There are 2 measures of opacity:
Contrast ratio (TAPPI opacity) is the ratio of the diffuse reflectance of light (R0), when the paper is backed by a very black material, to the diffuse reflectance when it is backed by a very white material (eg: 89% reflectance in TAPPI Test Method T 425). This dependence on the paper's opacity can be seen by placing a printed sheet of paper upside down on both a black surface and a white surface at the same time. The show through is much greater for a transparent paper than for an opaque paper.
Printing opacity (ISO opacity) is the ratio of the diffuse reflectance of light (R0), when the paper is backed by a very black material, to the diffuse reflectance when it is backed by sufficient layers of the same paper for the number of layers not to affect the reading (reflectivity, R). This measure approximates more closely to the visual effect of interest.