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In this paper, a novel quantitative assessment approach for image mosaicing algorithms is proposed by evaluating the quality of an output mosaic image and comparing with the input images. Five evaluation indices-entropy, clarity, registration error, peak signal-to-noise ratio, and structural similarity-are calculated and combined with the analytic hierarchy process for comprehensive assessment. This approach can overcome the lack of panoramic image and is easy to implement. Experimental results show the objectivity and the validity of the proposed approach.

A speed up technique for the SUSAN edge detector based on random sampling is proposed. Instead of sliding the mask pixel by pixel on an image as the SUSAN edge detector does, the proposed scheme places the mask randomly on pixels to find edges in the image; we hereby name it randomized SUSAN edge detector (R-SUSAN). Specifically, the R-SUSAN edge detector adopts three approaches in the framework of random sampling to accelerate a SUSAN edge detector: procedure integration of response computation and nonmaxima suppression, reduction of unnecessary processing for obvious nonedge pixels, and early termination. Experimental results demonstrate the effectiveness of the proposed method.

In many machine vision applications, a crucial step is to accurately determine the relation between the image of the object and its physical dimension by performing a calibration process. Over time, various calibration techniques have been developed. Nevertheless, the existing methods cannot satisfy the ever-increasing demands for higher accuracy performance. In this letter, an advanced geometric camera calibration technique which employs a frontal image concept and a hyper-precise control point detection scheme with digital image correlation is presented. Simulation and real experimental results have successfully demonstrated the superior of the proposed technique.

This PDF file contains the editorial “Special Section Guest Editorial: Fiber Lasers” for OE Vol. 50 Issue 11

Fiber dispersion plays a significant role in spectral broadening of incoherent continuous-wave light. We develop a self-consistent stochastic model for spectral broadening of incoherent continuous-wave light through nonlinear wave mixing and apply this model to numerical simulations of spectral broadening in a continuous-wave fiber Raman laser. The results of these numerical simulations agree very well with carefully conducted laboratory measurements. Under a wide range of operating conditions, these numerical simulations also exhibit striking features, such as damped oscillatory spectral broadening (during the initial stages of propagation) and eventual convergence to a stationary, steady-state spectral distribution at sufficiently long propagation distances. We analyze the important role of fiber dispersion in such phenomena. We also derive an analytical rate equation expression for spectral broadening, whose numerical evaluation is far less computationally intensive than the fully stochastic simulation, and a mathematical criterion for the applicability of this analytical expression.

Novel transverse techniques for measuring the refractive index profile and spontaneous emission of high-power optical fibers are described. These techniques are particularly attractive for measuring fiber samples incorporating axial variations, such as mode transformers, gratings, fusion splices, tapers, taps, or couplers. Computerized tomography of spontaneous emission is demonstrated as the first nondestructive method for estimating the spatial distribution of gain in a rare earth-doped fiber, which can be particularly useful when exploiting a spatially inhomogeneous dopant profile.

We perform modal characterization on an ytterbium-doped large mode area photonic-crystal-fiber (PCF) amplifier using spatial and spectral (S²) resolved imaging and compare results to conventional cutoff methods. We apply numerical simulations and step-index fiber experiments to calibrate our mathematical and experimental routines of our S² imaging system. We systematically analyze higher-order-mode (HOM) content of a polarizing 40-μm core double-clad PCF amplifier with various launching and coiling configurations. We demonstrate a HOM suppression of more than −24 dB with variance of 2.3 dB.

This paper outlines the most recent work at nLIGHT Oy (formerly Liekki Oy). We give an overview of the current state of the nLIGHT active fiber fabrication technology, discuss the capability of the manufacturing process, and review our results and the results of other groups on the reliability aspects of the manufactured fibers. We also present refractive index tailored and gain tailored Yb doped fibers as examples of recent fiber development.

We report >100 W output power by coherently combining two, three, and four high power lasers in all-fiber and all-passive configurations using Yb fiber laser cavities operating at a wavelength of 1080 nm with polarization maintaining fibers and 2×2 fused fiber couplers. We present the power and number scaling characteristics of the laser arrays and compare their beam combining efficiencies in different array configurations. We experimentally show beam combining efficiency degradation with increase of laser power and number of lasers. In addition, we discuss the effect of fiber nonlinearity on beam combining efficiency.

