Opacity of paper (15/d geometry, illuminant A/2?, 89% reflectance backing and paper backing)

Opacity is a fundamental optical property of paper as a whole, yet the measurement of opacity is determined by a ratio of reflectance measurements. The opacity of the sheet is influenced by thickness, the amount and kind of filler, degree of bleaching of the fibers, coating, and the like. The utility of bond, writing and book papers is enhanced by a high opacity.

The essential principle of this method for determining the opacity of paper is as follows: The reflectance of paper when combined with a white backing is higher than that of paper when combined with a black backing because, in the former case, light transmitted through the imperfectly opaque sheet is largely reflected by the white backing, and a portion of the light is transmitted through the paper a second time thus increasing the total reflection. Two types of “white” backing are used, leading to two measures of opacity: