Phase in Optical Image Processing

The use of phase has a long standing history in optical image processing, with early milestones being in the field of pattern recognition, such as VanderLugt's practical construction technique for matched filters, and (implicitly) Goodman's joint Fourier transform correlator. In recent years, the flexibility afforded by phase-only spatial light modulators and digital holography, for example, has enabled many processing techniques based on the explicit encoding and decoding of phase. One application area concerns efficient numerical computations. Pushing phase measurement to its physical limits, designs employing the physical properties of phase have ranged from the sensible to the wonderful, in some cases making computationally easy problems easier to solve and in other cases addressing mathematics' most challenging computationally hard problems. Another application area is optical image encryption, in which, typically, a phase mask modulates the fractional Fourier transformed coefficients of a perturbed input image, and the phase of the inverse transform is then sensed as the encrypted image. The inherent linearity that makes the system so elegant mitigates against its use as an effective encryption technique, but we show how a combination of optical and digital techniques can restore confidence in that security. We conclude with the concept of digital hologram image processing, and applications of same that are uniquely suited to optical implementation, where the processing, recognition, or encryption step operates on full field information, such as that emanating from a coherently illuminated real-world three-dimensional object

Phase in Optical Image Processing

The use of phase has a long standing history in optical image processing, with early milestones being in the field of pattern recognition, such as VanderLugt's practical construction technique for matched filters, and (implicitly) Goodman's joint Fourier transform correlator. In recent years, the flexibility afforded by phase-only spatial light modulators and digital holography, for example, has enabled many processing techniques based on the explicit encoding and decoding of phase. One application area concerns efficient numerical computations. Pushing phase measurement to its physical limits, designs employing the physical properties of phase have ranged from the sensible to the wonderful, in some cases making computationally easy problems easier to solve and in other cases addressing mathematics' most challenging computationally hard problems. Another application area is optical image encryption, in which, typically, a phase mask modulates the fractional Fourier transformed coefficients of a perturbed input image, and the phase of the inverse transform is then sensed as the encrypted image. The inherent linearity that makes the system so elegant mitigates against its use as an effective encryption technique, but we show how a combination of optical and digital techniques can restore confidence in that security. We conclude with the concept of digital hologram image processing, and applications of same that are uniquely suited to optical implementation, where the processing, recognition, or encryption step operates on full field information, such as that emanating from a coherently illuminated real-world three-dimensional object

Stereo vision based approach for extracting features from digital holograms

With digital holography one can record and reconstruct real world three-dimensional (3D) objects [1,2]. The recorded interference pattern includes information about both amplitude and phase of a wavefront reflected from or transmitted through the object. However, some of the hologram capture setups pose a problem for the reliable reconstruction of quantitative phase information. This can be because the twin image or noise corrupts the reconstructed phase. In such cases it is usual that only amplitude is reconstructed and used as the basis for metrology. A focus criterion is often applied to this reconstructed amplitude to extract depth information from the sensed 3D scene [3,4]. In this paper we present an alternative technique based on applying conventional computer stereo vision algorithms to amplitude reconstructions. We show the effectiveness of our technique using digital holograms of both macroscopic and microscopic real-world 3D objects. We discuss sensitivity to the depth of field of reconstructions, and which hologram capture setups are, and which are not, suitable for the technique

Numerical reconstruction of digital holograms for conventional 3D display

True hologram video displays are currently under development, but are not yet available. Because of this restriction, conventional 3D displays can be used with digital holographic data. However when using conventional 3D displays, holographic data has to be processed correctly to meet the requirements of the display. A unique property of digital holograms, namely that a single hologram encodes multiple perspectives, can be used to achieve this goal. Reconstructions from digital holograms at different perspectives are processed further to meet the requirements of the conventional 3D display, which are typically based on stereoscopic images of the scene

