What is opacity?

The degree of resistance of the paper to penetration of light incident on the paper layer. This property does not affect the printability of paper. In addition to reducing the printing contrast, the low-opacity printing paper still leaves ink printed on it. In addition, white paper is laminated on dark cardboard, or carbon paper is used on the lower layer of the white paper. Envelopes and gift wraps require high-opacity paper. When incident light hits the paper, part of the incident light is reflected back and the other part is absorbed. The rest of the light penetrates the paper layer. There are two types of light penetrating the paper layer, one is non-diffusing parallel light and the other is diffused light. The sum of the light penetrating in parallel and the penetrating light can determine the opacity of the paper. Pure cellulose fibers are transparent. The light entering the paper layer can be absorbed, or returned to the air layer through the air interface with the fiber, or filled with filler fibers and air. The diffused light produced by the interface between air and fillers and fibers can increase the opacity. The addition of dark pigments and dyes in the paper can absorb part of the incident light, thus increasing the opacity of the paper. Milled wood pulp and unbleached fibers can increase the opacity of paper because they absorb a greater amount of incident light. Fillers in filled paper can reflect more light and therefore increase the opacity of the paper. Titanium dioxide has the best hiding efficiency, and it can increase the paper's opacity in addition to increasing the whiteness of paper, but its disadvantage is that the price is relatively high. In general, printing paper is translucent, and for printability, it is sometimes desirable to add expensive titanium dioxide.