Optical Encryption Of Three-dimensional Object With Digital Holography

With the development of science and technology, people enter the information age, but accordingly face the increasingly serious information security problem. Initially, people mainly encrypt image by the electronic means such as computer or digital signal processor; however, this kind of methods is limited both to speed and cost. Luckily, optical encryption with such advantages as concurrency, high-speed, large-capacity information, a large key space, dimensions, etc. becomes an important area in information security.A large number of three-dimensional objects are seen in our daily life, so studying on the encryption them is of great importance. This thesis first briefly introduced the theory of Fourier transform, fractional Fourier transform, Fresnel diffraction and Fresnel holography, and then proposed three encryption techniques, together with analyses and optical experiments on these technical principles. The main research contents are presented as follows:First, this thesis presents a optical encryption of three-dimensional objects with digital holography based on Fourier transform, and then give the corresponding optical experiments. Firstly, digital holograms with the object information and key information are recorded respectively, and then in the digital Fourier holography, the useful information is extracted, eliminating the zero-order image and conjugate image. The three-dimensional object is reconstructed by inverse Fourier transform frequency domain information. Optical experiments results prove the feasibility of this method. In addition, by analyzing the robustness against occlusion and noise attacks of the encryption system can draw a conclusion that reducing the noise can improve the system’s the robustness against occlusion and noise attacks.Second, a optical encryption of three-dimensional objects with digital holography based on fractional Fourier transform is proposed. The three-dimensional object is reconstructed by filtering in spectrum domain and simple numerical calculation. Optical experiments results prove that the three-dimensional object is reconstructed when the keys are correct. The robustness against occlusion and noise attacks of the encryption system is analyzed. Compared with the optical encryption with digital holography based on Fourier transform, the optical encryption with digital holography based on fractional Fourier transform enhances the security of the system, because the fractional order is another key.Third, a optical encryption of three-dimensional object with digital holography in the Fresnel domain is proposed. The digital holograms witch are recorded by CCD are Fourier transformed by computer and the useful information is extracted. The three-dimensional object is reconstructed when the frequency domain information is inverse Fourier transformed and Fresnel diffraction. The robustness against occlusion and noise attacks of the encryption system is analyzed. Diffraction distance can be used as a key except the RPMs. The decrypted images with different diffractive distance are presented to have a stereoscopic effect on 3D object.