The reliability of low-index polymer coated double-clad (DC) fibers used in the manufacture of fiber lasers and amplifiers has not received adequate attention. This paper evaluates the mechanical reliability of fibers, using standard fiber optic test procedures, and compares the performance of the DC fibers to the GR-20-CORE standard adopted by the industry. An 85°C hot water soak test is proposed as an accelerated test to evaluate a low-index polymer coated DC fiber performance with prolonged exposure to temperature and humidity conditions experienced during storage and operation of fiber lasers. The test is used to evaluate DC fibers with three different coatings, including a specially engineered coating, and benchmark fibers from competitors. The data in this paper demonstrate that a dual acrylate coated DC fiber, using the specially engineered coating, has median failure stress values of over 700 kpsi and an average stress corrosion parameter of 21, well exceeding the recommended industry minimum values of 550 kpsi and 18, respectively. The accelerated temperature and humidity aging test clearly demonstrates that DC fibers with specially engineered coatings have 2 to 3 orders of magnitude better optical reliability. Such remarkable optical and mechanical performance significantly alleviates long term reliability concerns of fiber lasers and amplifiers.

Thulium-doped fiber lasers (TFLs) emitting retina-safe 2-μm wavelengths offer substantial power-scaling advantages over ytterbium-doped fiber lasers for narrow linewidth, single-mode operation. This article reviews the design and performance of a pump-limited, 600 W, single-mode, single-frequency TFL amplifier chain that balances thermal limitations against those arising from stimulated Brillouin scattering (SBS). A simple analysis of thermal and SBS limits is anchored with measurements on kilowatt class Tm and Yb fiber lasers to highlight the scaling advantage of Tm for narrow linewidth operation. We also report recent results on active phase-locking of a TFL amplifier to an optical reference as a precursor to further parallel scaling via coherent beam combining.

High-power fiber lasers and amplifiers have gained tremendous momentum in the last 5 years. Many of the traditional manufacturers of gas and solid-state lasers are now pursuing the fiber-based systems, which are displacing the conventional technology in many areas. High-power fiber laser systems require reliable fibers with large cores, stable mode quality, and good power handling capabilities-requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 100 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to megawatt power levels. Furthermore, we describe the novel airclad-based pump combiners and their use in a completely monolithic 350 W cw fiber laser system with an M² of less than 1.1.

We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations.

We have theoretically proposed a new type of active optical zoom system. It can change the magnification of an imaging system at any field angle, namely, it can change the effective focal length at a limited region of interest and simultaneously maintain high resolution. It is comprised of two static aspheric mirrors and two deformable mirrors; furthermore, by changing the curvature of mirrors the focal length can be adjusted from −399 to −558.6 mm. The system reduces the bandwidth of data transmission and may find potential applications in high speed photography.

A first-order formula for calculations of the direction cosines of the rays refracted by Risley prisms was derived. The formula was obtained by representing the deviation of the ray passing through a prism by the product of rotation matrices, and using the series expansion of the product. It can be applied to the system of an arbitrary number of prisms or a combination of Risley prisms. Related errors were discussed and some numerical calculations were made and compared with the exact solutions using the refraction equation. The scan patterns of a single Risley prism or a combination of two Risley prisms calculated using the generalized first-order formula are in good agreement with the exact solutions.

We present a novel 3-D recovery method based on structured light. This method unifies depth from focus (DFF) and depth from defocus (DFD) techniques with the use of a dynamic (de)focused projection. With this approach, the image acquisition system is specifically constructed to keep a whole object sharp in all the captured images. Therefore, only the projected patterns experience different defocused deformations according to the object's depths. When the projected patterns are out of focus, their point-spread function (PSF) is assumed to follow a Gaussian distribution. The final depth is computed by the analysis of the relationship between the sets of PSFs obtained from different blurs and the variation of the object's depths. Our new depth estimation can be employed as a stand-alone strategy. It has no problem with occlusion and correspondence issues. Moreover, it handles textureless and partially reflective surfaces. The experimental results on real objects demonstrate the effective performance of our approach, providing reliable depth estimation and competitive time consumption. It uses fewer input images than DFF, and unlike DFD, it ensures that the PSF is locally unique.

Conventional touch-based sensors cannot capture the fingerprint images of all five fingers simultaneously due to their flat surfaces because the view of the thumb is not parallel to the other fingers. In addition, touch-based sensors have inherent difficulties, including variations in captured images due to partial contact, nonlinear distortion, inconsistent image quality, and latent images. These degrade recognition performance and user acceptance to using sensors. To overcome these difficulties, we propose a device that adopts a contact-free structure composed of a charge-coupled device (CCD) camera, rotating mirror equipped with a stepping motor, and a green light-emitting diode (LED) illuminator. The device does not make contact with any finger and captures all five fingerprint images simultaneously. We describe and discuss the structure of the proposed device in terms of four aspects: the quality of captured images, verification performance, compatibility with existing touch-based sensors, and ease of use. The experimental results show that the proposed device can capture all five fingerprint images with a high throughput (in 2.5 s), as required at the immigration control office of a country. Also, on average, a captured touchless image takes 57% of a whole rolled image whereas the image captured from a conventional touch-based sensor only takes 41% of a whole rolled image, and they have 63 and 40 true minutiae on average, respectively. Even though touchless images contain 13.18-deg rolling and 9.18-deg pitching distortion on average, 0% equal error rate (EER) is obtained by using five fingerprint images in verification stage.

