Measured under TAPPI T 425, it divides R0, the reflectance of one sheet over a black body, by its reflectance over a backing of 89 percent absolute reflectance, then multiplies by 100. Using a fixed white backing instead of the paper's own pad gives a common reference across grades of different brightness. The result usually differs by a point or two from printing opacity on the same sheet.
Data sheet shows TAPPI opacity, but the European customer only accepts printing opacity, so run both.
Also heard as
- contrast ratio
- C89
- 89 percent backing opacity
Do not mix it up with
- Printing opacity: Printing opacity uses a pad of the same paper as the white backing, not a fixed 89 percent standard.
Related terms
Opacity
A sheet's ability to hide what lies behind it, expressed as a percentage from readings over black and white backings.
Printing opacity
Opacity calculated as single-sheet reflectance over black divided by the reflectance of an opaque pad of the same paper.
R0
Reflectance factor of one sheet measured over a black cavity, the starting point for every opacity calculation.
Black cavity
Light trap placed behind a single sheet so nothing reflects back from the backing during an R0 reading.