A security document is formed from a transparent plastics substrate having opacifying layers applied to its front and rear surfaces except in at least one window region. A diffractive device provided in the window region, the diffractive device operating in the near infrared (NIR) region of the electromagnetic spectrum, whereby when the security document is passed through an edge detection apparatus that operates in the near infrared (NIR) region, the diffractive device diverts a detector beam of edge detection apparatus away from an edge detector used to detect an edge of the security document, thereby avoiding false positive edge detection results.