Showing papers in "Optical Engineering in 2008"

Corner detector based on global and local curvature properties

Abstract: This paper proposes a curvature-based corner detector that detects both fine and coarse features accurately at low computational cost. First, it extracts contours from a Canny edge map. Second, it com- putes the absolute value of curvature of each point on a contour at a low scale and regards local maxima of absolute curvature as initial corner candidates. Third, it uses an adaptive curvature threshold to remove round corners from the initial list. Finally, false corners due to quantiza- tion noise and trivial details are eliminated by evaluating the angles of corner candidates in a dynamic region of support. The proposed detector was compared with popular corner detectors on planar curves and gray- level images, respectively, in a subjective manner as well as with a fea- ture correspondence test. Results reveal that the proposed detector per- forms extremely well in both fields. © 2008 Society of Photo-Optical

Accurate calibration method for a structured light system

Abstract: System calibration is crucial for any 3-D shape measurement system An accurate method is proposed to calibrate a 3-D shape measurement system based on a structured light technique The projector is treated as a camera to unify the calibration procedures of a structured light system and a well-established stereo vision system The key to realizing this method is to establish a highly accurate correspondence between camera pixels and projector pixels and generate digital micromirror device (DMD) image sets for projector calibration A phase-shifting method is used to accomplish this task A precalibrated lookup table and a linear interpolation algorithm are proposed to improve the accuracy of the generated DMD image, and accordingly improve the accuracy of the projector calibration and the 3-D measurement Some experimental results are presented to demonstrate the performance of the system calibration

Free-space optical system performance for laser beam propagation through non-Kolmogorov turbulence

Abstract: It is well know that free-space laser system performance is limited by atmospheric turbulence. Most theoretical treatments have been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately, several experiments have been reported recently that show that the Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular, in portions of the troposphere and stratosphere. We present a non-Kolmogorov power spectrum that uses a generalized exponent instead of constant standard exponent value 11/3, and a generalized amplitude factor instead of constant value 0.033. Using this new spectrum in weak turbulence, we carry out, for a horizontal path, an analysis of long-term beam spread, scintillation index, probability of fade, mean signal-to-noise ratio (SNR), and mean bit error rate (BER) as variation of the spectrum exponent. Our theoretical results show that for alpha values lower than =11/3, but not for alpha close to =3, there is a remarkable increase of scintillation and consequently a major penalty on the system performance. However, when alpha assumes a value close to =3 or for alpha values higher than =11/3, scintillation decreases, leading to an improvement on the system performance.

Detection of foreign bodies in chocolate with pulsed terahertz spectroscopy

Abstract: We demonstrate the detection of metallic and nonmetallic foreign bodies in chocolate using pulsed terahertz imaging. Investigating the shape of the temporal waveform allows for the discrimination between wanted ingredients like nuts on one hand and foreign bodies like stone, glass, or plastic particles on the other hand. Yet, the intensity image alone does not provide enough information to evaluate the quality of the chocolate bar. To achieve a low false-alarm rate it is important to measure the height profile of the sample and to include the measured results in the image-processing step. Our results show that terahertz imaging can be used for the detection of contaminations in chocolate bars. Furthermore, other kinds of dry food can be investigated with our technique.

Improved covariance matrices for point target detection in hyperspectral data

Abstract: Our goals in hyperspectral point target detection have been to develop a methodology for algorithm comparison and to advance point target detection algorithms through the fundamental understanding of spatial and spectral statistics. In this paper, we review our methodology as well as present new metrics. We demonstrate improved performance by making better estimates of the covariance matrix. We have found that the use of covariance matrices of statistical stationary segments in the matched-filter algorithm improves the receiver operating characteristic curves; proper segment selection for each pixel should be based on its neighboring pixels. We develop a new type of local covariance matrix, which can be implemented in principal-component space and which also shows improved performance based on our metrics. Finally, methods of fusing the segmentation approach with the local covariance matrix dramatically improve performance at low false-alarm rates while maintaining performance at higher false-alarm rates.

Polarization imaging for material classification

Abstract: A simple and stable method is proposed for distinguishing dielectric and metal material surfaces from the polarization images captured by a vision system consisting of a linear polarizer and a digital camera. The polarization state is determined by the transmitted light intensity through the polarizer as a function of polarizing orientation. The degree of polarization (DOP) is estimated from the image intensities through the polarizer. The DOP map is quite effective for material classification around specular highlight on an object surface. We prove that the DOP map is convex for a dielectric surface and concave for a metal surface. The problem of material classification is then reduced to a simple judgment of the convexity of the DOP map obtained around the highlight peak. The proposed method is not a pixelwise local method based on thresholding the Fresnel ratio computed at each pixel but an area-based method based on the DOP map in a highlight area. The feasibility of the method is confirmed in experiments under a variety of conditions.

Fading-loss assessment in atmospheric free-space optical communication links with on-off keying

Abstract: Link-budget calculations are a common way to assess system parameters, such as the required transmitter power and receiver sensitivity, in free-space optical (FSO) communication systems. One of the biggest challenges for long-range FSO deployment is its signal propagation under turbulent atmospheric conditions, which produce intensity fluctuations. Methods to estimate atmospheric-fading loss in radio-frequency systems cannot be adapted to the FSO channel. Until now no general closed-form methods have been developed to describe the fading loss in such a channel. A method to calculate the losses due to scintillation fading in the threshold approach, based on lognormal statistics of the received power, is presented.

