Showing papers in "Optical Engineering in 2006"

Novel method for structured light system calibration

Abstract: System calibration, which usually involves complicated and time-consuming procedures, is crucial for any 3-D shape measurement system. In this work, a novel systematic method is proposed for accurate and quick calibration of a 3-D shape measurement system we developed based on a structured light technique. The key concept is to enable the projector to "capture" images like a camera, thus making the calibration of a projector the same as that of a camera. With this new concept, the calibration of structured light systems becomes essentially the same as the calibration of traditional stereovision systems, which is well estab- lished. The calibration method is fast, robust, and accurate. It signifi- cantly simplifies the calibration and recalibration procedures of struc- tured light systems. This work describes the principle of the proposed method and presents some experimental results that demonstrate its performance. © 2006 Society of Photo-Optical Instrumentation Engineers.

High-resolution, real-time three-dimensional shape measurement

Abstract: We describe a high-resolution, real-time 3-D shape measurement system based on a digital fringe projection and phase-shifting technique. It utilizes a single-chip digital light processing projector to project computer-generated fringe patterns onto the object, and a high-speed CCD camera synchronized with the projector to acquire the fringe images at a frame rate of 120 frames/s. A color CCD camera is also used to capture images for texture mapping. Based on a three-step phase-shifting technique, each frame of the 3-D shape is reconstructed using three consecutive fringe images. Therefore the 3-D data acquisition speed of the system is 40 frames/s. With this system, together with the fast three-step phase-shifting algorithm and parallel processing software we developed, high-resolution, real-time 3-D shape measurement is realized at a frame rate of up to 40 frames/s and a resolution of 532×500 points per frame.

Wavelet-based pavement distress detection and evaluation

Abstract: An automated pavement inspection system consists of image acquisition and distress image processing. The former is accomplished with imaging sensors, such as video cameras and photomultiplier tubes. The latter includes distress detection, isolation, classification, evaluation, segmentation, and compression. We focus on wavelet-based distress detection, isolation, and evaluation. After a pavement image is decomposed into different-frequency subbands by the wavelet transform, distresses are transformed into high-amplitude wavelet coefficients and noise is transformed into low-amplitude wavelet coefficients, both in the high-frequency subbands, referred to as details. Background is transformed into wavelet coefficients in a low-frequency subband, referred to as approximation. First, several statistical criteria are developed for distress detection and isolation, which include the high-amplitude wavelet coefficient percentage (HAWCP), the high-frequency energy percentage (HFEP), and the standard deviation (STD). These criteria are tested on hundreds of pavement images differing by type, severity, and extent of distress. Experimental results demonstrate that the proposed criteria are reliable for distress detection and isolation and that real-time distress detection and screening is currently feasible. A norm for pavement distress quantification, which is defined as the product of HAWCP and HFEP, is also proposed. Experimental results show that the norm is a useful index for pavement distress evaluation.

Long-range three-dimensional imaging using range-gated laser radar images

Abstract: An approach to long-range 3-D imaging using laser illuminated range-gated viewing is presented. The basis for 3-D scene reconstruction is an image sequence acquired using a sliding gate delay time. Two different methods are suggested, and algorithm performance is investigated through Monte Carlo simulations using a simplified system and imaging model. Assumptions are justified by comparison with real measurements at range of 0.8 to 7.2 km. It is shown that range resolution and precision become significantly better than system design parameters such as gate length, gate transition length, and gate step length. The presented reconstruction methods thus enable high-precision range imaging using available long-range gated imaging systems.

Robust image watermarking using local invariant features

Abstract: This paper addresses a novel robust watermarking method for digital images using local invariant features. Most previous water- marking algorithms are unable to resist geometric distortions that desyn- chronize the location where copyright information is inserted. We pro- pose a watermarking method that is robust to geometric distortions. In order to resist geometric distortions, we use a local invariant feature of the image called the scale-invariant feature transform SIFT, which is invariant to translation and scaling distortions. The watermark is inserted into the circular patches generated by the SIFT. Rotation invariance is achieved using the translation property of the polar-mapped circular patches. Our method belongs to the blind watermark, because we do not need the original image during detection. We have performed an inten- sive simulation to show the robustness of the proposed method. The simulation results support the contention that our method is robust against geometric distortion attacks as well as signal-processing attacks. We have compared our results with those of other methods, and our method outperforms them. © 2006 Society of Photo-Optical Instrumentation