Study of imperfect keys to characterise the security of optical encryption

In conventional symmetric encryption, it is common for the encryption/decryption key to be reused for multiple plaintexts. This gives rise to the concept of a known-plaintext attack. In optical image encryption systems, such as double random phase encoding (DRPE), this is also the case; if one knows a plaintext-ciphertext pair, one can carry out a known-plaintext attack more efficiently than a brute-force attack, using heuristics based on phase retrieval or simulated annealing. However, we demonstrate that it is likely that an attacker will find an imperfect decryption key using such heuristics. Such an imperfect key will work for the known plaintext-ciphertext pair, but not an arbitrary unseen plaintext-ciphertext pair encrypted using the original key. In this paper, we illustrate the problem and attempt to characterise the increase in security it affords optical encryption

Compression of encrypted three-dimensional objects using digital holography

We present the results of applying data compression techniques to encrypted three-dimensional objects. The objects are captured using phase-shift digital holography and encrypted using a random phase mask in the Fresnel domain. Lossy quantization is combined with lossless coding techniques to quantify compression ratios. Lossless compression alone applied to the encrypted holographic data achieves compression ratios lower than 1.05. When combined with quantization and an integer encoding scheme, this rises to between 12 and 65 (depending on the hologram chosen and the method of measuring compression ratio), with good decryption and reconstruction quality. Our techniques are suitable for a range of secure three-dimensional object storage and transmission applications

Using disparity in digital holograms for three-dimensional object segmentation

Digital holography allows one to sense and reconstruct the amplitude and phase of a wavefront reflected from or transmitted through a real-world three-dimensional (3D) object. However, some combinations of hologram capture setup and 3D object pose problems for the reliable reconstruction of quantitative phase information. In particular, these are cases where the twin image or noise corrupts the reconstructed phase. In such cases it is usual that only amplitude is reconstructed and used as the basis for metrology. A focus criterion is often applied to this reconstructed amplitude to extract depth information from the sensed 3D scene. In this paper we present an alternative technique based on applying conventional stereo computer vision algorithms to amplitude reconstructions. In the technique, two perspectives are reconstructed from a single hologram, and the stereo disparity between the pair is used to infer depth information for different regions in the field of view. Such an approach has inherent simplifications in digital holography as the epipolar geometry is known a priori. We show the effectiveness of the technique using digital holograms of real-world 3D objects. We discuss extensions to multi-view algorithms, the effect of speckle, and sensitivity to the depth of field of reconstruction

Framework for task scheduling in heterogeneous distributed computing using genetic algorithms

An algorithm has been developed to dynamically schedule heterogeneous tasks on heterogeneous processors in a distributed system. The scheduler operates in an environment with dynamically changing resources and adapts to variable system resources. It operates in a batch fashion and utilises a genetic algorithm to minimise the total execution time. We have compared our scheduler to six other schedulers, three batch-mode and three immediate-mode schedulers. Experiments show that the algorithm outperforms each of the others and can achieve near optimal efficiency, with up to 100,000 tasks being scheduled

DSEARCH: sensitive database searching using distributed computing

Summary: We present a distributed and fully cross-platform database search program that allows the user to utilise the idle clock cycles of machines to perform large searches using the most sensitive algorithms. For those in an academic or corporate environment with hundreds of idle desktop machines, DSEARCH can deliver a âfreeâ database search supercomputer. Availability: The software is publicly available under the GNU general public licence from http://www.cs.may.ie/distributed Contact: [email protected] Supplementary Information: Full documentation and a user manual is available from http://www.cs.may.ie/distribute

Study of imperfect keys to characterise the security of optical encryption

In conventional symmetric encryption, it is common for the encryption/decryption key to be reused for multiple plaintexts. This gives rise to the concept of a known-plaintext attack. In optical image encryption systems, such as double random phase encoding (DRPE), this is also the case; if one knows a plaintext-ciphertext pair, one can carry out a known-plaintext attack more efficiently than a brute-force attack, using heuristics based on phase retrieval or simulated annealing. However, we demonstrate that it is likely that an attacker will find an imperfect decryption key using such heuristics. Such an imperfect key will work for the known plaintext-ciphertext pair, but not an arbitrary unseen plaintext-ciphertext pair encrypted using the original key. In this paper, we illustrate the problem and attempt to characterise the increase in security it affords optical encryption