Optical Brighteners in Substrates & Instrument Errors: M0, M1, M2, M3

In today's printing market, over 90% of coated and uncoated high-brightness papers contain optical brighteners. This introduces a fundamental challenge for instrumental color control. Press operators frequently encounter a situation where the spectrophotometer indicates a perfect match on paper, yet the printed result visually drifts into a muddy yellow or an unnatural blue tint. The cause lies in the interaction between the ultraviolet (UV) light spectrum, paper fluorescence, and legacy measurement modes of color measurement devices.

The Physics: Optical Brighteners and Metamerism

Optical Brightening Agents (OBAs) are organic compounds added to paper pulp to visually increase whiteness. Their mechanism is based on photoluminescence:

1. The agent absorbs invisible ultraviolet (UV) radiation in the 300–400 nm range.
2. The absorbed energy is re-emitted in the visible blue-violet spectrum in the 400–500 nm range (peaking around 440 nm).
3. This excess blue light compensates for the natural yellowish tint of cellulose. As a result, the human eye perceives the paper as brighter, whiter, and cooler.

The core issue is that the intensity of OBA fluorescence depends directly on the amount of UV radiation in the light source. Paper that looks neutral white under daylight (rich in UV) will appear yellowish in a showroom illuminated with cheap LEDs (which emit no UV). This is a classic case of spectral metamerism driven by fluorescence.

From a colorimetric standpoint, high concentrations of OBAs shift the coordinates of unprinted paper along the $b^*$ axis deep into the negative (blue) region of the CIE $L^a^b^$ color space (where $b^$ values can reach $-6$ or even $-10$).

Spectrophotometric Standards (ISO 13655): M0, M1, M2, M3

To standardize measurement conditions for fluorescent materials, the ISO 13655 standard defines four measurement conditions, known as M-modes.

1. Condition M0 (Legacy Mode / Uncontrolled UV)

• Technical Definition: The light source is a legacy gas-filled tungsten lamp (CIE Illuminant A) with a color temperature of approximately 2856 K. The UV component of this source is not standardized and remains completely uncontrolled.
• The Issue: Because the proportion of UV in the M0 lamp is undefined, instruments from different manufacturers (and even different units of the same model) excite OBA fluorescence differently. This makes measurements of OBA-heavy papers non-repeatable.
• Application: Only permitted for legacy workflows and substrates guaranteed to be free of brighteners ($b^* \ge -2$). In modern color management, M0 is considered technically obsolete.

2. Condition M1 (D50 UV-Included / Controlled UV)

• Technical Definition: The recommended modern standard for graphic arts. The instrument's light source is closely matched to the spectral distribution of standard daylight CIE D50 (5000 K), including a strictly controlled, calibrated ratio of UV radiation below 400 nm.
• Advantage: Ensures that the spectrophotometer "sees" OBA fluorescence in exactly the same way a human observer does under a standardized ISO 3664 D50 viewing booth. This eliminates discrepancies between instrument readings and visual assessment.
• Application: Profiling and quality control on any paper with moderate to high OBA levels (e.g., Fogra 51 and Fogra 52 characterizations).

3. Condition M2 (UV-Cut / UV-Excluded)

• Technical Definition: The instrument uses a physical or software-based UV-cut filter that eliminates all radiation below 400 nm.
• Advantage: Completely isolates OBA fluorescence from the measurement. Paper coordinates are read as if no optical brighteners were present.
• Application: Profiling for environments completely devoid of UV light (e.g., museums, galleries with UV-blocked lighting) or behind UV-blocking glass. M2 is also widely used in UV inkjet printing on plastics to stabilize sensor readings, as detailed below.

4. Condition M3 (Polarized / Polarizing Filter + UV-Cut)

• Technical Definition: Combines M2 properties (UV-cut) with mutually perpendicular polarizing filters on the emitter and receiver.
• Advantage: Polarization eliminates specular reflection (gloss) from the wet ink film surface. This allows density measurements of wet sheets directly at the press console to accurately predict dry-down color shifts.
• Application: Offset printing, real-time press run density control. Rarely, if ever, used to build ICC profiles in digital printing.

Impact of Measurement Errors on ICC Profiling & Proofing

When profiling a digital press on paper with high OBA concentrations using the legacy M0 mode, two critical errors occur:

1. The "Yellowing" Effect in Paper Compensation

The profiling algorithm detects the deep blue shift of the substrate (due to OBA excitation under M0 being misread by the device). To pull the gray wedge coordinates to neutral relative to the paper white point, the profiling engine starts adding yellow ink ($Y$) to the light neutral highlights. This produces prints with muddy, jaundiced highlights and skin tones.

2. Contract Proof Verification Failure

Modern proofing standards are built on Fogra 51 (PSO Coated v3) and Fogra 52 (PSO Uncoated v3) characterization targets, which mandate M1 measurements. If you attempt to verify a standard Ugra/Fogra Media Wedge printed on OBA paper using a device in M0 or M2 mode, you will trigger a critical tolerance failure, with $\Delta E_{00}$ often exceeding 4.0 (against the standard proofing limit of $\le 1.5$ average). The proof will fail formal certification, even if it looks visually acceptable.

Practical Recommendations for RIP Configuration (Onyx, Caldera, ColorGate)

To eliminate brightener-induced print rejects, align your prepress and RIP workflows using the following rules:

1. Selection of Target Profiles

• For gloss and matte coated papers with moderate OBAs, use Fogra 51 (PSOcoated_v3.icc) instead of legacy ISO Coated v2 (Fogra 39).
• For uncoated offset papers with high whiteness (high OBA), use Fogra 52 (PSOuncoated_v3_FOGRA52.icc) instead of legacy PSO Uncoated ISO12647 (Fogra 47).

2. Spectrophotometer Settings in the RIP

• When using modern instruments (X-Rite i1Pro 2, i1Pro 3, Barbieri Spectro LFP) during linearization and profiling, always manually enforce M1 mode.
• In Onyx (Media Manager) or Caldera (EasyMedia), select the option Illuminant D50 / Measurement Condition M1 when reading target charts.

3. UV Printing on Rigid Plastics

When printing UV-curing inks directly onto plastic sheets (PVC, polystyrene, acrylic), substrate fluorescence can be erratic due to the high-heat curing lamps of the print engine:

• If an opaque white ink layer is printed first, it blocks the plastic's OBA fluorescence. In this scenario, configure the CMYK layer on top to use M1 mode.
• If printing directly onto a fluorescent polymer without a white base layer, using M2 (UV-Cut) is often more effective for profile calculation. This filters out unstable spectral polymer noise, yielding smoother gradients and avoiding banding in highlights.

Technical Audit Checklist for Technologists

If you encounter color drift between runs, inspect the following critical control points:

1. Instrument Compatibility: Do your spectrophotometers (including inline systems in HP Indigo or Konica Minolta engines) physically support M1? Legacy i1Pro (v1) devices are physically limited to M0.
2. Software Configuration: Check the measurement import settings in your RIP. Verify that M1 data is not being silently converted to M0.
3. Viewing Environment: Are your viewing booths equipped with D50 lamps that have a calibrated UV component? Evaluating M1-profiled prints under generic shopfloor LEDs renders color control useless, as the human eye will not see OBA fluorescence under UV-free light.

Standardizing on M1 measurement is the only way to ensure digital print color matches predictably, regardless of paper type or ambient lighting conditions.