We describe the encryption and decryption of a two-dimensional (2-D) image by using a sandwich diffuser in the Fourier plane of the first lens of a 4-f imaging system. The diffuser is made with two random phase diffusers, sandwiched together. The encrypted image is holographically recorded in a photorefractive BaTiO3 crystal. Decryption is done by generating a phase conjugate image through the phase conjugation technique. During decryption, if only one of the two random phase diffusers constituting the sandwich diffuser is used, it becomes impossible to retrieve the image. Also no decryption is possible if any of the diffusers constituting the sandwich is shifted in-plane with respect to the other. It is possible to generate the image only if the diffusers are sandwiched together again after matching point to point, and then the sandwich diffuser is kept at the same position and orientation in which it was kept during the encryption. Experimental method has been described to overcome the practical difficulty for the generation of the key in the case of the sandwich diffuser. Simulation and experimental results are presented in support of the proposed idea. To evaluate reliability of the technique, the mean-square-error (MSE) between decrypted and original image has been calculated.
"Optical encryption and decryption using a sandwich random phase diffuser in the Fourier plane," Optical Engineering 46(5), 055201 (1 May 2007). https://doi.org/10.1117/1.2739627