In optics, the extent to which an object or surface will impede the transmission of light through it. A completely opaque object is one which allows no light to pass through it. See also Translucency and Transparency.

In papermaking, opacity is a property of paper that describes the amount of light which is transmitted through it. Paper that has a high degree of opacity does not let much light pass through it, while paper that has a low degree of opacity is more translucent, or allows much light to pass through it. A paper's opacity determines the extent to which printing on a particular side of paper will be visible from the reverse side (called show-through).

Cellulose fibers, the primary constituents of paper, are transparent, but the piling up of them in a paper web diffuses the light passing through the sheet, imparting a paper's opacity. Fillers such as clay, titanium dioxide, and calcium carbonate are added to increase the diffusion of light through a paper, thereby increasing its opacity. The best opacifiers, however, tend to be the most expensive, so the majority of papers are at best translucent. Tinting and dyeing of papers also increase their opacity, as does increasing a paper's basis weight, bulk, or coating. Increasing a paper's whiteness tends to decrease its opacity, as does increasing its degree of calendering, supercalendering, or fiber refining.

The measurement of a paper's opacity is accomplished utilizing an opacimeter employing either the contrast ratio method or the diffuse opacity method. The contrast ratio method compares the amount of light reflected back through a paper when it is backed by a black sheet of paper to the amount reflected back though a paper when it is backed by a white surface. The diffuse opacity method differs from the contrast ratio method in that the white surface is replaced by a stack of the paper being tested.

The degree to which printing will show through to the reverse side of a sheet of paper may be the result not of diminished opacity but of an increase in ink absorption and penetration through the paper, being not an optical problem but a structural one.

The term opacity also refers to the extent to which a printed ink will permit or prevent the transmission of light through it, either blotting out or allowing to be seen what is beneath it. Opaque pigments do not allow light to pass through them, instead reflect it back, and are used to print solid colors. Transparent pigments allow varying amounts of light to pass through them, revealing the background printing or substrate. Transparent pigments are used to reduce color strength of more opaque inks, aid dispersion of color pigments, and "extend" a quantity of more expensive pigments. (See Pigment.) Ink opacity is also referred to as its covering power.

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