Lens distortion correction for digital image correlation by measuring rigid body displacement

Abstract: A method of lens distortion correction is proposed in order to improve the measurement accuracy of digital image correlation for two-dimensional displacement measurement. The amounts of lens distortion are evaluated from displacement distributions obtained in a rigid body in-plane translation or rotation test. After detecting the lens distortion, its coefficient is determined using the method of least squares. Then, the corrected displacement distributions are obtained. The effectiveness of the proposed method is demonstrated by applying the correction method to an in-plane translation test and tension tests. The experimental results show that the proposed distortion correction method eliminates the effect of lens distortion from measured displacements.

Human-tracking systems using pyroelectric infrared detectors

Abstract: We design and develop a low-cost pyroelectric detector- based IR motion-tracking system. We study the characteristics of the detector and the Fresnel lenses that are used to modulate the visibility of the detectors. We build sensor clusters in different configurations and demonstrate their use for human motion tracking. © 2006 Society of Photo-

Color phase-shifting technique for three-dimensional shape measurement

Abstract: A color phase-shifting technique has been recently developed for high-speed 3-D shape measurement. In this technique, three sinusoidal phase-shifted images used for a measurement cycle in a traditional grayscale phase-shifting technique are encoded into one color image. Therefore, only a single color image is needed for reconstructing the 3-D surface shape of an object. The measurement speed can then be increased up to the frame rate of the camera. However, previous experimental results showed that the measurement accuracy of this technique was initially low, due largely to the coupling and imbalance of color channels. In this paper, two solutions, one software-based and one hardware-based, are proposed to compensate for these errors. Experimental results show that the second solution—modification of the camera together with an imbalance compensation algorithm—would effectively reduce the errors and produce better measurement results than the software-based compensation method. This technique has many potential applications in high-speed measurement, such as highway inspection and dynamic measurement of human body.

Strehl ratio and scintillation theory for uplink Gaussian-beam waves: beam wander effects

Abstract: First-order weak-fluctuation Rytov theory predicts that the longitudinal (on-axis) component of the scintillation index of an uplink collimated beam will become significantly smaller as the size of the transmitter aperture increases up to around 100 cm. However, the results of recent computer simulations are at odds with this behavior, and we believe that this discrepancy is due to the fact that the conventional Rytov theory does not correctly account for the effects of beam wander on the scintillation index. We present a theoretical structure that accurately describes far-field irradiance fluctuations caused by uncorrected beam wander. This new theory is validated by demonstrating excellent agreement between the predicted scintillation index and computer code results for both tracked and untracked beams. For many applications of practical interest, such as free-space optical communications, a good understanding of the time-average Strehl ratio is also essential simulation results for this parameter are presented and shown to be in good agreement with the theory.

Strain transferring analysis of fiber Bragg grating sensors

Abstract: We develop an analytical model for the relationship between the strain measured by a fiber Bragg grating sensor and the actual structural strain. The values of the average strain transfer rates calculated from the analytical model agree well with available experiment data. Based on the analytical model, the critical adherence length of an optical fiber sensor can be calculated and is determined by a strain lag parameter, which contains both the effects of the geometry and the relative stiffness of the structural components. The analysis shows that the critical adherence length of a fiber sensing segment is the minimum length with which the fiber must be tightly bonded to a structure for adequate sensing. The strain transfer rate of an optical fiber sensor embedded in a multilayered structure is developed in a similar way, and the factors that influence the efficiency of optical fiber sensor strain transferring are discussed. It is concluded that the strain sensed by a fiber Bragg grating must be magnified by a factor (strain transfer rate) to be equal to the actual structural strain. This is of interest for the application of fiber Bragg grating sensors.

