Brightness vs whiteness
Two different measurements, two different standards, routinely conflated on datasheets. Here is what each number actually means.
Brightness and whiteness are not synonyms. A paper can score high on one and low on the other. Mixing them up produces mis-specified substrates and unexpected print results.
Brightness: a narrow-band measurement
ISO brightness (ISO 2470-1) measures diffuse blue reflectance at 457 nm, a single wavelength in the blue end of the visible spectrum. The result is expressed as a percentage of a perfect reflector (0-100+, since optical brightening agents can push values above 100%).
The 457 nm wavelength was chosen because it discriminates well between pulp grades: natural unbleached pulp reflects poorly here; bleached pulp reflects strongly; mechanical pulp (which yellows under UV) falls in between. It is a proxy for "how well was this pulp bleached," not "how white does this look under all light sources."
TAPPI brightness (T452) is nearly identical in principle, also 457 nm, but uses a slightly different geometric measurement condition (directional rather than diffuse). US mills quote TAPPI brightness; European and Asian mills typically quote ISO. The two are close but not identical: ISO values tend to run ~0.5-1.5 points lower.
Whiteness: a full-spectrum integration
ISO whiteness (ISO 11475, CIE whiteness) integrates reflectance across the entire visible spectrum, weighted to mimic how the human eye perceives whiteness under a standard illuminant (D65, representing average daylight). The CIE formula also accounts for the tint (blue-red balance).
A high-whiteness paper looks white to a human observer under daylight. A high-brightness paper may look strongly blue-tinted under fluorescent office lighting, not because it is dirty, but because it contains optical brightening agents (OBAs) that re-emit UV as visible blue light.
Why the difference matters
| Scenario | Which matters | |---|---| | Specifying uncoated office paper for black-only printing | Brightness (contrast with toner/ink) | | Matching off-press proofs to press output | Whiteness (visual match in daylight) | | Checking bleaching efficiency at a mill | ISO brightness | | Archival documents (no OBAs allowed) | Neither, look for ISO 9706 permanence | | Newsprint with no optical brighteners | Brightness (OBA-free baseline) |
OBAs complicate both measurements. They boost perceived whiteness under UV-rich daylight and brightness meter readings, but fade over time and degrade under high temperature or humidity. For specifications where longevity matters, archival books, legal documents, insisting on OBA-free furnish means rejecting papers with high brightness-by-OBA numbers.
Newsprint WPI-g-000869 is a clear case: it carries no OBAs, runs at ISO brightness ~57-60, and no one expects it to look white. The relevant spec is contrast with black ink, not visual whiteness.
Art printing papers WPI-g-000498 are often specified with both figures: ISO brightness ≥94 and CIE whiteness ≥130. When you see only one figure on a datasheet, ask which standard was used.
Which standard to cite
- ISO 2470-1, ISO brightness, diffuse geometry. Use for international specifications.
- TAPPI T452, US domestic brightness standard. If you're buying from a North American mill, expect this number.
- ISO 11475 / CIE, Whiteness with tint information. Use when the visual appearance under daylight conditions matters more than instrument-room brightness.
WPI editorial note: If a supplier quotes a single "brightness" figure above 100, they are including OBA contribution. That is valid under the standard but worth flagging, papers with heavy OBA loading can exhibit metameric mismatch between samples evaluated under different light sources.
For questions about how surface treatment affects these readings, see opacity.