Hierarchical secret image sharing using phase modulation based on the Fresnel transform

ABSTRACT

In idealized secret image sharing (SIS), all of the shared images (shadows) play the same role, but in real life, the shared images are often hierarchical. This paper proposes a hierarchical secret image sharing (HSIS) method by means of optical imaging. To implement the scheme, a generalized single-phase modulation algorithm is proposed in Fresnel transform domain. Its features that multiple secret images can be recovered through combination of some shared images though each participant only holds one share of images. Theoretically, the secret images are divided into multiple phase encodings (shared images) by phase modulation, each secret image can be directly captured by the intensity detector when illuminating some of the shared images orderly displayed with parallel light. Experimental simulation shows that the shared images and the restored secret images do not have any size distortion, and further verifies the feasibility of the proposed scheme.