Improvements in security substrates that are used in the production of security documents are provided. The security substrate includes a base substrate having a plurality of openings extending through a thickness of the base substrate. The plurality of openings define a frame of the base substrate with an island of the base substrate lying within the frame. The island is connected to the frame by a plurality of spurs formed from the base substrate, with the spurs being spaced around a periphery of the island and being separated by the plurality of openings. The frame has an inner periphery, where at least 50% of the inner periphery lies adjacent to the plurality of openings.