At present time methods of optical encryption are actively developed. The majority of existing methods of optical encryption use not only light intensity distribution, easily registered with photosensors, but also its phase distribution which require application of complex holographic schemes in conjunction with spatially coherent light. This leads to complex optical schemes and low decryption quality. To eliminate these disadvantages it is possible to implement optical encryption using spatially incoherent illumination which requires registration of light intensity distribution only. However this applies new restrictions on encryption keys: Fourier spectrum amplitude distribution of encryption key should overlap Fourier spectrum amplitude distribution of image to be encrypted otherwise loss of information is unavoidable. Therefore it seems that best key should have white spectrum. On the other hand due to fact that only light intensity distribution is registered, spectra of image to be encrypted and encryption key always have peaks at zero frequency and their heights depend on corresponding total energy. Since encrypted image contains noise, ratio of its average spectrum energy to noise average energy determines signal to noise ratio of decrypted image. Therefore ratio of amplitude at zero frequency to average spectrum amplitude (RZA) of encryption key defines decrypted images quality. For generation of encryption keys with low RZA method of direct search with random trajectory (DSRT) was used. To estimate impact of key RZA on decrypted images error numerical experiments were conducted. For experiments keys with different RZA values but with same energy value were generated and used. Numerically simulated optical encryption and decryption of set of test images was conducted. Results of experiment demonstrate that application of keys with low RZA generated by DSRT method leads to up to 20% lower error in comparison to keys generated by means of uniform random distribution.

Generation of keys for image optical encryption in spatially incoherent light aimed at reduction of image decryption error

Optical encryption of arrays of binary digits in spatially incoherent light is experimentally implemented. Successful optical encryption and numerical decryption of images of binary arrays using a developed setup is demonstrated. Faultless decryption of arrays with normalized average energies (NAEs) up to 0.3 inclusive has been implemented. It is demonstrated that a decrease in the NAE of arrays to be encrypted leads to a decrease in the encryption error rate.

Optical Encryption of Arrays of Binary Digits in Spatially Incoherent Light

Diffraction image correlator based on commercial digital SLR photo camera was reported earlier. The correlator was
proposed for recognition of external scenes illuminated by quasimonochromatic spatially incoherent light. The
correlator hardware consists of digital camera with plugged in optical correlation filter unit and control computer. The
kinoform used as correlation filter is placed in a free space of the SLR camera body between the interchangeable camera
lens and the swing mirror. On the other hand, this correlator can be considered as a hybrid optical-digital imaging
system with wavefront coding. It allows not only to recognize objects in input scene but to restore, if needed, the whole
image of input scene from correlation signals distribution registered by SLR camera sensor. Linear methods for image
reconstruction in the correlator are discussed. The experimental setup of the correlator and experimental results on
images recognition and input scenes restoration are presented.

Input scene restoration in pattern recognition correlator based on digital photo camera

Performance capabilities of commercial digital cameras are demonstrated by the example of a Canon EOS 400D camera in measuring and detecting spatial distributions of laser radiation intensity. It is shown that software extraction of linear data expands the linear dynamic range of the camera by a factor greater than 10, up to 58 dB. Basic measurement characteristics of the camera are obtained in the regime of linear data extraction: the radiometric function, deviation from linearity, dynamic range, temporal and spatial noises (both dark and those depending on the signal value). The parameters obtained correspond to those of technical measuring cameras.

http://iopscience.iop.org/article/10.1070/QE2010v040n04ABEH014202/pdf

Extension of the possibilities of a commercial digital camera in detecting spatial intensity distribution of laser radiation

At present time methods of optical encryption are actively developed. The majority of existing methods of optical
encryption use not only light intensity distribution, easily registered with photosensors, but also its phase distribution
which require application of complex holographic schemes in conjunction with spatially coherent monochromatic
illumination. This leads to complex optical schemes and low decryption quality. To eliminate these disadvantages it is
possible to implement optical encryption using spatially incoherent monochromatic illumination which requires
registration of light intensity distribution only.
Encryption is accomplished by means of optical convolution of image of scene to be encrypted and encryption
diffractive optical element (DOE) point spread function (PSF) which serves as encryption key. Encryption process is
described as follows. Scene is illuminated with spatially-incoherent monochromatic light. In the absence of encryption
DOE lens forms image of scene in photosensor plane. DOE serves as encryption element, its PSF - encryption key.
Light passing through DOE forms convolution of object image and DOE PSF. Registered by photosensor convolution is
encrypted image. Decryption was conducted numerically on computer by means of inverse filtration with regularization.
Kinoforms were used as encryption DOE because they have single diffraction order. Two liquid crystal (LC) spatial light
modulators (SLM) were used to implement dynamic digital information input and dynamic encryption key change. As
input scene amplitude LC SLM HoloEye LC2002 with 800×600 pixels 32×32 μm 2 and 256 gray levels was used. To
image synthesized encryption kinoforms phase LC SLM HoloEye PLUTO VIS with 1920×1080 pixels 8×8 μm 2 and 256 phase levels was used. Set of test images was successfully optically encrypted and then numerically decrypted.
Encrypted images contents are hidden. Decrypted images despite quite high noise levels are positively recognizable.
Results of optical encryption and numerical decryption are presented.