In order to improve underwater range-gated imagery quality, image restoration and super-resolution reconstruction techniques based on maximum a posteriori framework are combined with the point spread function (PSF) of the system. A model based on beam propagation considering time-dependent backscatter is established including the extreme attenuation properties of water and the diffraction limit of sensor for obtaining the PSF. The proposed method is applied for an underwater range-gated imaging system. Performance of the approach is compared with the traditional restoration and reconstruction algorithms. Objective image quality metrics are used to quantify the effectiveness of the restoration and reconstruction. Experimental results showed that the proposed method can effectively improve the resolution and quality of underwater range-gated imaging.

Against a background of newly emerged security threats, the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept into a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the image generation software. The camera concept is based on a Cassegrain-type mirror optics, an optomechanical scanner, an array of 20 superconducting transition-edge sensors operated at a temperature of 450 to 650 mK, and a closed-cycle cryogen-free cooling system. The main figures of the system include: a frequency band of 350±40 GHz, an object distance of 7 to 10 m, a circular field of view of 1.05 m diameter, and a spatial resolution in the image center of 2 cm at 8.5 m distance, a noise equivalent temperature difference of 0.1 to 0.4 K, and a maximum frame rate of 10 Hz.

Traditionally, most camera calibrations rely on a planar target with well-known marks. However, the localization error of the marks in the image is a source of inaccuracy. We propose the use of high-resolution digital displays as active calibration targets to obtain more accurate calibration results for all types of cameras. The display shows a series of coded patterns to generate correspondences between world points and image points. This has several advantages. No special calibration hardware is necessary because suitable displays are practically ubiquitious. The method is fully automatic, and no identification of marks is necessary. For a coding scheme based on phase shifting, the localization accuracy is approximately independent of the camera's focus settings. Most importantly, higher accuracy can be achieved compared to passive targets, such as printed checkerboards. A rigorous evaluation is performed to substantiate this claim. Our active target method is compared to standard calibrations using a checkerboard target. We perform camera, calibrations with different combinations of displays, cameras, and lenses, as well as with simulated images and find markedly lower reprojection errors when using active targets. For example, in a stereo reconstruction task, the accuracy of a system calibrated with an active target is five times better.

We describe a system for performing high-accuracy, noncontact rms roughness measurements of flat and curved machined parts in the industrially relevant range of ∼0.05 to 0.35 μm. The system uses a near infrared (NIR) super-continuum laser to measure the intensity of specular reflection versus wavelength, at relatively long (∼1 m) stand-off distances and has the potential to be used in high speed, in-line manufacturing applications. The surface roughness value is extracted from the slope of the normalized specular intensity using the Beckmann-Kirchhoff (BK) model. According to the BK model, the normalized specular intensity in the NIR mostly depends on the surface roughness parameter alone and is independent of the absolute reflectance due to the normalization process. We discuss the benefits of performing the reflectance measurements in the NIR versus the commonly used visible spectrum. These include measurements at lower angles of incidence and the lack of need for a reference of the same metal composition. The roughness measurements performed by this system are in very good agreement with comparative data from a stylus profilometer and a white light interferometer. A potential industrial application is also demonstrated where the system is used to detect polishing defects in automotive engine crankshaft journals.

The NASA Goddard Space Flight Center (GSFC) Radiometric Calibration Laboratory (RCL) maintains several large integrating sphere sources covering the visible to the shortwave infrared wavelength range. Two critical, functional requirements of an integrating sphere source are short- and long-term operational stability and repeatability. Monitoring the source is essential in determining the origin of systemic errors, thus increasing confidence in source performance and quantifying repeatability. If monitor data falls outside the established parameters, this could be an indication that the source requires maintenance or recalibration against the National Institute of Science and Technology irradiance standard. The GSFC RCL has developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of its integrating sphere calibration sources in the 400 to 2400 nm region. Sphere output change mechanisms include lamp aging, coating (e.g., BaSO₄) deterioration, and ambient water vapor level. The FRMS wavelength bands are selected to quantify changes caused by these mechanisms. The FRMS design and operation are presented, as well as data from monitoring four of the RCL's integrating sphere sources.

