OBA, M1, M2, and M3 — and the Things That Could Go Wrong

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 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.

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.

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:

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?

What are the disadvantages of using M0?

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.

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

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.

Part two of this two-part series continues here.