This PDF file contains the editorial “Humility” for OE Vol. 46 Issue 05

A novel binary level set method for boundary-based image segmentation is proposed, which is extended from region-based binary level set methods. The proposed binary level set method is based on the geometric active contour framework, which is a traditional level set method applied in boundary-based image segmentation. However, being different from the geometric active contour, the proposed binary level set method replaces the traditional level set function with a binary level set function to reduce the expensive computational cost of redistancing the traditional level set function. The experiments and complexity analysis show that the proposed binary level set method is more efficient than the geometric active contour for image segmentation while giving similar results to the geometric active contour.

This PDF file contains the editorial “Optical Metrology in the Transportation Industry” for OE Vol. 46 Issue 05

The calibration of 3-D optical sensors normally requires the use of a calibration artifact of known dimensions. By labeling regions within the measured point clouds as belonging to a known region of the artifact, camera and projector parameters can be optimized. A novel 3-D Hough transform has been developed to extend the well-known strategy for detecting circles in 2-D images to detecting spheres in a 3-D point cloud. In its standard form, the Hough transform suffers from excessive memory storage requirements for the intermediate Hough accumulator space, which can make its application to 3-D problems impractical. We describe an accumulator implementation using an optimized sparse 3-D matrix model that provides compact data storage and efficient data access. Application of this method to experimental shape data for spheres is discussed, demonstrating its memory-saving benefits, computational efficiency, and 3-D feature detection capability.

Fractures in ductile thin-sheet structures, such as a fuselage or automobile panels, often occur under complex loading conditions. In particular, under remote mixed-mode I/III loading conditions, a cracked structure is subjected to a combination of in-plane tension and large out-of-plane tearing deformation, which may lead to crack tip fields consisting of all three fracture modes (modes I, II, and III). Understanding such fracture events in ductile materials is an important component of the structural integrity analysis of load-bearing structures containing ductile, thin sheets. Due to the complex nature of mixed-mode I/III fracture in ductile thin-sheet materials, reports of experimental investigations are very limited in the literature. We configure three-dimensional digital image correlation (3D-DIC) systems to acquire full-field deformations during the loading and stable tearing processes. The full-field deformation measurements are used to characterize the stable crack extension behavior of an aluminum alloy undergoing quasistatic and dynamic mixed-mode I/III loading. Results confirm that 3D-DIC is an excellent methodology for measuring 3-D deformations in the presence of large out-of-plane warping and motion, both dynamically and statically. Data obtained during the fracture process indicate that the introduction of a mode III component into the loading process alters the crack tip displacement and strain fields relative to those measured in the nominally mode I loading. Furthermore, the measured crack-opening displacement (COD) values during quasistatic and impact mixed-mode I/III fracture show that (1) COD is nearly constant for crack extension beyond 2 mm and (2) COD under combined-mode I/III loading is four times larger than observed during mixed-mode I/II or mode I fracture of the same material, indicating that the magnitude of the critical COD is a function of loading mode in highly ductile, thin-sheet materials.

Among the different optical measurement techniques for full-field analysis of displacements and strains, digital image correlation (DIC) has proven to be very flexible, robust, and easy to use, covering a wide range of different applications. DIC measurement results, among others, are influenced by the systematic errors of the measurements system, a major source of which is due to imaging system calibration. We present a 3-D DIC system that provides online error information concerning diverse error sources, and even more important, the propagation of errors throughout the calculations to the resulting contours, displacements, and strains. On the basis of this system we discuss error sources, error propagation, and the impact on correlation results. Performance tests for studying the impact of calibration errors on the resulting data are shown for static and dynamic applications.

An improved method to compute the displacement field and strain field from the speckle recorded from a CCD camera is proposed. Previous studies have shown that the displacement field can be determined from two neighboring speckle images using the fast Fourier transform. Moreover, the accuracy of determining the displacement field in this manner relies on the ability to determine the peak position of the Fourier spectrum precisely. We present an improved method that can measure the displacements to an accuracy of 2 μm, provided that the distance between two pixels corresponds to 0.1 mm. The algorithm developed to obtain the displacement field uses the usual assumption that the displacements within each subelement of the image are constant and evaluated at the center of the subelement. However, instead of moving the entire subelement in each loop of the algorithm, a fraction of the subelement size is adopted to generate a family of overlapping subelements, resulting in an enhanced resolution of the resulting displacement field.

