Optiks Window
A complex security feature, which consists of:
1. a hidden wide security thread (up to 18 mm), inserted into the paper during its manufacturing process and seen in transmitted light;
2. a hole in the paper substrate with uneven edges and fibers along its perimeter forming a kind of fringe;
3. a transparent polymer strip with metallized coating covering the hole on the front or back side (fig. 1).

a

b

c

d
Fig. 1. Optiks window. 100 Pa’anga of Tonga (2008):
a – thread under reflected light; b – thread in transmitted light; c – window; d – zoomed edge of the window