Phase-only Filter Research Articles

Optimization of decision making for face recognition based on nonlinear correlation plane

We report on a specific procedure in the correlation plane which allows us to make more robust and discriminating decision for face recognition applications. In this scheme, we multiply the correlation plane by a nonlinear function which is chosen to increase the correlation peak, reduce the autocorrelation noise, and increase the inter-correlation noise. In our work we present tests using a VanderLugt correlator (VLC) fabricated with different filters (phase-only filter (POF), composite POF) and we also discuss the efficiency of the protocol using peak-to-correlation-energy measures. Rewardingly, our technical results demonstrate that this method is remarkably efficient to increase both robustness and discrimination performances of a VLC.

Phase filter design for sharper focus of radially polarized beam

Based on vector diffraction theory, a phase-only filter with cosine function is proposed and a sharper focal spot of radially polarized beam is obtained on the focal plane. The function parameters of the filter are optimized and a series of optimized filters for different Strehl ratio S are given. The optimization results show that there is a significant improvement on obtaining sharper focal spot with this new type of filter. Compared with other results, this type of filters has superior supperresolution effect and higher energy utilization ratio. The numerical calculation results about the phase filter may facilitate new approaches to get superresolution.

Sensitive test for sea mine identification based on polarization-aided image processing

Techniques are widely sought to detect and identify sea mines. This issue is characterized by complicated mine shapes and underwater light propagation dependencies. In a preliminary study we use a preprocessing step for denoising underwater images before applying the algorithm for mine detection. Once a mine is detected, the protocol for identifying it is activated. Among many correlation filters, we have focused our attention on the asymmetric segmented phase-only filter for quantifying the recognition rate because it allows us to significantly increase the number of reference images in the fabrication of this filter. Yet they are not entirely satisfactory in terms of recognition rate and the obtained images revealed to be of low quality. In this report, we propose a way to improve upon this preliminary study by using a single wavelength polarimetric camera in order to denoise the images. This permits us to enhance images and improve depth visibility. We present illustrative results using in situ polarization imaging of a target through a milk-water mixture and demonstrate that our challenging objective of increasing the detection rate and decreasing the false alarm rate has been achieved.

Optical moving target indicator for synthetic aperture radar images

An optical indicator is designed to detect the moving targets in complex-valued synthetic aperture radar (SAR) images and estimate their azimuth velocity components. The indicator consists of two subsystems, and each of them is composed of one phase-only filter and two cylindrical lenses. The subsystems refocus the SAR image by changing the parameters of the phase-only filters to get two refocused images in which the background is defocused to the same extent while the moving targets are defocused differently. The indicator involved detects moving targets by comparing the sharpness of two defocused SAR images piece-by-piece, and then estimates the azimuth velocity of each detected target from its sharpness difference curve. The effectiveness of the optical architecture is investigated, and the proposal is confirmed by theoretical analysis and experiments with simulated and field data.

A novel design for maskless direct laser writing nanolithography: Combination of diffractive optical element and nonlinear absorption inorganic resists

Maskless laser direct writing lithography has been applied in the fabrication of optical elements and electric-optical devices. With the development of technology, the feature size of the elements and devices is required to reduce down to nanoscale. Increasing the numerical aperture of converging lens and shortening the laser wavelength are good methods to obtain the small spot and reduce the feature size to nanoscale, while this will cause the reduction of the depth of focus. The reduction of depth of focus will lead to some difficulties in the focusing and tracking servo controlling during the high speed laser direct writing lithography. In this work, the combination of the diffractive optical elements and the nonlinear absorption inorganic resist thin films cannot only extend the depth of focus, but also reduce the feature size of the lithographic marks down to nanoscale. By using the five-zone annular phase-only binary pupil filter as the diffractive optical elements and AgInSbTe as the nonlinear absorption inorganic resist thin film, the depth of focus cannot only extend to 7.39 times that of the focused spot, but also reduce the lithographic feature size down to 54.6nm. The ill-effect of sidelobe on the lithography is also eliminated by the nonlinear reverse saturable absorption and the phase change threshold lithographic characteristics.