Optical encryption in spatially-incoherent light using two LC SLMsfor both information input and encryption element imaging

Diffraction image correlator based on commercial digital SLR photo camera is described. The correlator is proposed for recognition of external 2-D and 3-D scenes illuminated by quasimonochromatic spatially incoherent light. Principal optical scheme of the correlator is analogous to that of incoherent holographic correlator by Lohmann. The correlator hardware consists of digital camera with attached optical correlation filter unit and control computer. No modifications have been introduced in units of commercial digital SLR photo camera. Digital camera was connected via camera interface to computer for controlled camera shooting, transfer of detected correlation signals and post-processing. Two ways were used for correlation filter unit mounting. In the first case, correlation filter was attached to the front of the camera lens. In the second one, it was placed in a free space of the SLR camera body between the interchangeable camera lens and the swing mirror. Computer generated Fourier holograms and kinoforms were used as correlation filters 
in experiments. The experimental setup of the correlator and experimental results on images recognition are presented. The recognition of test objects of direct and reversed contrast with the same correlation filter was performed.

Pattern recognition correlator based on digital photo camera

Since the moment of laser appearance and till now the attention concentrated on great potentiality realization of coherent optical data processing systems. However, the difficulties occurred in creating the workable systems of this type were the reasons of spreading the field of investigation on diffraction systems in which not only the coherent, but the partially coherent and non-coherent illumination is used. Spatial coherence and light spectrum components as well as amplitude, phase and polarization of light passed through the optical system can be modulated by the information to be processed. The transformation of spatial coherence allows to perform Fourier and correlation analysis of images or temporal signals formed by spatially incoherent quasimonochromatic light, when the traditional holographic matched filter is used. The input of two-dimensional data arrays can be performed by the acousto-optic deflector. Developed method of bipolar holographic filter pulse response synthesis increases image recognition reliability. If the light is non- monochromatic it is possible to involve the spectral components as the information parameters in correlation signal formation due to the special modulating mask placed in reference beam when the filter is recorded. A possibility of optical system creation with no lens for image formation based on spatial coherence transform is considered.

Optical information processing systems with various structure incoherent radiation

Fourier holograms are commonly used for reference images storing in diffraction correlators with spatially coherent or spatially incoherent illumination. Kinoforms can be a real alternative to Fourier holograms in the correlators. The kinoform represents a computer-synthesized optical element which performs only a phase modulation of a light wave. The kinoform restores true intensity of the recorded image and random distribution of phase. Therefore, it can be utilized for storing reference images, first of all, in correlators with spatially incoherent illumination. The absence of carrier frequency reduces demanded number of pixels of the spatial light modulator being used. Since the kinoform provides reconstruction of reference image in zero diffraction order, requirement on monochromaticity of illumination are decreased as well. The diffraction correlator with the kinoform used as spatial frequency filter is considered. The 2-D acoustooptic deflector was employed to form input images in real time by monochromatic spatially incoherent light. The reference images were recorded on the commercially available kinoforms. The input and reference images were of 256×256 pixels and 200×200 pixels respectively. Since input images were consisted of approximately 400 pixels with non-zero 
brightness, the image update frequency was gained at 200 Hz. The experimental setup and experimental results on images recognition are presented.

Incoherent acousto-optic image correlator with the kinoform

A possibility of creation of optical-electronic system with no lines for image formation based on light field spatial coherence transform and of detecting nonstationary wave fronts with the help of wide-aperture photodetectors is considered. Methods of useful signal independent time modulation implementation are considered. Experimental results on image formation of 1D objects are shown.

Ivan V. Solyakin

Papers

Input scene restoration in pattern recognition correlator based on digital photo camera

Pattern recognition correlator based on digital photo camera

Optical information processing systems with various structure incoherent radiation

Incoherent acousto-optic image correlator with the kinoform

Image formation in an optical-electronic system with spatial coherent transformation of light field