This PDF file contains the editorial “Asymmetries” for OE Vol. 48 Issue 04

Optical foveated imaging using liquid crystal (LC) spatial light modulators (SLMs) has received considerable attention in recent years as a potential approach to reducing size and complexity in fast wide-angle lenses. We cover a theoretical study quantifying the diffraction efficiency and image quality of foveated optical systems (FOSs) based on transmissive LC SLMs. A practical design example of a fast wide-angle FOS based on the current transmissive LC SLM technology is proposed.

High-speed flier velocity measurement is one of the key technologies in investigating collision sites on the surfaces of spacecraft structures impacted by high-speed space debris. We have designed and constructed an optoelectronic system to accurately determine the average velocity of a flier impacting on a spacecraft structure. The system is based on two parallel laser screens, which are crossed by the fliers before impact. This system utilizes scattered light as the start and the end signal to measure the time of flight between the two screens. A wideband optical sensor has been designed and evaluated, and an electronic circuit is used to accurately record the time of flight and calculate the velocity. Experimental results show this system is adequate to measure the velocity of a flier larger than 100 μm, in the range from 0.1 to 10 km/s, with accuracy better than 1.3%, and with low cost, simplicity, and high reliability.

A ballistic chronograph with an expanded uncertainty of less than ±0.2 m/s has been designed, constructed, and tested. Because of this low uncertainty, this chronograph can be used as a reference to calibrate and/or characterize the performance of commercially available chronographs.

We describe a scanning system for shape recovery that employs a pair of one-dimensional interlaced axilenses encoded onto a reflective liquid-crystal spatial light modulator. The design of these axilenses with opposite depth presets allows the generation of a high-quality laser line pattern with extended depth of focus, which is applied in the shape recovery. Other attributes of the system allow the scanning of objects whose lateral dimensions are larger that those of the spatial light modulator itself. The high performance of the scanning system is demonstrated by its application to the shape recovery of ceramic objects.

We report on a mode-mismatched pump-probe photothermal lens experiment aimed at determination of the thermal diffusivity coefficient of optical samples. In the scheme, the probe beam is collimated and the pump beam is focused. Using a Fresnel diffraction approximation, we show that under these conditions the time dependence of the signal does not depend on the Rayleigh parameters and waist positions of the light beams. This fact allows a more simple and reliable calibration of the experiment in comparison to other schemes. We conduct studies on liquid and solid samples that confirm the predictions of the model. Using the proposed method, we measure the thermal diffusivity coefficient of distilled water, ethanol, methanol, chloroform, nitrobenzene, ethylene glycol, and a solid piece of acrylic plastic. The results are in good agreement with previous measurements.

Peak-to-valley departure (PV) is entrenched in optics design and manufacture as a characterization of an optical figure; modern interferometers commonly use 1k×1k detectors, the output of which may not be well represented by two points. PVr is a newly proposed robust amplitude parameter that combines the PV of a 36-term Zernike fit and the root mean square of the residual. This provides automatic filtering, is insensitive to system resolution, and relates directly to imaging performance via the Marechal criterion. Use of PVr in place of PV is recommended.

This study proposes a method for measuring small displacements. To this purpose, a circularly polarized heterodyne light beam, reflected from a mirror, impinges into a semispherical prism in a radial direction and is reflected at the flat base of the prism passing then through a properly oriented analyzer for interference. The phase difference between s- and p-polarized light results are sensitive to the impinging angle when it is equal to the internal reflection polarization angle. If the mirror is displaced, then it causes a small variation of the impinging angle and phase changes. The phase difference obtained allows evaluating the mirror displacement through heterodyne interferometry. The feasibility of this method was demonstrated, and the displacement measurement resolution is ~46 nm.

A vibration-displacement measurement system based on tracking the phase variation of an optical fiber Michelson interferometer with electronic feedback loops is presented. The measurement system includes two sets of electronic feedback loops. One electronic feedback loop is used to compensate for the low frequency drifts in the phase of the interferometric signal that results from environmental disturbances, while the other one is used to track the phase variation induced by the measured vibration displacement, and thus realize the measurement of the vibration displacement and provide a sense of direction of the displacement simultaneously. The measurement system is designed to be capable of measuring vibration displacement with frequencies ranging from 1.5 to 200 Hz, with measurement resolution reaching 13 nm.