We present a new, high-accuracy digital image correlation measurement system based on a double long-focus CCD microscope. This has many advantages, such as noncontact measurement, high accuracy, quasi real time, and convenient operation. It can be used in the measurement of strain (especially in strain measurement of flexible high-molecular synthetic material, where the strain measured is averaged data) and linear expansibility of new material that has low expansibility. It can measure strain that is less than 1 με when it is used in strain measurement. The conventional digital image correlation technology can measure strain that should be more than 20 με. An array optical element is used to set the mark of 0.05 mm. The time-average method is used to eliminate CCD noise and bad effects caused by liberation, so the correlation measurement sensitivity is up to 0.01 pixel. The result of the strain test for metal material is listed. Moreover, we illustrate that the digital image correlation measurement method is the technology of a high-point diffraction resolving ratio in a super optics system.

To reduce costs and facilitate automation in the automotive industry, adhesive bonding has gained popularity as a replacement for conventional mechanical fasteners such as bolts, screws, rivets, and welding. Adhesive bonding is particularly useful for bonding parts made of plastics and polymer composites, which are playing an increasing role in reducing vehicle weight. However, the adhesive bonding process is more susceptible to quality variations during manufacturing than traditional joining and fastening methods. Shearography and pulsed thermography are full-field, noncontact, nondestructive testing methods that are widely used in the aerospace industry, offering significant potential as practical tools for in-process inspection of adhesive bond quality. The two techniques are often used to address a common set of aerospace applications, e.g., delaminations or skin-to-core disbonds in composite structures. However, they are fundamentally different, based on different flaw detection mechanisms: Shearography measures the sample's mechanical response to mechanical stresses, while pulsed thermography measures the sample's thermal response to an instantaneous thermal excitation. For the convenience of potential users and readers, the authors review shearography and pulsed thermography. The potential of these techniques for inspecting adhesive bonding is demonstrated and compared.

The use of thermography as a nondestructive testing method has increased significantly in recent years. However, implementation is usually based on visual interpretation of image results by a trained inspector. Although this is accepted practice in the aerospace industry, where thermography is widely used, it is not appropriate for automotive manufacturing, where higher production rates demand higher inspection throughput that can only be accomplished with automated detection. The logarithmic derivatives of flash thermography time histories provide an excellent basis for automated detection. A unique model, based on the logarithmic derivatives of a series solution for the surface temperature of a flash heated plate with adiabatic boundary conditions, provides a template for identifying pixels that deviate from uninterrupted diffusion that is characteristic of a defect-free sample. We demonstrate automated defect detection using this model on a composite sample.

Digital speckle pattern interferometry (DSPI) and digital shearography (DS) are two independent useful whole-field noncontacting optical methods for nondestructive flaw detection and precision measurements. We describe a (1,N) spatial phase-shifting technique in DSPI and DS for nondestructive evaluation (NDE) of quasidynamic behavior of objects subject to slowly varying loads. The technique employs a double-aperture arrangement in front of the imaging system to introduce spatial carrier fringes within the speckle. The prominent advantage of the proposed technique is it requires only a single frame prior to the object deformation and a number N of frames during the object deformation for NDE. Quantitative measurement of a defect and its behavior in loading conditions are studied by recording spatially phase shifted frames before and during thermal stressing of the object for continuous deformation variation with time. Experimental results on a polymethyl methacrylate (PMMA) panel using an error-compensating five-phase-step algorithm for quantitative NDE using both DSPI and DS are demonstrated.

Field instrumentation is investigated on an in-service highway bridge over a 2-year period. Extrinsic Fabry-Pérot interferometric (EFPI) strain sensors provide a permanent health-monitoring capability. The bridge is a reinforced-concrete (RC) structure that was repaired and strengthened using fiber-reinforced-polymer (FRP) wraps. A sensor network monitors the load-induced strain in the FRP reinforcement and the steel rebar. Colocated electrical resistance strain gauges and a finite element analysis are used for comparison. Both dynamic and static load characteristics are analyzed for a near-capacity truck. The fiber optic measurements are generally consistent with the comparison measurements and the analytical results; and they show no failure or degradation as opposed to the electrical resistance gauges. We demonstrate the implementation and the performance of in situ EFPI sensors in a long-term field environment. Embedded fiber optic sensors can provide the required information for the intelligent management of a transportation infrastructure.

A multiplexed optical fiber Michelson interferometer system that is self-referenced with a stabilizing feedback loop is presented. This system employs fiber Bragg gratings and wavelength division multiplexing technique to combine two optical fiber interferometers that share the same optical path in the main part of the optical system. When one Michelson interferometer, which uses the fiber Bragg gratings as reflective mirrors and is used as reference interferometer, is stabilized by an electric feedback loop, the other interferometer, which is used for the measurement, is also stabilized. This system is therefore suitable for online precision measurement. An active phase-tracking technique is applied for signal processing to achieve high resolution.