Photoconductive metal-semiconductor-metal ultraviolet detectors were fabricated based on the polycrystalline MgZnO films, which were grown on SiO₂/Si substrates by radio frequency magnetron sputtering. The peak response of the detector was at 300 nm, which lay in the solar-blind spectrum range (220 to 300 nm). And the peak responsivity was 34.02 A/W. The ratio of I_ph/I_d was as high as 2659.7 at 5 V bias under a low pressure Hg lamp illumination. And a rise time of 44.48 μs (90%) and fall time of 120.2 ms (10%) of the detector were acquired.

A new multifocus three-dimensional display which gives full parallax monocular depth cue and omni-directional focus is developed with the least parallax images. The key factor of this display system is a slanted array of light-emitting diode light source, not a horizontal array. In this system, defocus effects are experimentally achieved and the monocular focus effect is tested by four parallax images and even two parallax images. The full parallax multifocus three-dimensional display is more applicable to monocular or binocular augmented reality three-dimensional display in the modification to a see-through type.

By introducing a line defect and a V-shaped defect into two-dimensional triangular lattice photonic crystals, a 60 deg beam bend, a Y-shaped beam slitter, and a one-to-three beam splitter are designed based on the self-collimation effect of photonic crystals (PCs). By analyzing the equi-frequency contours calculated with the plane wave expansion method, the frequency for self-collimation in the PC is determined. By using the finite-difference time-domain method, the beam bending efficiency and splitting ratios of the device as a function of the defect radius are calculated. These results are confirmed by computational simulations that provide field distributions of light propagating in the structures. The designed beam bend and splitters offer more choices for beam control in the design of photonic integrated circuits.

This paper presents a comparative study on second harmonic conversion and saturation behavior of three copper vapor laser (CVL) beams of same average power up to 11 W (510 nm), however with different spatial coherence width of 17%, 35%, and 100% of the full beam cross-section. The nonlinear crystal employed is beta barium borate. It is observed that the trends in second harmonic (SH) conversion efficiency and its saturation are widely different for the three CVL pump beams. As expected, the conversion efficiency scaled up with increase in pump power. However, very interesting trends are observed in SH saturation characteristics with scaling of pump beams spatial coherence. For lower coherence CVL beam, the saturation of SH efficiency is observed at lower pump flux and at higher average power. Also, the saturation occurred at lower SH efficiency for CVL beam of lower spatial coherence. The fall in the SH efficiency after the peak is more rapid for lower spatial coherence pump beam. The experimental trends are explained in terms of combined effect degree of spatial coherence of pump beams and the thermal dephasing in the nonlinear crystal by high pump average power.

Efficient diode-pumped Yb:LuY₂SiO₅ laser mode locked by single walled carbon nanotube saturable absorber is reported for the first time. Under the absorbed pump power of 12.83 W, continuous wave mode-locked pulses were generated with the maximum average output power of 1.50 W. We obtained pulses as short as 5.2 ps around a center wavelength of 1058.6 nm. The peak power and the single pulse energy of the mode-locked laser were up to 2.9 kW and 15.0 nJ, respectively.

A simple and effective method of encapsulation of a fiber Bragg grating (FBG) sensor for use at elevated temperatures and harsh and corrosive environments using a rigid probe technique is presented in this paper. A thorough characterization has been carried out on the encapsulated, as well as bare, FBGs. The experimental studies reveal that encapsulated FBGs are superior in their responsivity, linearity, sensitivity, and repeatability when compared with unencapsulated ones.

In recent years, miniature spectrometers have been found to be useful in many applications to resolve spectrum signatures of materials. In this paper, algorithms are proposed to realize a miniature spectrometer using a low-cost filter-array spectrum sensor. Conventionally, the filter-array spectrum sensor can be modeled as an overdetermined problem, and the spectrum can be reconstructed by solving a set of linear equations. In this paper, we instead model the spectrum reconstruction process as an underdetermined problem, and bring up the concept of template-selection by sparse representation. l1 − norm minimization is introduced to achieve a high reconstruction resolution. Both simulation and experimental results show that a superior quality of spectrum reconstruction can be made possible from the presented underdetermined approach.

A novel monitoring scheme based on the fiber Bragg grating sensor to achieve real-time, long-distance, and passive-support monitoring of the power transmission line galloping is proposed and experimentally demonstrated. Based on the relationship between galloping amplitude and horizontal tension of a transmission line, numerical simulation and experiments are carried out. In the experiment the galloping amplitude monitored by this novel scheme is 0.3418 m, while the actual galloping amplitude is 0.35 m, which shows that the galloping amplitude can be obtained accurately by using this monitoring scheme.

