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.