From patent application FR 2 804 784, an identifying member made of a transparent curable resin is known, which incorporates gas bubbles randomly scattered within the identifying member. The gas may optionally be colored in order to increase the contrast between the bubbles and the remainder of the identifying member. This identifying member requires a specific manufacturing process, which is appropriate only for certain types of material.
Also known from patent application FR 2 425 937 is a fibrous structure containing metal fibers. In order to authenticate or identify the fibrous structure, a signature formed by metal fibers dispersed within the fibrous structure is used by relying, for example, on the nature, size, concentration and spatial location of the metal fibers (three-dimensional distribution). Said metal fiber signature may be measured, for example, by an appropriate sensing system in response to a magnetic stimulus.
Moreover, when a paper sheet is formed, the fibers assume an orientation and a distribution, on which the look-through and other optical properties of the sheet depend. The manufacturer will generally attempt to make the fiber distribution homogeneous in order to create a so-called well-closed formation. The presence of air bubbles is generally considered to be a defect, as these air bubbles lead to surface irregularities when forming the sheet, and/or to the presence of minute holes in the sheet. These defects are conspicuous by transmission or look-through, as they generate a locally smaller material density.
The manufacturer may also have to deposit on the paper sheet surface one or more non-fibrous, notably binder-based, surface layers, so as to provide printability characteristics and/or specific properties. Here again, air bubbles within these layers cause defects known as “pinholes” leading to visible light-colored spots, notably in transmitted light.
Devices for removing the air bubbles before they reach the area where the paper layer is formed have been developed. In particular, paper machines are equipped with circuits for stock deaeration so as to reduce the air bubbles.