The phase-to-coordinate conversion is an important step for accurate 3-D shape measurement by use of structured light methods. We propose to use an absolute phase map in the Fourier transform method to achieve better accuracy in the phase-to-coordinate conversion. A cross-shaped marker is embedded in the fringe pattern that is projected onto the object. The position of the marker in the captured fringe image is detected and utilized to calculate the absolute phase map. For phase analysis of the fringe image, the marker is removed and the sinusoidal intensity distribution of the fringe pattern is restored before the forward and inverse Fourier transforms are applied. Experimental results of absolute phase retrieval and 3-D reconstruction are presented to show the feasibility of the proposed method.

A lenticular system provides optimal display quality when a homogeneous lenticular sheet is precisely aligned on LCD subpixels and when a viewer is located within a predetermined viewing zone. In practice, however, many lenticular systems suffer from image distortion due to misalignment or inhomogeneity of the lenticular sheet. To alleviate such distortion, we propose a new multiplexing algorithm. The proposed algorithm first obtains the geometric relationship between LCD subpixels and lenticules by using various pattern images. Then, it generates a mapping matrix between LCD subpixels and multiple-view images based on the obtained relationship. While the proposed scheme is quite simple, it effectively compensates any misalignment or inhomogeneity of the lenticular sheet, and it significantly improves 3-D image quality. In addition, by simply adjusting the mapping parameters, the algorithm can change the predetermined viewing zone to include a viewer's actual position. Experimental results demonstrate that the proposed compensation scheme successfully works in a real 3-D lenticular display system.

We present a method for measuring all eight parameters (including the signs) of the Jones matrices of liquid-crystal displays. The method can be applied to measure the Jones matrices for all addressable gray levels thus delivering the specifications needed for calculating characteristic curves for arbitrary input and output polarizations. Unlike other approaches, we do not rely on a physical model of the LCD. Thus, it is possible to measure the Jones matrices of a more complex optical system in one step (e.g., when a reflective LCD is used in combination with a beamsplitter). Though the method presented is, in principle, applicable for transmissive and reflective LCDs, calculations and experiments are only shown using the example of a reflective liquid-crystal-on-silicon display.

To evaluate the influence of wavefront aberrations on antenna received power in free-space laser communications, an aberration attenuation factor is proposed, based on which the power penalty at the receiver due to misalignment and primary aberrations is investigated. It is shown that antenna received power decreases gradually with increasing misalignment and aberrations. A comparison shows that tilt (misalignment) has greater influence than other primary aberrations. When the rms aberration value is 0.1λ, the received power penalties caused by tilt, astigmatism, coma, curvature, and spherical aberrations are about 40%, 36%, 35%, 24%, and 23%, respectively. In addition, the obscuration ratio of the transmitter antenna has a noticeable but relatively minor influence on the aberration attenuation factor.

Precisely frequency-stabilized semiconductor lasers produce physical-random numbers, allowing for optimal noise-suppression with one notable exception: white frequency-modulation (FM) noise. However, this component can also be converted into physical-random numbers. The interference created between two beams of rubidium (Rb) gas-cell-stabilized lasers allows for laser frequency noise to be extracted. Using this technique, we produce physical-random numbers from the relative frequency noise. Finally, we confirm that the numbers produced are, in fact, random, by means of the standard National Institute of Standards and Technology Federal Information Processing Standard (NIST FIPS) 140-2 test.

We propose a simple ultraviolet (UV) writing technique for the fabrication of buried waveguide devices, which employs a UV lamp as the light source and commercial benzocyclobutene (BCB) and epoxy OPTOCAST 3505 as the core and cladding materials, respectively. We show that the light emitted from the UV lamp can induce a large refractive-index change (up to 0.012) in BCB, while a negligible index change in the epoxy, which makes possible the writing of buried waveguide devices in BCB by exposing an epoxy-clad BCB film to the UV light through a mask. We demonstrate this technique experimentally with straight channel waveguides, Y-junction waveguide branches, and long-period waveguide gratings. The propagation loss of a typical fabricated waveguide is 1.6 dB/cm at 1550 nm and the polarization-dependent loss is 0.4 dB/cm.