In optical measurement techniques, such as holographic and shearographic interferometry, digital speckle pattern interferometry, and 3-D computer vision, phase is used as a carrier for different physical quantities. For example, phase could stand for information concerning shape and deformation. The useful information is coded and wrapped into the range of −π to +π rad in a phase map. Phase unwrapping is the basic technique to resolve 2π ambiguities and recover expected physical quantities. We describe a novel method for unwrapping phase in a noise-disturbed phase map, which is characterized by broken fringes induced by different sources, such as a poor coherence length, high light or weak reflection on the sample surface, too dense fringes due to stress concentration, etc. In our method, broken fringes are detected and estimated. The phase map is segmented into different regions based on the quality of phase. Regions are ranked and arranged to construct a systematic unwrapping sequence. Unwrapping errors brought by broken fringes are minimized by the proposed unwrapping scheme. Several applications are presented to validate the new method.

The detection of cracks in rails is a challenging problem, and much research effort has been spent in the development of reliable, repeatable crack detection methods for use on in-service rails. While crack detection in the rail head and shear web is reliably achieved using ultrasonic and eddy current methods, neither technique is particularly effective for the detection of cracks in the rail foot. The authors present a new crack detection method for rail, which utilizes the change in infrared emission of the rail surface during the passage of a train wheel. Initial data from this infrared method are presented, from studies of both a laboratory-based three-point bend specimen and a short section of rail. The results of these two studies confirm the ability of the proposed method to locate and quantify surface-connected notches and cracks.

We present a microcontrolled optoelectronic system to measure online the average velocity of a projectile impacting on aircraft and spacecraft structures. The projectile velocity can vary in the range from subsonic to supersonic. The implemented optical system is based on three optical barriers that are crossed by the projectiles before impacting on the structural elements. A simple optoelectronic prototype is constructed and tested. The flight times among the three optical barriers are obtained by the microcontroller system. The measured velocity accuracy in test conditions is better than 1%. A high-brightness liquid crystal display is used to show messages of the system configuration and also the measured projectile velocity. To ensure safety in the experimental impact tests, serial RS-232 communication can be used to monitor this information using a remote computer. The system is highly robust since it is able to measure projectile velocity even when an optical barrier is missed by failure in either the optical transmitter or the receptor.

The hatches of airplanes and boats are made of Plexiglas. There are three types of stresses in the structure made of Plexiglas or glass. Those stresses are (1) residual stresses from the casting, (2) residual stresses due to manufacturing process involving sheet forming structure, and (3) the stresses from the serving period. The stresses and the testing method are studied by Isodyne method. Frequency domination phase shift method is presented to recognize scattered light patterns automatically for the directional Plexiglas plate and structure. The results show that the Isodyne method is a powerful tool of stress analysis for Plexiglas.

A single method for simultaneously measuring 2-D rotational angles is proposed by using a collimation laser. Compared with the traditional autocollimator measurement method, the proposed method can also be applied to longer distance and higher precision angular measurement. The 2-D angular drift of the reference datum beam is suppressed by a factor of 10 via expansion. A traditional single lens in front of the detector is replaced by combination lenses to increase the sensitivity and compactibility. The measurement principle of the 2-D angles is also analyzed. The feasibility and stability of the presented system are verified by comparison with the Renishaw ML10 laser interference measuring system. The experimental results show that the linear correlativity is 0.99999 and the point-to-point standard deviation error is less than ±0.2 arcsec in a ±60-arcsec range and 2000-mm distance. This proposal is most advantageous for simultaneous long distance and high accuracy of 2-D angular errors measurement of a linear guide manner.

Although wavefront aberration in stepper projection optics has been classified with respect to its spatial frequency on the pupil, the physical meaning of this classification has not been clarified. In this paper, we show that wavefront aberration can be classified into figure aberration, random aberration, and nonconserved aberration, by taking into consideration the theoretical effect of undulated wavefront aberration with respect to its spatial frequency. We also show that the predictions of this theoretical classification coincide with both the results of numerical simulations for random aberration and the experimentally measured values of local flare size. Since our classification has a clear physical meaning, it will be valuable and applicable in developing not only stepper projection optics but also other more general optics.

An effective simulation of speckle with tilted surfaces illuminated by short-coherence-length lasers is presented. Two new tools for assessing speckle and/or its contrast under these conditions are developed and validated. The first is a simulation of the time-domain tilted-surface effects that provides speckle imagery. The second is a simple intuitive model for contrast derived from speckle reduction due to averaging. Field results of speckle imagery for a laser-illuminated target and a short-wave infrared imager validate the simulation. Simulated speckle is compared visually with the actual speckle. Also, contrasts of the speckle generated by simulation and actually imaged in the field are compared for one tilt angle. Existing analytical models of the contrast also validate the simulated speckle contrast. Contrast-versus-angle characteristics of simulated speckle are compared with the general analytical contrast model for speckle from tilted surfaces.

