Optimization of Hologram for Security Applications

Abstract

Optical pattern recognition for validation and security verifications has been one of the important issues of optical image processing for the past decade. As one technique, a method of joint transform correlation (JTC) has frequently been used for optical security systems for identification of biometric images. In that method, an image such as a fingerprint pattern is encrypted by a random key mask, and the joint Fourier transform of the image to be encrypted and the key pattern conforms an encrypted hologram. The hologram is printed on a card or a document for authenticity, such as a credit card or a passport. The hologram is read when and where necessary and decoded by using the same key that is used for the encryption. In practical optical security systems, a digital technique is used for the image encryption, since the time required to calculate an encryption pattern is not a critical factor and the encryption of an image and printing of the encrypted hologram on a card may be done offline. On the other hand, fast processing is required to decode an encrypted image and verify it. Accordingly, the optical technique is very suitable for such processing. In this chapter, we discuss optical security systems suitable for the use of holograms. For the congeniality of the method with real optical systems, binary holograms are frequently used in those systems. By the binarization of hologram, the reconstructed hologram is greatly degraded and, therefore, the optimization of hologram is required for the identification of a reference image. We study the method of the optimization of binary holograms based on a simulated annealing technique. The method of the simulated annealing usually takes a long time to reach a correct estimate, so that the fast optimization for binary hologram is applied. We also demonstrate an image decryption by the optimization of a binary hologram when both the hologram and the key for decryption are embedded in real electronic displays with periodic lattice structures. Finally, we discuss a technique to obtain an exact decryption image based on a phase-encoding technique, which enables easier realization for the practical applications in optical security systems. Even in this technique, the optimization of phase-encoded hologram plays an important role to obtain a good image-reconstruction.