We investigate the effect of thermal aging on the optical properties of multimode silica fibers. Samples were submitted to several heating-cooling cycles of 8 h each. Analyses were performed using an optical spectrum analyzer and an optical time-domain reflectometer to determine the signal loss. We proved that the attenuation coefficient is closely related to Rayleigh backscattering. Results show an increase of the attenuation coefficient with aging time. The obtained results show a degradation of the optical properties of fibers from 1680 h on, due to local variation of the refractive index induced by fluctuation of the material density.

A successive interference cancellation scheme is applied to optical code-division multiple-access (OCDMA) systems with spectral amplitude coding (SAC). A detailed analysis of this system, with Hadamard codes used as signature sequences, is presented. The system can easily remove the effect of the strongest signal at each stage of the cancellation process. In addition, simulation of the prose system is performed in order to validate the theoretical results. The system shows a small bit error rate at a large number of active users compared to the SAC OCDMA system. Our results reveal that the proposed system is efficient in eliminating the effect of the multiple-user interference and in the enhancement of the overall performance.

We present a realization of an optically controlled pulse-burst position modulator to be used for a radio-frequency photonic circuit aiming to produce beam forming for a Radar-transmitting antenna. The configuration uses a set of fiber ring resonators that contain erbium-doped fiber amplifiers. By controlling the pumping in each loop, the gain of the doped fibers is changed, which results with a change in the resonators' finesses. In the end of each optical path, the optical signal is sampled and converted to an electronic signal while an electronic subtraction is performed between the outputs of the two resonators. Because each resonator has different and controlled finesse, the subtraction results in an output pulse burst with varied position.

We propose a new system of the dark-bright solitons conversion using a micro- and nanoring resonators incorporating an optical add/drop filter, where the add/drop filter can be used to convert the dark soliton to a bright soliton. The key advantage of the system is that the detection of the dark soliton pulse is normally difficult due to the low level of input power. First, a dark soliton pulse is input into a microring resonator and then propagated into smaller micro- and nanoring resonators, respectively. Second, the add/drop filter is applied (connected) into the ring system, where the bright and dark solitons are obtained via the drop and through (or throughput) ports of the add/drop filter, respectively. The results obtained have shown that the detected soliton power can be controlled by the input soliton power and the ring resonator coupling coefficient, which is enough power to use in the transmission link. Thus, significant conversion-amplified signals can be achieved.

A geometrical and phasor representation technique is presented to illustrate the modulation of the lightwave carrier to generate quadrature amplitude modulated (QAM) signals. The modulation of the amplitude and phase of the lightwave carrier is implemented using only one dual-drive Mach-Zehnder interferometric modulator (MZIM) with the assistance of phasor techniques. Any multilevel modulation scheme can be generated, but we illustrate specifically, the multilevel amplitude and differential phase shift keying (MADPSK) signals. The driving voltage levels are estimated for driving the traveling wave electrodes of the modulator. Phasor diagrams are extensively used to demonstrate the effectiveness of modulation schemes. MATLAB Simulink models are formed to generate the multilevel modulation formats, transmission, and detection in optically amplified fiber communication systems. Transmission performance is obtained for the multilevel optical signals and proven to be equivalent or better than those of binary level with equivalent bit rate. Further, the resilience to nonlinear effects is much higher for MADPSK of 50% and 33% pulse width as compared to non-return-to-zero (NRZ) pulse shaping.