Quasi-achromatic superresolving phase only pupil filters

Reported in this paper is a new design of the quasi-achromatic phase-only superresolving pupil filters (QAPSFs) relatively independent of wavelength based on the zero order Bessel function of the first kind. The effect of design parameters of the new filters on the superresolving performance parameters such as the normalized spot size (G) and the Strehl ratio (S) is discussed and analyzed in detail. As an example, the performance parameters of QAPSFs made of optical quartz glass are simulated and compared with phase only pupil filters, which indicate that this kind of filters is less independent of wavelength in the range from 300nm to 589nm for superresolution, and a high Strehl ratio is also obtained. The equivalent performance parameters are derived to characterize the superresolution performance of the quasi-achromatic filter.

Spectral optimized asymmetric segmented phase-only correlation filter

We suggest a new type of optimized composite filter, i.e., the asymmetric segmented phase-only filter (ASPOF), for improving the effectiveness of a VanderLugt correlator (VLC) when used for face identification. Basically, it consists in merging several reference images after application of a specific spectral optimization method. After segmentation of the spectral filter plane to several areas, each area is assigned to a single winner reference according to a new optimized criterion. The point of the paper is to show that this method offers a significant performance improvement on standard composite filters for face identification. We first briefly revisit composite filters [adapted, phase-only, inverse, compromise optimal, segmented, minimum average correlation energy, optimal trade-off maximum average correlation, and amplitude-modulated phase-only (AMPOF)], which are tools of choice for face recognition based on correlation techniques, and compare their performances with those of the ASPOF. We illustrate some of the drawbacks of current filters for several binary and grayscale image identifications. Next, we describe the optimization steps and introduce the ASPOF that can overcome these technical issues to improve the quality and the reliability of the correlation-based decision. We derive performance measures, i.e., PCE values and receiver operating characteristic curves, to confirm consistency of the results. We numerically find that this filter increases the recognition rate and decreases the false alarm rate. The results show that the discrimination of the ASPOF is comparable to that of the AMPOF, but the ASPOF is more robust than the trade-off maximum average correlation height against rotation and various types of noise sources. Our method has several features that make it amenable to experimental implementation using a VLC.

Optimizing the optical field distribution of near-field SIL optical storage system using five-zone binary phase filters

Five-zone binary phase filters (FBPFs) are proposed for decreasing the spot size and/or increasing the focal depth of the near-field optical storage system with a hemisphere solid immersion lens (SIL). The design of filters is based on the vector diffraction theory and the MATLAB optimizing toolbox. Three FBPFs with rotationally symmetrical pupil function have been designed, where the one FBPF is for increasing the focal depth as big as possible, the second FBPF is for improving the resolution as high as possible, and the third FBPF integrate the increase of focal depth with the improvement of resolution. Numerical results show that compared with the three-zone amplitude filter, the designed five-zone binary phase-only filters have more prominent performances in improving the focal depth and the resolution of the near-field SIL optical storage system.

Analysis of amplitude fluctuation and timing jitter performance of spectrally periodic Talbot filters for optical pulse rate multiplication

We analyze simultaneous amplitude fluctuation and timing jitter performance of a set of commonly believed equivalent spectrally periodic phase-only filters for implementing pulse repetition rate multiplication. Whereas amplitude noise and time jitter mitigation is observed in all cases, our analysis reveals different noise performance to that obtained with the classical Talbot filter implementation based on a single dispersive medium. Moreover, different noise improvements are achieved depending on the filter's spectral period and a mutual interaction between amplitude noise and timing jitter is also observed.

Use of phase diversity and modified phase congruence for edge enhancement in ultrasonic imaging

Enhancement of boundaries is essential for the classification of suspicious regions in ultrasonic medical B scan images of internal organs so that malignancies can be identified and detected. Phase congruence has been suggested as a means to delineate the boundaries in images. Estimation of phase congruence requires the use of log-Gabor filters. In this work, spatial phase-only filters are used to generate the phase and magnitude information necessary for the estimation of phase congruence. Taking into account the multiplicative non-Rayleigh nature of speckle in these images, a modified version of phase congruence is used alongside pre-filtering with median filters to reduce speckle. Processing was undertaken on two computer-synthesized images and two B scan images of tissue-mimicking phantoms acquired using a commercial ultrasonic imaging unit. The results show that the modified phase congruence estimated after the filtering with four spatial phase-only filters enhances the boundaries of target-like regions in these images. Since these phase-only filters require fewer parameters than log-Gabor filters, the technique demonstrated in this work can provide a simple means to use phase congruence to highlight boundaries in ultrasonic images corrupted by speckle.