Displacement/strain measurements using an optical microscope and digital image correlation

Abstract: We conduct displacement/strain measurements on the micro- scale using light microscopy and digital image correlation DIC. Errors in the measurements attributed to the optical arrangement and aberration induced at high magnification are identified using a warping function. Coefficients of the warping function are determined using a simple tech- nique that employs a precisely made orthogonal cross-grating plate. By acquiring images of the grating and identifying the nodes using subpixel techniques, a relationship between the object and the image planes is established. Thus, the displacement/strain derived by means of DIC is corrected by converting the displacement components in the image plane to the coordinate system existing on the object's surface. The approach is validated through a determination of the elastic properties of common metals; errors in estimation of the elastic modulus were within 4%. Although surface preparation generally plays a critical role in suc- cessful application of DIC, it is found to be of minimal importance under high magnification. Instead, the natural surface texture can be used with adjustment of the light incident angle. Results of the study show that DIC is a powerful tool in performing displacement/strain measurements on the microscale using a light microscope provided that an adequate cor- rection is employed for image distortion. © 2006 Society of Photo-Optical Instru-

Encryption quality analysis of the RC5 block cipher algorithm for digital images

Abstract: We investigate the implementation and application of the RC5 block cipher algorithm for digital images and provide testing, verification, and encryption efficiency of the RC5 block cipher for digital images. We describe briefly the basic design parameters of the RC5 block cipher and its implementation for digital images. A complete specification for the method of application of the RC5 block cipher to digital images is given. Several test images are used for inspecting the validity of the encryption and decryption algorithms. Also, we provide and introduce a mathemati- cal measure for encryption efficiency, which we will call the encryption quality instead of visual inspection, and apply it to several images. The encryption quality of the RC5 block cipher algorithm is investigated along its several design parameters, such as word size, number of rounds, and secret key length, and the optimal choices for the best values of these design parameters are given. © 2006 Society of Photo-Optical Instrumentation

Novel method for patterned fabric inspection using Bollinger bands

Abstract: This paper introduces a new application of Bollinger bands for defect detection of patterned fabric A literature review on previous de- signed methods for patterned fabric defect detection will be depicted For data analysis, Bollinger bands are calculated based on standard devia- tion and are originally used in the financial market as an oversold or overbought indicator for stock The Bollinger bands method is an effi- cient, fast and shift-invariant approach, that can segment out the defec- tive regions on the patterned fabric with clear and crystal clean images The new approach is immune of the alignment problem that often hap- pens in previous methods In this paper, the upper band and lower band of Bollinger bands, which are sensitive to any subtle change in the input data, have been developed for use to indicate the defective areas in patterned fabric The number of standard deviation and length of time of Bollinger bands can be easily determined to obtain excellent detection results The proposed method has been evaluated on three different patterned fabrics In total, 165 defect-free and 171 defective images have been used in the evaluation, where 9859% accuracy on inspection has been achieved © 2006 Society of Photo-Optical Instrumentation

Advanced continuous wavelet transform algorithm for digital interferogram analysis and processing

Abstract: An advanced continuous wavelet transform algorithm for digital interferogram analysis and processing is proposed. The algorithm is an extension of the traditional wavelet transform; the mother wavelet and normalization parameter are selected based on the characteristics of optical interferograms. To reduce the processing time, a fast Fourier transform scheme is employed to implement the wavelet transform calculation. The algorithm is simple and is a robust tool for interferogram filtering and for whole-field fringe and phase information detection. The concept is verified by computer simulation and actual experimental interferogram analysis.

Modeling of phase volume diffractive gratings, part 1: transmitting sinusoidal uniform gratings

Abstract: A detailed model of the diffraction of plane and Gaussian beams on plane uniform phase Bragg gratings based on Kogelnik's theory of coupled waves is presented. The model describes transmitting gratings with arbitrary orientation in a plane-parallel plate taking into ac- count spectral width and angular divergence of laser beams along with material dispersion of a photosensitive medium. The model results are compared with experimental data for high-efficiency Bragg gratings in a photothermorefractive PTR glass. © 2006 Society of Photo-Optical Instrumenta- why our modeling is illustrated for those variations of grat- ing parameters that are typical for PTR VBGs: the refrac- tive index is in the range of 1.5 at wavelengths ranging from 0.4 to 2.7 m, refractive index modulation is up to 1000 ppm 10 3 , and grating thickness is from 0.2 to 20 mm. The goal of this work is to reduce Kogelnik's theory to practical formulas that enable practical modeling and de- sign of diffractive optical elements based on VBGs. This part of modeling considers diffraction of plane monochro- matic, divergent, and polychromatic laser beams on uni- form sinusoidal lossless transmitting volume gratings and compares the model with experimental results in PTR Bragg gratings. Further parts will describe modeling of re- flecting volume gratings holographic mirrors as well as the application of both transmitting and reflecting grating for spectral beam combining for different types of lasers.

