mean M1, Part 2 only.

It’s very important to be aware that an instrument simply listed as offering M1 may or may not offer Part 1, so if you need to read fluorescence in inks or toners, clarify a particular device’s capabilities with the manufacturer before purchasing. The X-Rite eXact was the first instrument to market that can capture both parts of M1.

The Promise and the Reality of M1

While M1 addresses issues of brightening agents causing mismatches, it is not yet as straightforward as originally hoped. This is because of some significant issues regarding mismatching in the M1 workflow.

Who should use M1 and why?

M1 is being touted as the goal for most workflows, particularly in press houses. In theory, any printer who doesn’t need the specialized capabilities of M2 or M3 should be using M1. Given the challenges described above, M0 remains the choice of most printers.

However, if customers require compliance with certain contemporary standards, if the customers’ work is created on substrates with a high OBA Index value, or too much rework is being done to get acceptable output, M1 will probably be the most accurate and cost-efficient solution.

The Advantages of M1 Workflows

1. Using the M1 standard to measure substrates will more precisely account for the presence of optical brightening agents in both proofing and printing substrates.
2. Measuring and managing these optical brighteners in proof and press substrates helps to deliver a better match between the two under current D50 standard lighting.
3. Using the M1 standard works better (though it may not work perfectly) for creating output to be viewed with lighting sources that have an ultraviolet component.
4. There will be fewer surprises when jobs printed with optically brightened ink or paper go to other viewing environments where ultraviolet is present, such as daylight.
5. Agreement between multiple instruments will be closer.
6. Industry groups currently release data sets requiring the use of M1. However, using the substrate relativity calculator allows you to use either M0 or M1 with any data set.
7. Properly used, matches and consistency will continue to improve as various tools and recommendations are updated and implemented.

What are the disadvantages of using an M1 Workflow?

Just as with a major software upgrade, there are costs and changes associated with the adoption of a new standard. In addition, technical challenges must be addressed to perfect the M1 workflow — to the degree it can be perfected currently.

1. Viewing stations might have to be upgraded and old viewing lamps replaced. Simply switching lamps may be all that’s needed, depending on how well current viewing environments meet the new viewing standard needed for an M1 workflow. See the complete details needed in ISO 3664:2009, which deals not only with lamp specifications, but with the total viewing condition and environment.
2. If using M1 measurements to print or proof — and evaluating the printed piece in a viewing environment without UV components in the lighting — the printed piece with OBAs will probably look dull and dingy, with a yellow cast. This can be a very serious mismatch, requiring multiple sets of proofs or prints to match multiple conditions, a very challenging proposition.
3. Older instruments should be replaced with instruments able to make M1 measurements. Note: To read inks as well as substrates in an M1 workflow, be certain the instrument you purchase can read M1, Part 1.
4.  Measurements of OBA-brightened stocks won’t match measurements from other conditions, such as M0.
5. Proofing papers require very careful matching to the press stocks for good matching, including OBA content for both proof and press sheet.
6. Legacy work that has printed well in the past will require careful attention. If meeting new standards is not needed for that client’s work, it may be easiest to try recreating the previous viewing environment while continuing to print the job the same way. Next, equipping new bulbs with UV-blocking filters will recreate legacy viewing lighting sources as closely as possible. Contact the manufacturer of your viewing environment for UV-blocking options but be prepared for the possibility of a less than perfect match. The filters don’t provide a perfect match to previous conditions and may introduce other issues.
7. If all work currently being printed needs to comply with contemporary standards, legacy jobs may have to be re-characterized, using M1 measurements to comply with contemporary printing standards.
8. Making M1 measurements will take longer to measure patches with some instruments in prepress than measuring with M0 because some instruments used in prepress workflows require two passes to measure with M1. Pressroom instruments capable of making M1 measurements should be expected to do so in one pass.
9. In an industry beset by diminishing profits and shrinking margins, it can be tough to justify the investment in new lighting and new instruments that M1 demands, as well as the testing and measuring required to implement an M1 workflow.

Part two of this two-part series continues here.