In two-wavelength contouring by difference holographic interferometry, the test object is illuminated holographically by real images of the master object belonging to the two different wavelengths. In this process, however, the illuminating holograms have to reconstructed together. The illuminations belonging to the other wavelengths are unwanted, but fortunately they are reconstructed with some direction shift. Thus they can be filtered out by a proper aperture in the Fourier plane of a lens. Because the shift of the corresponding spot in the Fourier plane is relatively small, the similarly small filtering aperture leads to a significant reduction of the master object wave intensities in the recording steps. This in practice may result in a low-quality master hologram and consequent poor holographic illumination as well. To overcome this, the paper suggests two techniques to increase the light intensity of the filtered master object waves. First, a shape change of the aperture is proposed, and its extension to an aperture system. Second, a special optimization of the optical arrangement geometry is suggested to maximize the applicable aperture size in the Fourier plane. The two techniques provide an order of magnitude of intensity increase, even up to more than half of the nonfiltered value.

We examine the potential of subsea free-space optics (FSO) for sensor network applications leveraging the emerging technologies of highly sensitive photon-counting detectors and semi-conductor LED and laser light sources in the UV solar blind. Monitoring oil and gas production installations is the niche application discussed. The merits of FSO include the capacity for broadband communication that would enable the transmission of video data in real time, which is not possible with other technologies at present. However, subsea FSO is challenged by high extinction and the immense variability of background illumination in shallow waters. This has stimulated us to investigate the potential of underwater FSO in the UV solar-blind spectral range, where background illumination is nearly nonexistent and considerable scattering occurs. The achievable performance is compared to transmission at 520 nm, where, in Clear Ocean, data rates of 100 Mbps can be transmitted over distances of ~170 m, falling to under 15 m in harbor waters. It is anticipated that ranges of 12 m can also be obtained with UV solar-blind wavelengths, although experimental corroboration is not yet available.

Traditional industrial computed tomography systems generally calibrate the central ray by scanning a wire phantom. That is time-consuming and slow. This paper presents a direct central-ray determination method using the projection data of the object to be examined. The basic idea is that with a fan-beam arrangement, any straight line on the object slice will align with the x-ray point source at only two times during a 360-deg scan. By measuring the mismatching of the two sets of projection data that correspond to these two alignments, the true central ray can be identified at the minimum of the measurement. Our test has proven that this automated method is fast, reliable, and accurate.

This paper presents a novel reversible fragile watermarking scheme based on a pyramidal structure. The similarities among the detail components of a pyramid-structure image are exploited to select appropriate embedding areas, and then the cryptographic watermarks are embedded into selected wavelet coefficients using a difference-expansion method. An adaptive embedding method is proposed to avoid needing extra space to store parts of the original image and the watermark location map. The cryptographic signatures are created using a watermark generation function that comprises a hashing function and a public-key encryption function. To resist counterfeiting attack, block relationships are created using toral automorphism to increase the security and implementation practicability. Experimental results show that the proposed reversible fragile watermarking scheme can successfully obtain the original image if the protected image is unaltered, and unauthorized manipulations can be correctly identified and localized even when protected images undergo a cropping attack.

With the rapid development of numerous imaging sensors, a variety of image fusion algorithms have been proposed. However, these methods only transfer absolute information into the fused image and neglect the relative information contrast. Nevertheless, the contrast is the key stimulus variable represented by a neuron's activity. We propose a new image fusion method inspired by the human visual system. The new method uses the contrast to represent the salient features of an image. First, we use the polyharmonic local sine transform to decompose the source image into two components: the polyharmonic component and the residual. Next, we compute the contrast of every pixel by dividing the residual by the average value of the polyharmonic component. The polyharmonic component and residual are fused separately with different fusion rules. We can easily obtain the fused image through directly adding the composite polyharmonic component and the composite residual. Compared with the existing image fusion methods, such as the Laplace pyramid, shift-invariant wavelet, and contourlet, the inverse transformation is easy. The results demonstrate that the proposed algorithm is effective and is superior to the conventional image fusion algorithm in terms of the mutual information.

