There is described a method of creating a transparent polymer window (W) with a field of lenses (L) in a security paper substrate (1), the method comprising the steps of (i) providing a security paper substrate (1), (ii) forming an opening (10) into the security paper substrate (1), (iii) laminating a transparent film (5; 5*) onto a first side (I) of the security paper substrate (1) in such a way as to close the opening (10) at one end, and (iv) filling the opening (10) with transparent polymer material (2). In one embodiment, the transparent film (5) comprises a field of lenses (L) and is laminated onto the first side (I) of the security paper substrate (1) in such a way as to form lenses (L) on the first side (I) of the security paper substrate (1) in register with the opening (10). In another embodiment, the field of lenses (L) is replicated into the transparent polymer material (2) applied in the opening (10) in such a way as to form lenses (L) on a second side (II) of the security paper substrate (1), opposite to the first side (I), in register with the opening (10). Also described is a device designed to fill the opening (10) formed into the security paper substrate (1) with the transparent polymer material (2) and a processing machine comprising the same.