Tampering attacks on binary phase only filter–based watermarking schemes for image authentication

Recently, a category of watermarking techniques based on binary phase-only filter (BPOF) has been proposed for image authentication. In such techniques, the authentication is implemented by evaluating the correlation between Fourier phase information and the hidden watermark bitplane. In this paper, we reveal the security flaws of BPOF-based watermarking algorithms and propose sophisticated tampering attacks against them. We show how the attacker can easily tamper with a watermarked image without being detected. Experimental results demonstrate that our attacks are successful in tampering watermarked images. The watermarking schemes are proven to be fundamentally flawed.

Generation of a high depth of focus with constant transversal spot size using a phase-only pupil filter

A novel approach is proposed to obtain an extended depth of focus (DoF) in white light with constant transversal spot size within the DoF. It combines a phase-only pupil filter based on multiplexed radial zones with alternating quartic phase functions. The design is first tested via numerical simulations of the point spread function (PSF) based on the scalar diffraction theory. The results for a fourfold gain of the depth of focus are experimentally verified with a phase-only spatial light modulator liquid crystal device combined with a 3D PSF measurement system. A close conformity between the experimental and simulation results proves the effectiveness of the proposed approach.

All optical phase-only filtering correlation with binarized inputs by a ferroelectric liquid-crystal spatial light modulator

All optical phase-only filtering correlator is constructed with an optically addressed ferroelectric liquid-crystal spatial light modulator (FSLM) for a binarized input object and a twisted-nematic liquid-crystal spatial-light modulator for a computer-generated phase-only filter. In order to improve the discrimination capability of a phase-only filtering correlator, a binarized amplitude with maximum contrast in a FSLM is used for an object input. The contrast of binarized intensities can be maximized by adjusting the rotation angles of a FSLM and polarizers. The threshold level of binarized input objects can be controlled by changing the power of a laser incident on a FLSM. Experimental results with a high discrimination capability are presented to support the numerical correlation performance.

Simultaneous Optical Pulse Multiplication and Shaping Based on the Amplitude-Assisted Phase-Only Filter Utilizing a Fiber Bragg Grating

A novel all-optical simultaneous pulse multiplication and shaping approach is proposed, which is based on the simultaneous utilization of two amplitude-assisted phase-only spectral filters realizable by using a short fiber Bragg grating. This proposed method enables to create a high-repetition-rate rectangular pulse train with both an ideal rectangular envelop and a high energy efficiency. As a particular example, a repetition rate of 90 GHz rectangular pulse train is numerically demonstrated from an initial 10 GHz Gaussian pulse train.

Design and acceleration of phase-only filter-based optical pattern recognition for fingerprint identification

We present the use of phase-only filter-based correlation for fingerprint pattern identification. The main advantage of this approach is that it is distortion tolerant and can be realized in optical or electronic parallel hardware. Given that real-world fingerprints are almost never perfect, distortion tolerance can prove to be very important for this application. Our results indicate that the algorithm can identify prints with 58% of the data missing on average. With large fingerprint databases, identification can be a computationally challenging task. The high parallelism in the phase-only correlation filter makes it ideally suited to field programmable gate array (FPGA)-based hardware acceleration. We examine the FPGA-based acceleration of the fingerprint algorithm. On a Xilinx Virtex II Pro FPGA, we achieve speedups of about 47 times over an optimized C implementation of the algorithm on a 2.2-GHz AMD Opteron processor. Our FPGA implementation is optimized to allow efficient processing of large databases.