Demosaicking is an interpolation process that transforms a color filter array (CFA) image into a full-color image in a single-sensor imaging pipeline. In all demosaicking techniques, the interpolation of the green components plays a central role in dictating the visual quality of reconstructed images because green light is of maximum sensitivity in the human visual system. Guided by this point, we propose a new soft-decision demosaicking algorithm using directional filtering and embedded artifact refinement. The novelty of this approach is twofold. First, we lift the constraint of the Bayer CFA that results in the absence of diagonal neighboring green color values for directionally recovering diagonal edges. The developed directional interpolation method is fairly robust in dealing with the four edge features, namely, vertical, horizontal, 45-deg diagonal, and 135-deg diagonal. In addition, the proposed embedded refinement scheme provides an efficient way for soft-decision-based algorithms to achieve improved results with fewer computations. We have compared this new approach to six state-of-the-art methods, and it can outstandingly preserve more edge details and handle fine textures well, without requiring a high computational cost.

With the growth of digital video surveillance markets and requirements of high-quality surveillance data, an efficient video compression technique that is suitable for surveillance applications and is compatible with emerging coding standards (e.g., H.264) is required urgently. We propose multiple-priority region-of-interest H.264 video compression using constraint variable bitrate control for video surveillance. The proposed Multiple Moving region-of-interest (RoI) macroblock decision and an RoI-based Constraint Variable Bitrate Control, called MM-RoI-CVBC, are able to determine each macroblock (MB) coding priority and increase SNR of automatically or manually detected RoI. In comparison to H.264 JM fixed-quantization parameter coding, H.264 JM constant bitrate control, and other RoI coding algorithms, the proposed MM-RoI-CVBC method can obviously enhance the quality of RoI with less coding complexity in subject to target bits.

We introduce a new deblocking algorithm that can remove the blockiness without blurring image details. After modeling the blocking artifacts as well as the real edges, the proposed algorithm controls the suppression of the blocking artifact based on the human visual system. Through simulations, we show that our algorithm can successfully reduces the blocking artifacts without excessive blurring.

With the increasing need for security systems, iris recognition is one of the reliable solutions for biometrics-based identification systems. In general, an iris recognition algorithm includes four basic modules: image quality assessment, preprocessing, feature extraction, and matching. This work presents a whole iris recognition system, but particularly focuses on the image quality assessment and proposes an iris recognition scheme with an improved empirical mode decomposition (EMD) method. First, we assess the quality of each image in the input sequence and select clear enough iris images for the succeeding recognition processes. Then, an improved EMD (IEMD), a multiresolution decomposition technique, is applied to the iris pattern extraction. To verify the efficacy of the proposed approach, experiments are conducted on the public and freely available iris images from the CASIA and UBIRIS databases; three different similarity measures are used to evaluate the outcomes. The results show that the presented schemas achieve promising performance by those three measures. Therefore, the proposed method is feasible for iris recognition and IEMD is suitable for iris feature extraction.

We propose a complete application capable of tracking multiple objects in an environment monitored by multiple cameras. The system has been specially developed to be applied to sport games, and it has been evaluated in a real association-football stadium. Each target is tracked using a local importance-sampling particle filter in each camera, but the final estimation is made by combining information from the other cameras using a modified unscented Kalman filter algorithm. Multicamera integration enables us to compensate for bad measurements or occlusions in some cameras thanks to the other views it offers. The final algorithm results in a more accurate system with a lower failure rate.

Recently, fake fingerprints have become a serious concern for the use of fingerprint recognition systems. We introduce a novel fake-fingerprint detection method that uses multiple static features. With regard to the usability of the method for field applications, we employ static features extracted from one image to determine the aliveness of fingerprints. We consider the power spectrum, histogram, directional contrast, ridge thickness, and ridge signal of each fingerprint image as representative static features. Each feature is analyzed with respect to the physiological and statistical distinctiveness of live and fake fingerprints. These features form a feature vector set and are fused at the feature level through a support vector machine classifier. For performance evaluation and comparison, a total of 7200 live images and 9000 fake images were collected using four sensors (three optical and one capacitive). Experimental results showed that proposed method achieved approximately 1.6% equal-error rate with optical-based sensors. In the case of the capacitive sensor, there was no test error when only one image was used for a decision. Based on these results, we conclude that the proposed method is a simple yet promising fake-fingerprint inspection technique in practice.

