There is described a substrate (S) for security documents, such as banknotes, comprising one or more paper layers (11, 12) and a polymer layer (20) which is made to adhere to a side of at least one of the paper layers (11, 12), which polymer layer (20) is substantially transparent in at least one region of the substrate (S) which is not covered by the paper layer or layers (11, 12) so as to form a substantially transparent window (W) in the substrate (S) which is formed and closed by the polymer layer (20). The polymer layer (20) exhibits in the region of the window (W) a thickness (T) which is greater than a thickness (t) of the polymer layer (20) outside of the region of the window (W). The thickness (T) of the polymer layer (20) in the region of the window (W) is substantially equal to the added thickness of the paper layer or layers (11, 12) and of the polymer layer (20) outside of the region of the window (W) so that the substrate (S) exhibits a substantially uniform and constant thickness (T). The substrate (S) further comprises a micro-optical structure (30), in particular a lens structure, which is disposed in the region of the window (W) on at least one side of the polymer layer (20).