Quantitative analysis of pansharpened images

Abstract: Pansharpening is a pixel-level fusion technique used to increase the spatial resolution of the multispectral image using spatial information from the high-resolution panchromatic image, while preserving the spectral information in the multispectral image. Various pansharpening algorithms are available in the literature, and some have been incorporated in commercial remote sensing software packages such as ERDAS Imagine® and ENVI®. The demand for high spatial and spectral resolutions imagery in applications like change analysis, environmental monitoring, cartography, and geology is increasing rapidly. Pansharpening is used extensively to generate images with high spatial and spectral resolution. The suitability of these images for various applications depends on the spectral and spatial quality of the pansharpened images. Hence, the evaluation of the spectral and spatial quality of the pansharpened images using objective quality metrics is a necessity. In this work, quantitative metrics for evaluating the quality of pansharpened images are presented. A performance comparison, using the intensity-hue-saturation (IHS)-based sharpening, Brovey sharpening, principal component analysis (PCA)-based sharpening, and a wavelet-based sharpening method, is made to better quantify their accuracies.

Commutative watermarking and encryption for media data

Abstract: A commutative watermarking and encryption scheme is proposed for media data protection. In the scheme, the partial encryption algorithm is adopted to encrypt the significant part of media data, while some other part is watermarked. The commutative property brings conveniences to practical applications in secure media transmission or distribution.

Fabrication of diffractive optics by use of slow tool servo diamond turning process

Abstract: In recent years, it has become possible to fabricate complicated optical surfaces using multi-axis ultraprecision machines. Two diffractive optical designs were fabricated using an ultraprecision diamond turning machine equipped with four independent axes. Unlike the conventional clean-room-based micromachining process, this research demonstrates the development of two innovative diamond tool trajectories that allow the entire diffractive pattern to be machined in a single operation directly, without going through multiple steps, as commonly used in conventional lithography processes. The machined diffractive optical elements were measured for curve geometry and surface roughness. In addition, the optical performance was also evaluated. Finally, a simple welding test setup was utilized to test the 256-level diffractive optical elements (DOEs). Compared to conventional approaches where feature indexing is difficult and unreliable, the slow tool servo (STS) process can be utilized to produce DOEs with accurate geometry and optical surface finish; therefore, the process may be called non-clean-room or maskless micromachining. Unlike its predecessors, this micromachining process which is based on ultraprecision diamond machining can be used to produce true three-dimensional (3D) features in a single operation, thus making it a promising technology for micro-optical, electromechanical component fabrication. Moreover, the 3D micro features can be readily applied to a freeform substrate, making this process a unique approach for fabrication of complicated micro-optical devices.

Observation of the experimental propagation properties of Helmholtz-Gauss beams

Abstract: We present the experimental generation and characterization of each one of the four fundamental families of Helmholtz-Gauss beams: cosine-Gauss beams, stationary and helical Mathieu-Gauss beams, sta- tionary and traveling parabolic-Gauss beams, and Bessel-Gauss beams. Both the transverse intensity profile and power spectrum that each one of the beams exhibits upon propagation is observed and compared to the theoretical model with good quantitative agreement. Emphasis is made on the fact that each of the four families of HzG beams is complete and orthogonal, and thus of fundamental relevance. © 2006 Society of Photo-

Use of one-dimensional iris signatures to rank iris pattern similarities

Abstract: A one-dimensional approach to iris recognition is presented. It is translation-, rotation-, illumination-, and scale-invariant. Traditional iris recognition systems typically use a two-dimensional iris signature that requires circular rotation for pattern matching. The new approach uses the Du measure as a matching mechanism, and generates a set of the most probable matches (ranks) instead of only the best match. Since the method generates one-dimensional signatures that are rotation-invariant, the system could work with eyes that are tilted. Moreover, the system will work with less of the iris than commercial systems, and thus could enable partial-iris recognition. In addition, this system is more tolerant of noise. Finally, this method is simple to implement, and its computational complexity is relatively low.