Real-time nondestructive testing (NDT) and nondestructive evaluation (NDE) of plate-type structures are important for structural health monitoring (SHM) applications. In this work, the wave scattering from horizontally oriented internal cavities or cracks in a plate is studied using the distributed point source method (DPSM). DPSM has gained popularity in the last few years in the field of ultrasonic field modeling. DPSM is a semianalytical technique that can be used to calculate the ultrasonic field (pressure, velocity, and displacement fields in a fluid, or stress and displacement fields in a solid) generated by ultrasonic transducers. So far, the technique has been used to model the ultrasonic field near a fluid-solid interface when a solid half-space is immersed in a fluid. This method has also been used to model the ultrasonic field generated in a homogeneous isotropic solid plate immersed in a fluid. The objective of this study is to present the theoretical modeling of the diffraction and scattering pattern of guided waves in the solid plate when transducers of finite dimension are used to generate guided waves in the defective plate.

As an important component of a projection display system, its illumination system determines the brightness and uniformity of the whole system. This paper presents a simple design for an illumination system, which has a compound parabolic retroreflector to efficiently utilize the light from a lamp. The performances of this ultrahigh-performance system is described with respect to light efficiency, and uniformity.

A new kind of radio-frequency-excited diffusively cooled gain slab waveguide array CO₂ laser is presented. In this type of laser, grooves are engraved on the top electrode to form a group of parallel subelectrodes that can produce the periodic gain distribution along the transverse direction of the waveguide. A copper feedback mirror with parallel grooves in the surface is used. By using this type of laser structure, a stable far-field intensity distribution with a spatially suppressed single peak is obtained.

By considering the Gaussian spatial distribution of the initial population-inversion density and the intracavity photon densities of the fundamental, the signal, and the idle lights, the rate equations of a laser-diode-pumped, doubly Q-switched, intracavity optical parametric oscillator (IOPO) with both acousto-optic modulator (AO) and C^r4+:YAG saturable absorber are derived. The influence of the pump rate, the thermal effect in the gain medium, and the change of the photon density along the cavity axis have also been taken into account the equations. In the experiment, a laser-diode-pumped, doubly Q-switched, Nd:YVO₄/KTP IOPO with an acousto-optic (AO) modulator and a Cr:YAG saturable absorber is realized. The shortest pulse width of 6.7 ns has been obtained at the maximum pump power of 6.3 W. The experimental results agree well with the theoretical calculations.

A 17.84-cm-long Littrow external cavity diode laser at 823 nm is mode-locked by actively modulating the current at a 840.843-MHz fundamental harmonic frequency. This mode-locked external cavity diode laser has an overall spectral width of 0.16 nm (or 71 GHz), roughly corresponding to 84 longitudinal modes, a temporal pulse width of 63 ps, and an average power of 1 mW. Using the Pound-Drever-Hall technique with both fast and slow servo corrections, this mode-locked laser is then frequency-stabilized to a 17.84-cm-long sealed optical cavity with a finesse of 6000. To the author's knowledge, this is the first frequency stabilization of a mode-locked diode laser with such a high-finesse cavity.

The effect of a spherical output surface on the performance of a truncated dielectric paraboloid in focusing the light from multiple diode lasers was studied with ray tracing. Especially the effects of the radius and the displacement of the output sphere were investigated. In addition, the performance of a parabolic concentrator with a spherical output surface in high-numerical-aperture focusing of diode laser light was demonstrated experimentally. The 1/e² diameter of the measured focus was less than 100 μm.

In order to ameliorate convergence of the algorithm to invert the particle-size distribution (PSD) from laser diffraction data, an improved conjugate gradient algorithm (ICGA) is proposed. This method is independent of any given a priori information of the particle-size distribution. In the algorithm, each objective function is constructed according to an equation of the system of equations. Then iterations are carried out continuously between objective functions by choosing conjugate gradient directions, and thus the objective functions are tied up. An iteration step-adjusting parameter is introduced, which depends on the row index vectors of the matrix equation. Two narrowly distributed particulate-certified reference materials, their mixture, and a widely distributed particle plate are used as samples to verify the algorithm. Experimental results show that the ICGA is sufficiently convergent and that the convergence points are stable. The presented method can be used to invert unimodal and multimodal PSD with high precision.