We report here a balanced Mach-Zehnder interferometer-based integrated optical E-field sensors (MZI IOES) configuration, where the output Y-branch is replaced by a 2×2 directional coupler. Based on detailed theoretical and experimental investigations, we prove such IOES configuration has a simple linear transfer function, as well as much improved optical bias control, highly desirable for practical field applications. The optical biases of the fabricated devices deviate from the ideal bias by 3.3°, 7.9°, and 14.5°, respectively. The theoretical analysis and experiment results show that this type of sensor inherits the advantages of a conventional MZI sensor and directional coupler sensor: with a sinusoidal transfer function and a better optical bias.

An optical tilt sensor based on a phase-sensitive diffraction grating with direct intensity-modulated scheme is proposed in this paper. The tilt sensor consists of a coherent light source, an integrated grating, a mechanical sensing part, and an optical signal processing circuit. The mechanical sensing part consists of a bulk proof mass and two cantilevers that can be fabricated with a two-mask process on a silicon-on-insulator substrate. A phase-sensitive diffraction grating, which was formed with the integrated grating and the upper surface of the proof mass, acts as the sensing element for the tilt measurement. Experimental results show this tilt sensor provides the optimal tilt sensitivity of 1.15 V/° and resolution of about 0.0046°, and a measurement range of ±20° in a rotational plane that is perpendicular to the direction of the earth's gravity.

We propose a new type of 3 dB optical power splitter by splitting reflected field in photonic bandgaps (PBGs) based on photonic crystal waveguides (PCWs) with a triangular lattice of air holes. By employing PBGs effect, the propagation fields with frequencies under the cutoff frequency of guided modes in a modulated PCW (MPCW) are reflected back and output from two symmetrically placed waveguides. As a consecutive section of input PCW, the MPCW is realized by varying the size of the border holes adjacent to the PCW core. A nearly full transmission with a wide bandwidth is achieved only by introducing a defect hole in the MPCW. It provides a new method and a compact model to split input power in PCW devices and can find practical applications in future photonic integrated circuits.

A design strategy is proposed to optimize the structure of photonic crystal (PC) slabs in order to achieve high reflection on its interface. A broad band (Δλ/λ > 19%) and ultrahigh (>95%) reflection spectrum can be realized for one-dimensional (1D) and two-dimensional (2D) PC slabs. Tolerance analysis of design parameters reveals distinct merits for different PC structures: as to maintain high reflection, 1D PC slabs show better structural tolerance, while 2D PC ones have a much greater tolerance for the material parameter. Such a difference may provide a criterion for the design of PCs-assist mirror structure in optoelectronic devices.

For the butt-joint coupling between a single mode fiber (SMF) and a single mode dielectric slab waveguide (SMDSW) with a spherical fiber microlens (SFML), because the SMF and the rectangular-section SMDSW have different symmetry, it is difficult to calculate the coupling efficiencies using the traditional analytic method. In this paper, a Monte Carlo model of the coupler is proposed. This model incorporates the geometrical surface of both the SFML and the SMDSW, the relative position of the SMF and the SMDSW, and both the aberration and diffraction effects into consideration. The propagation characteristics of the coupler are simulated. And the coupling efficiencies and misalignment loss are investigated.

We present a new technique for measuring ultraprecise absolute optical surfaces. The technique combines the lateral shearing method but using a Fizeau interferometer. It achieves faster reconstructions than with a deflectometric system and without the influence of the reference surface. The system is limited by the imprecision of the linear stage and those of the estimation of the curvature of the reference surface. Regarding the guidance errors, we propose a new method to estimate pitch and roll based on data redundancy analysis. Numerical simulation results of pitch-and-roll estimations are given for realistic errors. Reconstructions using the sequential lateral shearing are also provided achieving nanometer accuracy.

We propose a simple technique to enlarge the reconstructed three-dimensional (3D) optical image and shorten the reconstructed distance simultaneously in real time holographic projection using a conventional lens or concave reflecting mirror based on the optical reversibility theorem. The main factors causing the longitudinal and transversal distortions of a 3D enlarged optical image are analyzed, and the 3D optical images are enlarged where severe distortions are precompensated by constructing objects with distortions directly instead of computing the precompensated phase iteratively so that it does not increase the computing time. Numerical simulations and optical experiments are performed for magnifying a simple cubic model. The results show that a 3D enlarged optical image is achieved successfully without any distortion and the reconstructed distance is shortened simultaneously. It is believed that this proposed technique is useful for 3D real time holographic projection in the future.

