A further study on an optical watermarking scheme

Recently, authentication of valuable documents has been widely studied and applied all over the world. One of the solutions for high-security authentication is optical watermark, which is different from traditional digital watermark in the way of watermark extraction. Optical watermark is detected by some optical and visual means. This paper discuss the optical watermark based on Moire phenomenon. We first elucidate the Moire phenomenon mathematically. Then we give an in-depth exploration of optical watermark based on phase modulation in a theoretical way. In addition, a security improvement with random noise is introduced to make the watermark harder to be cracked.