A black-lined box or tube, usually angled inside or lined with velvet, that swallows essentially all light passing through the sample. Its own reflectance has to be close to zero, otherwise R0 and every opacity value calculated from it read high. Dust in the cavity and worn lining are a common cause of opacity readings that creep upward over weeks.
Opacity has been creeping up all week, so check the black cavity for dust before anybody chases the furnish.
Also heard as
- black body
- light trap
Related terms
R0
Reflectance factor of one sheet measured over a black cavity, the starting point for every opacity calculation.
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.