A low cost physical artifact with traceability to the national standard - meter is used for the metrological comparison of two terrestrial laser scanning systems: Riegl LMS Z390i and Trimble GX. The artifact is based on five spheres equidistantly situated and seven cubes of different dimensions. Accuracy and repeatability are evaluated using least squares fitting (LSF) and random sample consensus algorithms, for the study case of the spheres, and plane LSF and statistical analysis for the cubes. The horizontal resolution is evaluated using a modulated transfer function approach and the vertical one with the accuracy and repeatability data along Z axis. The Trimble system shows better results for all parameters, artifact parts, and algorithms under study.

This paper presents a novel approach to tracking a large number of closely spaced objects (CSO) in image sequences that is based on the particle probability hypothesis density (PHD) filter and multiassignment data association. First, the particle PHD filter is adopted to eliminate most of the clutters and to estimate multitarget states. In the particle PHD filter, a noniterative multitarget estimation technique is introduced to reliably estimate multitarget states, and an improved birth particle sampling scheme is present to effectively acquire targets among clutters. Then, an integrated track management method is proposed to realize multitarget track continuity. The core of the track management is the track-to-estimation multiassignment association, which relaxes the traditional one-to-one data association restriction due to the unresolved focal plane CSO measurements. Meanwhile, a unified technique of multiple consecutive misses for track deletion is used jointly to cope with the sensitivity of the PHD filter to the missed detections and to eliminate false alarms further, as well as to initiate tracks of large numbers of CSO. Finally, results of two simulations and one experiment show that the proposed approach is feasible and efficient.

The Naval Research Laboratory has developed a method for simulating atmospheric turbulence and a testbed that simulates its aberrations using a liquid crystal (LC) spatial light modulator (SLM). This testbed allows the simulation of so-called atmospheric seeing conditions ranging from very poor to very good and different algorithms may be easily employed on the device for comparison. Many models for simulating turbulence often neglect temporal transitions along with different seeing conditions. Using the statistically independent set of Karhunen-Loeve polynomials in conjunction with Kolmogorov statistics in this approach provides a spatial and temporal model for simulating turbulence. An added benefit to using an LC SLM is its low cost; and multiple devices can be used to simulate multiple layers of turbulence in a laboratory environment. Current testing using multiple LC SLMs is under investigation at the Naval Research Laboratory.

Information security has attracted much current attention due to the rapid development of modern technologies, such as computer and internet. We propose a novel method for optical image encryption using multilevel Arnold transform and rotatable-phase-mask noninterferometric imaging. An optical image encryption scheme is developed in the gyrator transform domain, and one phase-only mask (i.e., phase grating) is rotated and updated during image encryption. For the decryption, an iterative retrieval algorithm is proposed to extract high-quality plaintexts. Conventional encoding methods (such as digital holography) have been proven vulnerably to the attacks, and the proposed optical encoding scheme can effectively eliminate security deficiency and significantly enhance cryptosystem security. The proposed strategy based on the rotatable phase-only mask can provide a new alternative for data/image encryption in the noninterferometric imaging.

We propose a new and effective tone reproduction technique that preserves the visibility, contrast, and natural impression of scenes when rendering high dynamic range images on conventional low dynamic range display devices. The method is simple and easy to implement. Our new approach uses a modified mapping function that takes into account the block-level log-average, as well as the log-average of the whole image. This allows for increased visibility in dark regions. For those images with sharp edges that may cause a halo effect, the halo effect can be reduced by using bilateral filtering when the block-level log-average is computed. Furthermore, the local details of the image can be enhanced by using a median filter. Experimental results show that the technique gives satisfactory results for a variety of images and avoids common artefacts, such as halos, gradient reversals, and the loss of local contrast. In particular, our method produces excellent results when an image has both dark and bright regions due to new block-level processing during tone mapping.

A noniterative image reconstruction method is proposed by the subtraction of diffraction intensity patterns based on the Fraunhofer diffraction theory. According to the linearity theorem of the Fourier transform, the process of the noniterative image reconstruction is performed by numerical calculation, that is, the object wavefront can be extracted by the subtraction of three diffraction intensity patterns of an object and two hybrid masks. Furthermore, the way the noise impacts the quality of the reconstructed image is evaluated and the noise is suppressed by a median filtering algorithm. The results certify that the reconstructed image is consistent with the image of the object. The method provides us with an easy way for retrieving the wavefront with high quality in a wide class of wave fields such as optical, X-ray, electron, and atomic waves.

