Opacity compares how much light a single sheet reflects over a black cavity with how much it reflects over a white background, either a pad of the same paper or an 89 percent reflectance backing. A sheet at 100 percent hides everything behind it. Opacity rises with grammage, with light scattering from fines, filler and fiber surfaces, and with absorption from dyes or dark fiber, and it falls when calendering or wetting collapses the air spaces inside the sheet.
Dropping to 45 gsm cost us almost three points of opacity, so we're back to adding TiO2.
Opacity is hard to buy back: once refining and calendering have closed up the sheet, the quick fixes are filler, TiO2 or more grammage.
Do not mix it up with
- Show-through: Show-through is the visible defect in print; opacity is the instrument value that predicts it but ignores ink oil wetting the sheet.
Related terms
TAPPI opacity
Contrast ratio opacity: single-sheet reflectance over black divided by reflectance over an 89 percent white backing.
Printing opacity
Opacity calculated as single-sheet reflectance over black divided by the reflectance of an opaque pad of the same paper.
Show-through
Print on the back of a sheet, or on the page beneath, visible from the front because too much light passes through.
Light-scattering coefficient
Per-grammage measure of how strongly a paper redirects light, written s and given in square meters per kilogram.
TiO2
Titanium dioxide: the most powerful white pigment for adding opacity, used where every point of opacity counts.