Opacity C/2° (%) ISO 2471
Opacity is a measure of a material’s ability to obstruct the passage of light. Sufficient opacity is important to prevent printed text from showing through in a harmful manner on the reverse side of a paper. The opacity of a completely opaque paper is 100%.
Opacity is measured with an instrument that emits C light, i.e. a light that is related to daylight but without UV content. The sample is illuminated vertical to its surface and the detector is located 2° from the vertical axis, as shown in the figure below. Opacity is the ratio between the Y value of paper measured under conditions C/2° and the Y value of a single sheet measured under the same conditions against a black backing.