In the present work, a novel signal denoising technique for piecewise constant or linear signals is presented termed as "signal split." The proposed method separates the sharp edges or transitions from the noise elements by splitting the signal into different parts. Unlike many noise removal techniques, the method works only in the nonorthogonal domain. The new method utilizes Stein unbiased risk estimate (SURE) to split the signal, Lipschitz exponents to identify noise elements, and a polynomial fitting approach for the sub signal reconstruction. At the final stage, merging of all parts yield in the fully denoised signal at a very low computational cost. Statistical results are quite promising and performs better than the conventional shrinkage methods in the case of different types of noise, i.e., speckle, Poisson, and white Gaussian noise. The method has been compared with the state of the art SURE-linear expansion of thresholds denoising technique as well and performs equally well. The method has been extended to the multisplitting approach to identify small edges which are difficult to identify due to the mutual influence of their adjacent strong edges.

We present a novel color-invariant depth feature descriptor for color-shift-model (CSM)-based image processing. Color images acquired by a single camera equipped with multiple color-filter aperture (MCA) contain depth-dependent color misalignment. The amount and direction of the misalignment provides object's distance from the camera. The CSM-based image processing, which represents the combined image-acquisition and depth-estimation framework, requires a color-invariant feature descriptor that can convey depth information. For improving depth-estimation performance, color boosting is performed on a color image acquired by the MCA camera, and CSM-based channel-shifting descriptor vectors, or channel-shifting vectors (CSVs), are generated by using the feasibility test. Color-invariant features are also extracted in the luminance image. The proposed color-invariant three-dimensional (3-D) feature descriptor is finally obtained by combining the CSVs and luminance features. We present experimental analysis of the proposed feature descriptor and show that the descriptors are proportional to the depth of an object. The proposed descriptor can be used for feature-based image matching in various applications, including 3-D scene modeling, 3-D object recognition, 3-D video tracking, and multifocusing, to name a few.

Noncanonical illumination that is too dim or with color cast induces degenerated images. To cope with this, we propose a method for color-contrast enhancement. First, intensity, chrominance, and contrast characteristics are explored and integrated in the Naka-Rushton equation to remove underexposure and color cast simultaneously. Motivated by the comparison mechanism in Retinex, the ratio of each pixel to its surroundings is utilized to improve image contrast. Finally, inspired by the two color-opponent dimensions in CIELAB space, a color-enhancement strategy is devised based on the transformation from CIEXYZ to CIELAB color space. For images that suffer from underexposure, color cast, or both problems, our algorithm produces promising results without halo artifacts and corruption of uniform areas.

A problem of real time processing and forecast of dynamic images is discussed in this paper. A suggested approach to formation of the reference tunnel is based on the information processing method when laser beam centers are calculated with higher accuracy, and route images are classified to improve further forecast of their behavior.

Correlation filters for pattern recognition are commonly designed under the assumption that the shape and appearance of an object of interest are explicitly known. In this paper, we consider a signal model in which an object of interest is given at unknown coordinates in a cluttered reference image and corrupted by additive noise. The reference image is used to design filters for detecting a target in scenes with a nonoverlapping background and additive noise. An optimum correlation filter with respect to peak-to-output energy for object detection is derived. The shape and appearance of the target are estimated from the reference image. Two methods to estimate the frequency response of the derived filter are used. Computer simulation results obtained with the proposed filters are presented and discussed. The performance of the filters is evaluated in terms of discrimination capability and location accuracy for different statistics of the backgrounds and noise processes present in the signal model.

A new orientation-based face recognition method is proposed in this paper. The orientation analysis is performed with Gabor wavelet transform (GWT). The multispectral imagery includes the visible (RGB) and thermal (long-wave infrared) face images from the same group of subjects. The recognition performance of the new method is compared with that of three classical algorithms, principle component analysis, linear discriminant analysis, and elastic bunch graph matching. A score-level fusion of several algorithms versus multispectral images is explored and presented. Specifically, at each frequency band of GWT, an index number representing the strongest orientational response is selected, and then encoded in a binary number to favor the Hamming distance calculation. Multiple-band orientation codes are then organized into a face pattern byte (FPB) by using order statistics. With the FPB, Hamming distances are calculated and compared to achieve face identification. The FPB has the dimensionality of 8 bits per pixel and its performance will be compared to that of face pattern word (32 bits per pixel). The dimensionality of FPB can be further reduced down to 4 bits per pixel, called face pattern nibble. Experimental results with the multispectral faces of 96 subjects show that the proposed orientation-based face recognition method is very promising in contrast with three classical methods. Furthermore, the recognition performance with score-level fusion achieves 100% when tested on the entire multispectral database.