Use of two-dimensional phase-only filters and compounding for speckle reduction and edge detection in ultrasonic B-scan images

Phase-only filters (POF) are studied for speckle reduction and edge detection in ultrasonic images. A methodology is developed for selecting the filters and compounding the filtered outputs. Studies on four speckled images show that the parametric images of compounded phases highlighted the boundaries. Estimating the heterogeneity index defined as the ratio of the arithmetic to the geometric mean of the magnitudes, the boundaries were also highlighted, providing a second means of detecting boundaries. This technique based on diversity created through a bank of POF reduces the speckle as well as highlights the boundaries of targetlike regions.

High-rate pulse-train generation by phase-only filtering of an electrooptic frequency comb: Analysis and optimization

We investigate the generation of high-rate optical pulse trains by spectral phase-only filtering of a frequency comb derived from an electrooptically phase-modulated continuous-wave laser. The technique is initially analyzed as a two-step filtering process. First, a fundamental pulse-train with repetition-rate equal to the modulation frequency is obtained by line-by-line phase-cancellation of the electrooptic frequency comb. Second, the temporal Talbot-effect is considered so that the output pulse repetition-rate is an integer multiple of the electrooptic modulation frequency. Nonidealities found in the fundamental train lead, in general, to multiplied trains with important degradations. We numerically analyze optimum modulation conditions for generation of output pulse trains with minimum peak-to-peak variations and/or maximum extinction level. On the other hand, a genetic algorithm is considered to numerically find optimum line-by-line phase-only filters that generate output multiplied trains with minimized degradations. Numerical simulations show that, in general, this second approach allows for improvement in the quality of the resultant multiplied trains, in terms of the uniformity degree and/or noise-level, compared with the pulse trains resulting from the Talbot-effect-based approach.

All-optical magnetic superresolution with binary pupil filters

A method to produce magnetic superresolution in all-optical magnetic storage is proposed. Two-zone amplitude-only and phase-only filters are designed to improve the quality of the superresolved magnetization pattern, to increase the magnetic spot intensity, to reduce the magnetic spot size, and to control the sidelobe effect. A procedure for designing a rotationally symmetric pupil-plane mask to control the magnetization intensity distribution near focus is presented. As a practical implementation, we have applied our method to obtain superresolving two-zone phase filters that can improve the all-optical magnetic recording density.

Enhancing the capabilities of binary phase only filter

In this paper, we demonstrate that the capabilities of a binary phase-only filter (BPOF) can be enhanced to identify targets irrespective of rotation, scale or the imaging spectral band by utilizing the concept of log-polar transform and image fusion. Till date, BPOFs have been considered to be the simplest of all filters and incapable of identifying distorted images or images of different spectral bands like the visible or infrared (IR) bands. The novelty of this work lies in the approach adopted to demonstrate that a BPOF is equally capable of distortion-invariance like any other distortion-invariant complex matched filter. This is done by suitably fusing the images of visible and IR bands and then taking the log-polar transformation of the fused image to synthesize the BPOF. A single BPOF is thus sufficient to identify (0–360)° in-plane rotated images, (50–190)% scaled images, combination of rotation and scale changes of the target, noisy image of both the visible and IR spectral bands. A further enhancement of the correlation peak intensity (CPI) is achieved by modifying this BPOF with Mexican-hat wavelet. The designed filter was implemented in the hybrid digital–optical correlation scheme. Correlation peak intensity and peak correlation energy (PCE) have been calculated as metrics of goodness of the proposed approach. Experimental results have been presented.

Double fusion filtering based multi-view face recognition

Two-dimensional (2D) face recognition by correlation is a key challenge of telecommunication and optical information processing. Although this issue has been the focus of intense research, its utilization still has some drawbacks especially when the face is in rotation. In this paper, we propose an alternative method based on a newly designed optical correlation filter which allows recognizing faces with different view angles. This filter called “Multi-View Binary Phase-Only Filter” is based on a double fusion of reference images allowing an optimisation of the use of the spatial-bandwidth product (SBWP) in the filter Fourier plane. The first fusion is performed in the image (space) domain, and the second one is conducted in the spectral domain. Simulations results with the Pointing Head Pose Image Database illustrate the performance of the designed correlation filter for multi-view face recognition.