Optical brightening agents make paper appear whiter by absorbing ultraviolet radiation and re-emitting part of that energy in the blue visible region. Because the specimen adds emitted light to reflected light, its measured color depends on the spectral power reaching it. That is why UV control is central to modern paper and print measurement.
How optical brighteners change color
An OBA-treated sheet can have reflectance-factor values above 100% in the blue region because fluorescence contributes additional visible energy. More effective UV excitation generally produces a bluer, brighter-looking sheet; UV-poor illumination reduces that contribution. The effect carries into printed areas because many inks are partly transparent and the substrate remains part of the observed color.
Why apparently good instruments can disagree
Two instruments may be individually repeatable yet deliver different results if their illumination contains different UV energy or if their UV calibration targets differ. A viewing booth can also disagree with the instrument if its D50 simulation is poor in the UV region. This is a method mismatch, not necessarily a fault.
Choose the condition for the decision
| Condition | Use and limitation |
|---|---|
| M1 | D50-aligned graphic-arts measurement with controlled UV behaviour; preferred where OBA-containing stocks must align with modern viewing conditions. |
| M2 | UV-suppressed measurement used to remove most fluorescent contribution for diagnosis or a specified UV-cut workflow. |
| ISO brightness | Paper test calibrated to the ISO 2470-1 indoor-daylight UV condition, not simply an M1 print reading. |
| D65 brightness | Paper test with stronger UV excitation defined by ISO 2470-2; values should not be labelled ISO brightness. |
Use paired conditions to reveal fluorescence
When the instrument supports it, compare the same sheet under the specified UV-included condition and UV-cut condition. A larger difference in the blue region indicates stronger fluorescent contribution. Use this comparison diagnostically; it does not replace the official calculation or calibrated reference procedure required by a paper standard.
Do not correct ink before checking paper
If unprinted substrate and every printed patch shift together between sites, investigate UV condition, paper lot and backing before reformulating color.
Control the complete fluorescent workflow
- Specify the M or paper measurement condition with every target.
- Use certified fluorescent references and follow their handling and replacement rules.
- Align viewing booths with the applicable ISO 3664 condition.
- Track substrate spectral data by lot, not whiteness alone.
- Check inter-instrument agreement on representative OBA papers as well as nonfluorescent tiles.
Link the UV method to paper whiteness, ISO 13655 and printed-color measurement.
Last reviewed 22/09/2026 · Level: Advanced
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