We propose a novel scene categorization method based on multiscale category-specific visual words. The novelty of the proposed method lies in two aspects: (1) visual words are quantized in a multiscale manner that combines the global-feature-based and local-feature-based scene categorization approaches into a uniform framework; (2) unlike traditional visual word creation methods, which quantize visual words from the entire set of training, we form visual words from the training images grouped in different categories and then collate visual words from different categories to form the final codebook. This generation strategy is capable of enhancing the discriminative ability of the visual words, which is useful for achieving better classification performance. The proposed method is evaluated over two scene classification data sets with 8 and 13 scene categories, respectively. The experimental results show that the classification performance is significantly improved by using the multiscale category-specific visual words over that achieved by using the traditional visual words. Moreover, the proposed method is comparable with the best methods reported in previous literature in terms of classification accuracy rate (88.81% and 85.05% accuracy rates for data sets 1 and 2, respectively) and has the advantage in simplicity.

An eyegaze interface is one of the key technologies that serves as an input device in a ubiquitous-computing society. Recently, video-based techniques that do not require specific instruments have been studied. With these approaches, development of an accurate iris-extraction algorithm is very important to realize practical eyegaze tracking. For accurate iris extraction, it is necessary to achieve robustness, high speed, and high accuracy. Conventional iris-extraction algorithms experience difficulties in meeting all these requirements simultaneously. This study proposes an iris-extraction algorithm based on the parametric template matching method to satisfy all these requirements at the same time. The parametric template matching method achieves robustness by interpolating among some templates, and the method attains high accuracy by a subpixel matching technique. High-speed matching can be realized by coarse-to-fine matching. To verify the effectiveness of the proposed algorithm, we performed a basic experiment for eyegaze tracking. We show in this experiment that the processing time is approximately 1/900 of that of our previous method and that accuracy is acceptable with the new method. Then, we apply the proposed algorithm to an eyegaze keyboard, along with an imaging system for improving image quality, and we verify the effectiveness of this approach.

Feature extraction is a data analysis technique devoted to removing redundancy and extracting the most discriminative information. In face recognition, feature extractors are normally plagued with small sample size problems, in which the total number of training images is much smaller than the image dimensionality. Recently, an optimized facial feature extractor, maximum marginal criterion (MMC), was proposed. MMC computes an optimized projection by solving the generalized eigenvalue problem in a standard form that is free from inverse matrix operation, and thus it does not suffer from the small sample size problem. However, MMC is essentially a linear projection technique that relies on facial image pixel intensity to compute within- and between-class scatters. The nonlinear nature of faces restricts the discrimination of MMC. Hence, we propose an improved MMC, namely maximum neighborhood margin criterion (MNMC). Unlike MMC, which preserves global geometric structures that do not perfectly describe the underlying face manifold, MNMC seeks a projection that preserves local geometric structures via neighborhood preservation. This objective function leads to the enhancement of classification capability, and this is testified by experimental results. MNMC shows its performance superiority compared to MMC, especially in pose, illumination, and expression (PIE) and face recognition grand challenge (FRGC) databases.

An elliptic face segmentation algorithm, called a facial component extractor (FCExtractor), was recently proposed. The algorithm is based on a novel overcomplete wavelet template, a support vector machine (SVM) classifier, and wavelet entropy filtering. It is designed to consistently detect and segment the eyes-nose-mouth T-shaped facial region via ellipse. Thereafter, head orientation is estimated by using the ratio of cheeks. To evaluate the effectiveness of the FCExtractor, we introduce a face detection measure based on the distance between the expected and segmented eye-mouth triangle circumscribed circle areas. We then apply the local description of the segmented face through normalization, illumination normalization, log-polar mapping, and self-eigenface to achieve recognition. The novelty of this approach for face representation comes from the derivation of the likelihood fitness function for self-eigenface selection of a discriminative subset and the adaptive threshold value. The approach maximizes the differences amidst face images of different persons, and it also minimizes the expression and pose variations of the same person. Experimental results on available databases and a live sequence show that our method is superior to conventional methods based on rectangular face segmentation against complex scenes.