Also called paper-backing opacity, it is R0 divided by R-infinity times 100, measured with d/0 geometry on the luminance factor Y under illuminant C (ISO 2471). The paper-backed setup mimics a printed book, where the next page acts as the background. It is the preferred opacity for printing and writing grades in most markets.
Bible paper has to hold 80 printing opacity at 30 grams, so the TiO2 stays in the furnish.
On a very bright sheet, printing opacity can read below TAPPI opacity, because the sheet's own pad is a brighter background than the 89 percent backing.
Also heard as
- ISO opacity
- paper-backing opacity
Related terms
Opacity
A sheet's ability to hide what lies behind it, expressed as a percentage from readings over black and white backings.
TAPPI opacity
Contrast ratio opacity: single-sheet reflectance over black divided by reflectance over an 89 percent white backing.
R-infinity
Reflectance factor of a pad so thick that adding more sheets no longer changes the reading.
R0
Reflectance factor of one sheet measured over a black cavity, the starting point for every opacity calculation.
Luminance factor
Tristimulus Y: the overall lightness of a sample as the eye sees it under a given illuminant and observer.