This paper presents the proposal and experimental demonstration of a microwave photonic bandpass filter structure based on a multiwavelength semiconductor optical amplifier (SOA) ring laser, an optical phase modulator, and a segment of single-mode fiber as a dispersive element. Experimental results show multiple-tap microwave photonic bandpass filter by using a power-equalized multiwavelength SOA ring laser as the optical source. Two laser configurations, one with a linearly chirped fiber Bragg grating (LCFBG) to compensate the dispersion of the laser cavity and one without the LCFBG, are compared for their multiwavelength laser outputs and the corresponding microwave photonic bandpass filters' response. The two microwave photonic bandpass filters have free spectral ranges (FSRs) ~1.26 GHz and 1.85 GHz. The maximal main-to-secondary sidelobe ratio of the microwave filter is above 10 dB.

We investigate the effects of co-doping of boron on the index difference between the core and cladding of the optical fiber, the temperature and bending sensitivities of a long-period fiber grating (LPFG), and the resonance wavelength separation between the core mode and the first cladding mode of a fiber Bragg grating (FBG). We observe that the index difference between the core and cladding decreases with the slope of 1.69×10−⁴/SCCM and that the temperature sensitivity of the resonance wavelength shift of LPFG decreased with the slope of 0.01145 nm/°C/SCCM. The measurement results indicate that, as the amount of the co-doped boron is increased, the bending sensitivity of LPFGs increases, while the resonance wavelength separation between the core mode and the first cladding mode of FBGs decreases. These results may be used for design of Ge-B co-doped optical fibers with the desired characteristics suitable for optical fiber communication and sensing applications.

When a fiber Bragg grating (FBG) sensor is embedded in a structure to sense its strain, a portion of strain is absorbed by the protective interlayer of the fiber optical sensor. If an angle exists between the FBG sensor axis and external principal stress direction of the host material, the strain transfer from the host material to the sensor will be much different than that of the external stress parallel to the sensor axis. A suitable strain transfer model is developed for evaluating the interaction between the surrounding matrix and a length of optical fiber under nonaxial stress. A number of realistic assumptions are introduced to simplify the process of the mathematics rigor. Strain transfer rate is introduced to describe the level of strain loss within the protective interlayer and the amount transferred to the optical fiber core. Theoretical results show that the angle of the optical fiber sensor plays an important role in strain transferring from the surrounding materials to the optical fiber core. The theoretical findings are verified through a series of experiments with FBG sensors. The evaluation error of average strain transfer rate is discussed because of sensor-located angle deviation.

A fiber optic imaging sensor is presented that utilizes surface plasmon resonance (SPR) excitation. The configuration of this sensor allows for remote sensing and multiplexing. The sensor is advantageous in that it eliminates the traditional bulk optic prism in favor of a relatively simple and inexpensive design. Two sensor configurations are presented; the principle of one is based on using optical interference, and that of the other is based on using the optical intensity as in traditional bulk-optic SPR imaging systems. Experimental measurements indicate that the fiber optic sensor based on optical interference is compact and immune to noise due to ambient light, and is more powerful and reliable than the common SPR imaging sensor based on optical intensity. Using fiber optics, the theory of SPR, and interference, a new SPR imaging sensor is achieved, which is better fitted to applications.

A 33×33 thermo-optic tunable arrayed waveguide grating (AWG) is fabricated using the poly (2,3,4,5,6-pentafluorostyrene-co-glycidylmethacrylate) (PFS-co-GMA). The fabrication process of the device is depicted, and the measured results are presented. The measured insertion loss is between 8.5 dB for the central channel and 10 dB for the edge channels, the crosstalk is below −20 dB, the wavelength channel spacing is 0.8 nm, and the 3-dB bandwidth is 0.35 nm. The measured wavelength/temperature shift rate is about −0.12 nm/K. The central wavelength is 1545.21 to 1551.81 nm in the temperature range from 10 to 65°C. The tuning range is −6.6 nm.

The significant features of coherence multiplexing technique are applied to a turbo-coded optical pulse position modulated (PPM) code division multiple access (CDMA) system with differential detection, and its performance is evaluated in terms of bit-error rate (BER) by including the effects of thermal noise, beat noise, and shot noise. It is shown that around 98 users can be accommodated at a signal-to-noise ratio (SNR) of 2.0 dB for a fixed BER of 9.6688×10^-6. A significant reduction in BER of around 3.2355×10^-16 is achieved for an increased SNR of 7.2916 dB supporting 53 users for a source linewidth of 20 nm. On increasing the linewidth to 40 nm, the number of users is found to increase from 53 to 75 for the same BER and SNR. The reduction of BER of 3.2355×10^-16, 6.6473×10^-18, and 2.1089×10^-19 is obtained at an SNR of 7.29 dB for increasing interleaver lengths of 10, 32, and 100, respectively. The BER reduction by decreasing the user data rates is also presented. On comparison with the direct detection, the number of simultaneous users that can be accommodated at a BER of 10^-16 is found to increase from 31 to 54 users.