Automated three-axis gonioreflectometer for computer graphics applications

Abstract: We describe an automated three-axis gonioreflectometer, which can help increase the physical realism of computer graphics renderings by providing light scattering data for the surfaces in a scene. The gonioreflectometer performs rapid measurements of the bidirectional reflectance distribution function (BRDF) for flat, isotropic, sample surfaces over the complete visible spectrum and over most of the incident and reflection hemispheres. The instrument employs a broadband light source and a detector with a diffraction grating and linear diode array. Validation is achieved by comparisons against reference surfaces and other instruments. The accuracy and spectral and angular ranges of the BRDFs are appropriate for computer graphics imagery, while reciprocity and energy conservation are preserved. Measured BRDFs on rough aluminum, metallic silver paint, and a glossy yellow paint are reported, and an example rendered automotive image is included.

Feature-preserving regularization method for complex-valued inverse problems with application to coherent imaging

Abstract: We propose a method for feature-preserving regularized reconstruction in coherent imaging systems. In our framework, image formation from measured data is achieved through the minimization of a cost functional, designed to suppress noise artifacts while preserving features such as object boundaries in the reconstruction. The cost functional includes nonquadratic regularizing constraints. Our formulation effectively deals with the complex-valued and potentially random-phase nature of the scattered field, which is inherent in many coherent systems. We solve the challenging optimization problems posed in our framework by developing and using an extension of half-quadratic regularization methods. We present experimental results from three coherent imaging applications: digital holography, synthetic aperture radar, and ultrasound imaging. The proposed technique produces images where coherent speckle artifacts are effectively suppressed, and important features of the underlying scenes are preserved.

Penalized-likelihood image reconstruction for x-ray fluorescence computed tomography

Abstract: X-ray fluorescence computed tomography (XFCT) allows for the reconstruction of the distribution of elements within a sample from measurements of fluorescence x rays produced by irradiation of the sample with monochromatic synchrotron radiation. XFCT is not a transmission tomography modality, but rather a stimulated emission tomography modality; thus correction for attenuation of the incident and fluorescence photons is essential if accurate images are to be obtained. This is challenging because the attenuation map is, in general, known only at the stimulating beam energy and not at the various fluorescence energies of interest. We make use of empirically fitted analytic expressions for x-ray attenuation coefficients to express the unknown attenuation maps as linear combinations of known quantities and the unknown elemental concentrations of interest. We then develop an iterative image reconstruction algorithm based on penalized-likelihood methods that have been developed for medical emission tomography. Studies with numerical phantoms indicate that the approach is able to produce qualitatively and quantitatively accurate reconstructed images even in the face of severe attenuation. We also apply the method to real synchrotron-acquired data and demonstrate a marked improvement in image quality relative to filtered backprojection reconstruction.

Modeling of the temperature-dependent spectral response of In 1–χ Ga χ Sb infrared photodetectors

Abstract: A model of the spectral responsivity of In(1-x) Ga(x) Sb p-n junction infrared photodetectors has been developed. This model is based on calculations of the photogenerated and diffusion currents in the device. Expressions for the carrier mobilities, absorption coefficient and normal-incidence reflectivity as a function of temperature were derived from extensions made to Adachi and Caughey-Thomas models. Contributions from the Auger recombination mechanism, which increase with a rise in temperature, have also been considered. The responsivity was evaluated for different doping levels, diffusion depths, operating temperatures, and photon energies. Parameters calculated from the model were compared with available experimental data, and good agreement was obtained. These theoretical calculations help to better understand the electro-optical behavior of In(1-x) Ga(x) Sb photodetectors, and can be utilized for performance enhancement through optimization of the device structure.

Objective video quality assessment

Abstract: We propose a new method for an objective measurement of video quality. By analyzing subjective scores of various video sequences, we find that the human visual system is particularly sensitive to degradation around edges. In other words, when edge areas of a video sequence are degraded, evaluators tend to give low quality scores to the video, even though the overall mean squared error is not large. Based on this observation, we propose an objective video quality measurement method based on degradation around edges. In the proposed method, we first apply an edge detection algorithm to videos and locate edge areas. Then, we measure degradation of those edge areas by computing mean squared errors and use it as a video quality metric after some postprocessing. Experiments show that the proposed method significantly outperforms the conventional peak signal-to-noise ratio (PSNR). This method was also independently evaluated by independent laboratory groups in the Video Quality Experts Group (VQEG) Phase 2 test. The method consistently provided good performances. As a result, the method was included in international recommendations for objective video quality measurement.

