Color image single-channel encryption based on tricolor grating theory

Abstract

A method of color image single-channel encryption is proposed. The proposed method uses tricolor grating to encode a color image into a gray level image, then the gray level image is encrypted by double random phase encryption, so a color image is encrypted in a single-channel and its security is ensured. Computer simulations and the chromatic aberration analysis are given to prove the possibility of the proposed idea.The optical system is simpler and is easy to be applied into practice. The simulation results show that this method is efficiency to encrypt a color image, and it is robust.In recent years many optical image encryption techniques have been proposed, each has its meaningful and advantages in practical applications [1][2][3][4][5][6] . However, in these methods, the images are illuminated by monochromatic light, and the reconstructed images would lose their color information. In many cases, we need color information for its practical use, so color image encryption is becoming an important part of information security. More and more people have been attracted in this domain and some optical methods have been proposed in this area. Zhang and Karim introduced a method for color image encryption based on Fourier transform [7] . Zhao and Chen presented a multichannel color image encryption method by wavelength multiplexing based on lensless Fresnel transform holograms and double random phase encryption [8,9] . These methods can be used to obtain the encryption in real time with a high security. However, there are three channels in the optical setup, thus three lasers of different wavelengths will be needed. The system is complicated and expensive.A single-channel color image encryption method based on tricolor grating encoding is reported in this letter. By using a tricolor grating encoding, the color information of color image is encoded into gradational information of gray level image, and the three channels of R, G and B are converted * This work has been supported by the Opening Subject into a single channel; then the gray level image is encrypted with double-phase encryption. Compared with multichannel method, only one laser is needed in this method, thus the system is simplified and more practical. It has been already shown that tricolor grating has the character of multi-wave superposed in spatial domain and separated in frequency domain [10,11] . Tricolor grating is a special spatial light modulator. It is composed of three Ronchi subgratings in different orientations. The tricolor grating and its structure are shown in Fig.1. The R, G and B subgratings can act on the three colors selectively: the vertical subgrating is used for sampling the red information and only red light can pass it; the horizontal subgrating is used for sampling the green information; the sloping subgrating(135°) is used for sampling the blue information. Thus the spatial frequencies of R, G and B light-waves are modulated, and their diffractive transmission directions are changed, then the firstorder spectra of three colors ...

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