A combination of polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) in optical fibers may lead to anomalous pulse broadening. It raises the issue for more complete assessments when studying pulse propagation in the presence of polarization-dependent loss. A mathematical description is put forward in this work, where PMD is evaluated in the presence of XPM, PDL, and chirp. Simulation shows that the delay of a chirped Gaussian pulse depends not only on PDL, but also on the chirp of the pulse itself. To some degree, effective PMD can be controlled by PDL and chirp. The results give guidance for PMD compensation.

Wavelength conversion based on the cascaded sum-frequency generation and difference-frequency generation (cSFG+DFG) process in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) waveguides has the advantage of no pump occupation in the communication band, compared to the traditional cascaded second-harmonic generation (SHG) and DFG process. In cSFG+DFG-based wavelength conversion, two pump waves and a signal wave are required. The input pulse train, as a carrier of information, can be applied to either the signal or one of the two pump waves. Though the converted wave is in the form of pulses, its temporal and spectral characteristics are dependent on the arrangement of the pulsed wave. In this work, we target the wavelength conversion of 40-GHz picosecond pulse trains, and systematically analyze the temporal and spectral characteristics of the pump, signal, converted, and sum-frequency waves during their propagations in PPLN waveguides by using numerical simulation. In particular, pulse shapes, optical spectra, phases and conversion efficiency are compared when the picosecond pulse train is used as the signal and pump waves, respectively.

A novel 10-GHz picosecond pulse source consisting of an electroabsorption modulator (EAM) and a subsequent well-designed pulse compressor is demonstrated experimentally. Among its advantages are stable operation and a tunable repetition rate. A 1.1-ps transform-limited ultrashort pulse with no pedestal is obtained. As an illustration of its use, the obtained high-quality pulse is applied to measure other EAMs' optically or electrically induced temporal transmission windows.

The propagation of solitons through an optical fiber with dual-power-law nonlinearity, in the presence of a stochastic perturbation term, is studied. The Langevin equations are derived. It is thus proved that solitons travel down an optical fiber with a fixed mean free velocity.

We describe the encryption and decryption of a two-dimensional (2-D) image by using a sandwich diffuser in the Fourier plane of the first lens of a 4-f imaging system. The diffuser is made with two random phase diffusers, sandwiched together. The encrypted image is holographically recorded in a photorefractive BaTiO3 crystal. Decryption is done by generating a phase conjugate image through the phase conjugation technique. During decryption, if only one of the two random phase diffusers constituting the sandwich diffuser is used, it becomes impossible to retrieve the image. Also no decryption is possible if any of the diffusers constituting the sandwich is shifted in-plane with respect to the other. It is possible to generate the image only if the diffusers are sandwiched together again after matching point to point, and then the sandwich diffuser is kept at the same position and orientation in which it was kept during the encryption. Experimental method has been described to overcome the practical difficulty for the generation of the key in the case of the sandwich diffuser. Simulation and experimental results are presented in support of the proposed idea. To evaluate reliability of the technique, the mean-square-error (MSE) between decrypted and original image has been calculated.

The conventional measuring system of laser feedback noise for an optical pickup was specifically implemented only for laser diodes by using a static optical system. With no closed-loop servo control, it is impossible to measure the genuine laser noise distribution of a pickup while operating in an optical drive. By modifying the optical system of a commercial pickup, this study built up a system that was integrated with precision mechanical design, optical design, servo control design, and opto-electronic signal inspection for providing dynamic real-time laser feedback noise measurement. Using this system, some experimental results have been observed. The laser feedback noise is responsible to the focusing point within the linear range of an optical pickup head. It has the maximum value while the lens is on the best focus. The higher the environmental temperature, the noisier the laser output will be. Besides, the central frequency of noise is dependent on the disk rotation speed, and the noise level is reducible by increasing the disk rotation speed.