Reconfigurable image projection holograms

Abstract: We introduce reconfigurable image projection RIP holo- grams and a method for computing RIP holograms of three-dimensional 3-D scenes. RIP holograms project one or more series of parallax views of a 3-D scene through one or more holographically reconstructed projection surfaces. Projection surfaces are defined at locations at which the hologram reconstructs a variable number of real or virtual images, called holographic primitives, which collectively compose the surface and constitute exit pupils for the view pixel information. RIP holograms are efficiently assembled by combining a sweep of 2-D parallax views of a scene with instances of one or more precomputed diffractive elements, which are permitted to overlap on the hologram, and which reconstruct the holographic primitives. The technique improves on the image quality of conventional stereograms while affording similar efficient computation: it incorporates realistic computer graphic rendering or high-quality optical capture of a scene, it eliminates some artifacts often present in conven- tional computed stereograms, and its basic multiply-and-accumulate op- erations are suitable for hardware implementation. The RIP approach offers flexible tuning of capture and projection together, according to the sampling requirements of the scene and the constraints of a given dis- play architecture. © 2006 Society of Photo-Optical Instrumentation Engineers.

Waveguide-type optical passive ring resonator gyro using phase modulation spectroscopy technique

Abstract: We report the first demonstration of silica wave- guide optical passive ring resonator gyro OPRG based on the phase modulation spectroscopy technique. The ring resonator is composed of a 6-cm-long silica waveguide. Ob- served from the resonance curve, the free spectral range FSR of the resonator, the full width at half maximum FWHM of the resonance curve, the finesse F of the reso- nator, and the resonance depth are 3.4 GHz, 62 MHz, 54.8, and 70%, respectively. The detection sensitivity of this OPRG will be 7.310 5 rad/s. In the experiments, there is an acoustic-optical modulator AOM in each light loop. We lock the lasing frequency at the resonance frequency of the silica waveguide ring resonator for counterclockwise CCW lightwave; the frequency difference between the driving fre- quencies of the two AOMs is equivalent to the Sagnac fre- quency difference caused by gyro rotation. Thus, the gyro output is observed. © 2006 Society of Photo-Optical Instrumentation Engineers.

Quantum efficiency overestimation and deterministic cross talk resulting from interpixel capacitance

Abstract: Pixels in both hybridized and monolithic complementary metal-oxide semiconductor CMOS detector arrays may couple capaci- tively to their neighboring pixels. This "interpixel capacitance" can signifi- cantly distort the characterization of conversion efficiency and modula- tion transfer function MTF in CMOS devices. These effects have been largely unaccounted for in measurements to date. In this work, the ef- fects of this coupling are investigated. Compensation methods for these errors are described and applied to silicon P-I-N array measurements. The measurement of Poisson noise, traditionally done by finding the mean square difference in a pair of images, needs to be modified to include the mean square correlation of differences with neighboring pixels. © 2006 Society of Photo-Optical Instrumentation Engineers.

Gradient-doping negative electron affinity GaAs photocathodes

Abstract: A variety of gradient-doping reflection-mode GaAs photocathode materials are designed and are prepared into negative electron affinity (NEA) GaAs photocathodes by (Cs,O) activation technique for the first time. These gradient-doping photocathodes are grown by molecular beam epitaxy (MBE), in which from GaAs bulk to surface doping concentration is distributed gradiently from high to low. The activation experimental results show that for gradient-doping GaAs photocathodes that are grown over p-type GaAs (100) wafer, where the epitaxial layer doping concentration range is 1019 to 1018 cm–3 and the epitaxial layer thickness is 1 µm, can achieve high integral sensitivity. The highest integral sensitivity 1798 µA/lm is achieved for gradient-doping GaAs photocathodes, which is much higher than that of a common uniform-doping GaAs photocathode under identical cleaning and activation condition. The inherent mechanism responsible for the fact that a gradient-doping GaAs photocathode can obtain higher quantum efficiency is also discussed.