We propose a secure display that limits the viewing space three-dimensionally by use of optically decodable encryption.
A secret image is encrypted into a displayed image and a decoding mask. The decryption process is based on optical
logic and performed without calculation. The secret is visible within the limited viewing space. However, the viewed
results outside the viewing space look like random-dot pattern. When viewed at a distance different from the designed
viewing distance, only the central area of the secret is faintly visible for a limited direction. When a user watches the
secure display, in practice, the user's head prevents peeping at the secret. The relationships between the size and the
position of the displayed image and the viewing space have been analyzed and confirmed experimentally. The proposed
technique has been demonstrated by use of two-layered liquid-crystal displays. Furthermore, 3D displacement of the
viewing space have been realized only by renewing the displayed image, i.e., without any mechanical movements. By
using the decoding mask as the key for decryption, no one can view the secret image without the knowledge of the
decoding mask pattern, and the viewed image is secure against prying eyes behind the viewer.