Digital holography is applied to analyze the spatial distribution of the polarization state of light transmitted through anisotropic objects by successive hologram recording using polarization switching of the reference wave by an optical fiber Faraday rotator. For a preliminary object to confirm the principle of polarization analysis, a quarter wave plate (QWP) is used, and the distribution of the polarization ellipse over the object surface is analyzed for various azimuths of the QWP. By inserting a polarizer into a part of the object wave after the object as a reference area, a phase drift of the reference waves caused by polarization switching and/or environmental disturbances can be evaluated and compensated for by using the reference area. The principal axis and ellipticity are calculated and compared with their theoretical values. A good agreement between the experimental and theoretical values is observed. For an application, a photoelastic effect induced in a stressed PMMA specimen is also investigated.

We describe the performance of two-center hologram storage in In:Fe:Mn:LiNbO₃. The doping content of MnO and the oxidation state that yield the maximum nonvolatile diffraction efficiency in In:Fe:Mn:LiNbO₃ (0.080 wt % Fe₂O₃, 3 mol % In³⁺) are discussed.

The resolution of digital holography as an optical imaging system is described by the point spread function of the system. Here the point spread function of double-aperture digital holography is determined. It promises the possibility of a resolution increase by the use of synthetic apertures, where we combine the digital holograms of two CCD arrays in one large hologram and reconstruct, or we superpose coherently the two reconstructed wave fields after a proper shift. An experimental method for the determination of this shift with subpixel accuracy is given. Furthermore, experimental results on numerically reconstructed wave fields from synthetic holograms stemming from two mutually shifted digital holograms of the same scene are presented.

This research develops a model-based spectral image reconstruction (MBSIR) algorithm to reconstruct images collected from the Advanced Electro-Optical System (AEOS) Spectral Imaging Sensor (ASIS). The development of the algorithm requires two key elements: 1. the statistics of the photon arrival and 2. estimates of the spatial and spectral transfer functions. With these two elements, the MBSIR algorithm can, through image postprocessing, dramatically increase the resolution of the images as well as give insight into the performance of the imaging sensor itself. The MBSIR algorithm is designed to simultaneously improve both the spatial and spectral resolution, and is derived for the general case of a spectrally variant imaging system. While MBSIR algorithms can be developed for any spectral imaging system, this research focuses on ASIS, a new spectral imaging sensor installed with the 3.6-m AEOS telescope at the Maui Space Surveillance Complex (MSSC). The primary purpose of ASIS is to take spatially resolved spectral images of space objects. The low-light levels and object motion inherent in imaging some objects in space, such as satellites, lead to a sensor design with less spectral resolution than required for image analysis. However, by applying MBSIR to the collected data, the sensor will be capable of achieving a higher resolution, allowing for better spectral analysis. The algorithm is shown to work with simulated ASIS data and measured data from an ASIS-like sensor.

We propose an adaptive watermarking technique that is robust to JPEG compression and sensitive to malicious manipulations, including counterfeiting attacks. The watermarking technique includes three major functions: generating content-based watermarks, hybrid watermarking, and calibration. Two features, invariant to JPEG compression, used to generate content-based watermarks increase the resistance to counterfeiting attacks. The watermarks are embedded using various strategies, depending on the characteristics of discrete cosine transform (DCT) coefficients. Finally, two calibration functions are used to increase the accuracy of authentication. Experimental results demonstrate that the proposed watermarking technique can blindly localize tampered areas, is robust to JPEG compression, and is sensitive to malicious manipulations, including counterfeiting attacks.

A computed tomography (CT) imaging configuration with a straight-line trajectory is investigated, and a direct filtered backprojection (FBP) algorithm is presented. This kind of system may be useful for industrial applications and security inspections. Projections from a straight-line trajectory have a special property where data from each detector element correspond to a parallel-beam projection of a certain view angle. However, the sampling steps of parallel beams differ from view to view. Rebinning raw projections into uniformly sampled parallel-beam projections is a common choice for this type of reconstruction problem. However, the rebinning procedure suffers a loss of spatial resolution because of interpolations. Our reconstruction method is first derived from the Fourier slice theorem, where a coordinate transform and geometrical relations in projection and backprojection are used. It is then extended to 3-D scanning geometry. Finally, data-shift preprocessing is proposed to reduce computation and memory requirements by removing useless projections in raw data. In this method, the spatial resolution is better preserved and the reconstruction is less sensitive to data truncation than in the rebinning-to-parallel-beam method. To deal with limited angle problem, an iterative reconstruction reprojection method is introduced to estimate missing data and improve the image quality.