Color image segmentation largely depends on the color space chosen. Furthermore, spaces that show perceptual uniformity seem to outperform others due to their emulation of the human perception of color. We evaluate three perceptual color spaces, CIELAB, CIELUV, and RLAB, in order to determine their contribution to natural image segmentation and to identify the space that obtains the best results over a test set of images. The nonperceptual color space RGB is also included for reference purposes. In order to quantify the quality of resulting segmentations, an empirical discrepancy evaluation methodology is discussed. The Berkeley Segmentation Dataset and Benchmark is used in test series, and two approaches are taken to perform the experiments: supervised pixelwise classification using reference colors, and unsupervised clustering using k-means. A majority filter is used as a postprocessing stage, in order to determine its contribution to the result. Furthermore, a comparison of elapsed times taken by the required transformations is included. The main finding of our study is that the CIELUV color space outperforms the other color spaces in both discriminatory performance and computational speed, for the average case.

A divide sampling-based hybrid temporal-spatial prediction coding algorithm is designed to further improve the coding performance of the conventional H.264/AVC coding. In the proposed algorithm, a frame is first divided into four equal-sized subframes, and the first subframe is coded using the rate distortion optimization model with inter- or intraprediction adaptively. Then, the optimal prediction method of the macroblock in other subframes is selected flexibly and reasonably from intraprediction, the fast interprediction, and the spatial interpolation prediction. The simulation results show that compared with the conventional H.264/AVC coding, the average bit rate is reduced by 6.15% under the same peak signal-to-noise ratio (PSNR), the average PSNR is increased by 0.22 dB under the same bit rate, and the average coding time is saved by 12.40% in the proposed algorithm.

In this paper, we propose a fast intramode decision method in MPEG-4 AVC/H.264 based on the intraprediction cost. The rate-distortion function in MPEG-4 AVC/H.264 exhaustively computes the distortion and bit rate of every mode type, which results in high computational complexity. The target of a good, fast mode decision method is to reduce computational complexity with minimal loss of rate-distortion performance. The proposed algorithm is based on prediction cost estimation using the variances of input pixels and those of residual values to determine the minimal number of mode types for a given macroblock. The test results show that the total encoding time is reduced by 77.4%, on average, compared to that of the JM 13.2 reference encoder.

The recent H.264/AVC video coding standard provides a higher coding efficiency than previous standards. H.264/AVC achieves a bit rate saving of more than 50% with many new technologies, but it shows very heavy computational complexity. In this paper, a fast mode decision scheme for interframe coding is proposed to reduce the computational complexity for H.264/AVC video encoding system. To reduce the block mode decision complexity in interframe coding, we use the contextual information based on the co-located and neighboring macroblocks (MBs) to detect a proper MB that can be stopped early. Then, for the current MB, a priority information of the context is suggested for adding more mode types adaptively. The proposed algorithm shows the average speed-up factors of 59.11 to 77.41% for various sequences with a negligible bit increment and a minimal loss of image quality, in JM reference softwares.

The morphological joint fractional Fourier transform correlator (MJFTC), as a modified nonlinear joint fractional Fourier transform correlator (JFTC), is introduced in this paper. An optoelectronic setup that can implement such a nonlinear JFTC is advised and constructed. We investigate the correlation properties of the MJFTC using computer simulation and optical experiments. Numerical and experimental results show that the MJFTC exhibits remarkable improvement in terms of discriminability and peak sharpness compared with the classic JFTC. Moreover, the MJFTC provides better noise tolerance to the salt-and-pepper noise or normally distributed white additive noise than the JFTC and the other nonlinear processing or fractional-order filtering JFTC.

The ultrafast third-order nonlinear optical properties of C₆₀+ vinyltriethoxysilane H₂C=CHSi(OC₂H₅)₃ (C₆₀+VTES) sol-gel were investigated by the femtosecond optical Kerr gate (OKG) technique at 800 nm. The third-order optical susceptibility was measured to be 5.42 × 10−14 esu for C₆₀+VTES at a weight concentration of 0.06 wt.%. Using C₆₀+VTES as the OKG material, we acquired a series of narrow bandwidth and symmetric gated spectra continuously from the chirped white light continuum generated in water with femtosecond laser pulses. The gated spectra obtained using the C₆₀+VTES OKG have distinct superiorities compared with CS₂.

In this work, we developed a full-range Fourier domain optical coherence tomography (FD-OCT) imaging probe with a magnetic-driven resonant fiber cantilever. A galvanometer-driven reference mirror provides a linear phase modulation to each M-scan/B-scan frame to enable complex conjugate-free, full-range FD-OCT imaging. A fiber cantilever inside the probe is driven by a low-voltage miniature magnetic transducer. The fiber cantilever scans at its resonant frequency synchronized to the phase modulation induced by the reference mirror. Using a CCD line-scan camera-based spectrometer, we demonstrated real-time full-range FD-OCT at 34 frame/s (1024 pixel lateral × 2048 pixel axial).

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