scope:

The object of this method is to determine paper opacity by measuring the ratio of the reflectance of paper backed by black and white using an instrument equivalent to the geometry of the TAPPI brightness measurement instrument. Approximate TAPPI opacity is measured by contrast ratio of the specimen reflectance backed by a black cavity to white backing of 89%. The term "approximate" is used to differentiate the TAPPI opacity measured with this procedure to the TAPPI opacity measured using an instrument that conforms to the measurement of opacity as specified in the geometry in T 425 "Opacity of paper (15/d geometry, illuminant A/2o, 89% reflectance backing and paper backing)."

NOTE 1: Institute of Paper Chemistry (now Institute of Paper Science and Technology at Georgia Tech) Instrumentation Report 22, Part V, "Comparison of Printing Opacity Determined with Several Instruments [Tappi 53 (3): 436 (1970)] shows that the differences related to the changes in geometry are small in all cases; the differences related to the change in illuminant are small for white papers.