This paper presents a feasible system for image fingerprinting. Two important problems of fingerprinting are the design collusion-secure codes and the watermarking strategies for the fingerprints. We address these problems by proposing hierarchical fingerprint codes and a content adaptive watermarking scheme. It is challenging to find a practical collusion-secure code for fingerprinting because of the trade-offs between the fingerprint code's length, its collusion resiliency, and the size of the customer base. Therefore, instead of pursuing a high-performance code, we construct a new fingerprint code by concatenating existing codes using a hierarchical structure. The resulting code is efficient in decoding, much shorter than existing codes for similar parameters, and thus practical for real systems. In the fingerprinting, the codes are often very long if the applicable colluder-resisting ability and large customer bases are needed. The resulting acute degradation of fingerprinted content may be unacceptable. Hence the long-length watermark must be appropriately embedded into images to preserve the image quality. We design a strength-decision algorithm, on the basis of neural networks, to adaptively embed the long-length fingerprints with suitable magnitudes into different image regions. This adaptive watermarking technique maintains the combination of robustness and imperceptibility without the effort of dealing with visual models. In addition, we analyze the disturbance due to collusion attacks on images and propose an optimization algorithm that can select better embedding positions to resist collusion attacks and preserve acceptable transparency of the watermark according to different multimedia contents. Experimental results show the high detection correctness of the traitor tracing. It implies that our fingerprinting system, constructed by applying a hierarchical fingerprint code to the content-adaptive watermarking scheme, is effective for multimedia application.

Dynamic speckle techniques are useful tools for the study of temporal evolution. We use the difference histogram (DH) method for texture speckle image characterization. To test this proposal, two experiments were developed, concerning activity speckle images corresponding to live and dead seeds, and to drying of synthetic paints. The time history of the speckle pattern obtained from different stages of the samples was analyzed. The results are compared with those obtained from previous investigations performed with other methods. The comparison reveals that the proposed DH method is appropriate for this task.

We introduce fast image reconstruction algorithms for emission tomography, which provide not only edge-preserved reconstructions, but also their edge maps simultaneously. To explicitly model the existence of edges, we use the binary line-process model, which is incorporated as a Gibbs prior in the context of a Bayesian maximum a posteriori framework. To efficiently handle the problem of mixed continuous and binary variable objectives, we use a deterministic annealing (DA) algorithm. Since the DA algorithm is computer-intensive and requires many iterations to converge, we apply a block-iterative method derived from the well-known ordered-subset principle. The block-iterative DA algorithm processes the data in blocks within each iteration, thereby accelerating the convergence speed of the standard DA algorithm by a factor proportional to the number of blocks. Our experimental results indicate that, with moderate numbers of blocks and properly chosen hyperparameters, the accelerated DA algorithm provides good reconstructions as well as edge maps with only a few iterations.

In order to further improve edge location accuracies of charge-coupled-device laser autocollimators, we use rotation invariance and lower radial orders of orthogonal Fourier-Mellin moments to efficiently extract the characteristics of edges, set the edges in the vertical direction, and establish the expressions used for edge location. We accomplish this by analyzing the interrelationship of orthogonal Fourier-Mellin moments. We compensate errors caused by the differences between the masks selected (by correcting the locations in proportion to the dimensions of the masks selected) and by the differences between actual and ideal edge patterns (by correcting the bias errors in proportion to the distance from the center of the sampling area to the edge). Experimental results show that an edge location accuracy of 0.06 pixel can be achieved for straight lines, 0.08 pixel for curves, and ±0.12 pixel for an actual circular target acquired by a charge-coupled-device laser autocollimator. It is therefore concluded that the proposed method can be used as an efficient approach to achieve the higher edge location accuracies specified for such optoelectronic image measurements instruments as a charge-coupled-device laser autocollimator.

A feature-based, nonrigid image registration method using a Hausdorff distance-based matching measure is presented. The proposed method is robust to outliers and missing features, as no correspondence is established between the features. Utilizing a B-spline-based, nonrigid deformation model, the proposed method is able to handle nonrigid deformations between the images to be registered. A gradient descent-based optimization procedure is developed to maximize the matching measure under the nonrigid transformation model. With adaptively adjustable step sizes, the optimization procedure works in a coarse-to-fine manner so that first large nonrigid deformations are compensated and then the transformation parameters are gradually refined. In addition, two acceleration techniques are devised to greatly speed up the registration method, making it more practical for real applications. The performance of the proposed method is validated in various experiments from synthetic image registration to hand-drawn Chinese character registration and brain outline registration. The limitation of the method is also analyzed and exemplified. To partly alleviate the limitation of the method, we incorporate landmark information into the method and achieve promising results.

Dynamic noise elimination in 2-D periodic structures is achieved with a liquid crystal display used as a 2-D incoherent reconfigurable source. The method, which is based on the Lau effect, shows that under certain conditions it can be used for correcting, in real time, defects dues to noise in 2-D periodic structures. Experimental verification of the noise elimination is presented.