Paper opacity is the ability of a sheet of paper to prevent light transmission through it. The higher the opacity the less “show through” (when print on one side of a sheet of paper can be easily seen from the other side). Using a high opacity paper allows the reader to concentrate on the content, and reduces eyestrain.
Opacity is influenced by such characteristics as basis weight, color, and caliper. Non-cellulosic mineral fillers can be added during manufacturing to increase opacity. Some commonly used fillers are: clay, calcium carbonate, titanium dioxide, aluminum trihydrate and hydrated silicon dioxide. These fillers can not only increase opacity but also cost. Generally speaking the more opaque a sheet, the more costly it is.