Dynamic-range compression and contrast enhancement in infrared imaging systems

Abstract: Third-generation thermal cameras have high dynamic range (up to 14 bits) and collect images that are difficult to visualize because their contrast exceeds the range of traditional display devices. Thus, sophisticated techniques are required to adapt the recorded signal to the display, maintaining, and possibly improving, objects' visibility and image contrast. The problem has already been studied in relation to images acquired in the visible spectral region, while it has been scarcely investigated in the infrared. In this work, this latter subject is addressed, and a new method is presented that combines dynamic-range compression and contrast enhancement techniques to improve the visualization of infrared images. The proposed method is designed to meet typical requirements in infrared sensor applications. The performance is studied through experimental data and compared with that yielded by three well-established algorithms. Evaluation is performed through subjective analysis, assigning each algorithm a score on the basis of the average opinion of human observers. The results demonstrate the effectiveness of the proposed technique in terms of perceptibility of details, edge sharpness, robustness against the horizon effect, and presence of very warm objects.

Propagation of partially coherent vortex beams in a turbulent atmosphere

Abstract: A partially coherent vortex beam propagating in a turbulent atmosphere is investigated by using extended Huygens-Fresnel diffrac- tion integral. The influence of the turbulence, the spatial coherence, and the topological charges of the incident beam on the propagation charac- teristics of the beam is studied in great detail. It is shown that the spread- ing of the partially coherent vortex beam is less influenced by atmo- spheric turbulence than that of a partially coherent nonvortex beam, and that a partially coherent vortex beam of lower coherence is less influ- enced than one of higher coherence. © 2008 Society of Photo-Optical Instrumen-

Real-time vision-based defect inspection for high-speed steel products

Abstract: In the steel-making industry, both the quality and quantity of the products are critical. This work presents a real-time defect detection method for high-speed steel bar in coil (BIC). For good performance characteristics, the detection algorithm must be robust to problems associated with the cylindrical shape of BICs, the presence of noise and nonuniform brightness distribution of images, the various types of defects, and so on. Furthermore, because the target speed is very high, it should have a fast processing time. Therefore, a defect detection algorithm should satisfy the two conflicting requirements of reducing the processing time and improving the efficiency of defect detection. This work proposes an effective real-time defect detection algorithm that can satisfy these conditions. Moreover, to reduce cost, the proposed algorithm is implemented on a PC-based real-time defect detection system without a professional digital signal processing (DSP) board. Experimental results show that the proposed algorithm guarantees both real-time processing and accurate detection.

Selecting a composite correlation filter design: a survey and comparative study

Abstract: Many composite correlation filter designs have been proposed for solving a wide variety of target detection and pattern recognition problems. Due to the large number of available designs, however, it is often unclear how to select the best design for a particular application. We present a theoretical survey and an empirical comparison of several popular composite correlation filter designs. Using a database of rotational target imagery, we show that some such filter designs appear to be better choices than others under computational and performance constraints. We compare filter performance in terms of noise tolerance, computational load, generalization ability, and distortion in order to provide a multifaceted examination of the characteristics of various filter designs.

Three-dimensional shape measurement using a structured light system with dual cameras

Abstract: A structured light system for three-dimensional shape measurement with single camera has the shortcoming of camera occlusion. To alleviate this problem, this paper introduces a structured light system with dual cameras for three-dimensional shape measurement. We discuss (1) system description, (2) system calibration, (3) three-dimensional data registration using the iterative closest-point (ICP) algorithm, and (4) three-dimensional data merging using holoimage. The principle of the system is introduced, and experiments are presented to verify its performance.

Flexible method for structured light system calibration

Abstract: We propose a flexible technique to easily calibrate a structured light system, in which only two patterns are required: a paper checkerboard printed on a laser printer and attached to a surface plane, and a pattern image of a checkerboard, which is projected on the surface plane by a LCD projector. We treat the projector as a second camera; then the problem of calibrating the structure light system becomes equivalent to calibrating a stereovision system. The key to our technique is to make the projector able to see the calibration checkerboard like a camera. For that purpose, two approaches are proposed, called the black-and-white checkerboard method and the red-and-blue checkerboard method. Both computer simulation and real data are used to test the proposed technique, and very good results are obtained. (C) 2008 Society of Photo-Optical Instrumentation Engineers.

Optical fiber refractometer based on a Fabry-Pérot interferometer

Abstract: A fiber optic sensor for liquid refractive index measurement based on a Fabry-Perot interferometer is described. The interferometer is achieved between the reflection of a short fiber Bragg grating and the Fresnel reflection from the cleaved fiber end. This fiber end is then in contact with the liquid sample to provide refractive index measurements. The sensor is characterized by immersing the fiber tip in distilled water with different concentrations of ethylene glycol. A linear relation of the interferometer fringe visibility with refractive index variation is observed, and a resolution of ~10−3 is obtained. It is also shown that the sensor operation is independent of temperature effects, other than the one related to temperature-induced change of the liquid refractive index.

Thermal, structural, and optical properties of Cleartran ® multispectral zinc sulfide

Abstract: Thermal, mechanical, and optical properties of Cleartran® multispectral zinc sulfide were measured. Heat capacity, thermal conductivity, and thermal expansion were measured over the temperature range 213 to 643 K. Young's modulus and shear modulus were measured at 289 to 473 K. The characteristic four-point flexure strength for a 1-cm2 area was 75.4±0.5 MPa at 289 K and 89.5±0.4 MPa at 473 K, with Weibull moduli of 11.0±0.7 and 19.3±1.5, respectively. All 24 specimens tested at 289 K conformed to the Weibull curve. However, 2 of 23 specimens tested at 473 K had low, outlying strengths. The fracture origins of the outliers were surface flaws that were not detected by microscopic examination prior to mechanical testing. The only reliable means that we are aware of to identify weak material is a mechanical proof test. Infrared transmittance and reflectance at 297, 373, and 473 K were measured with an integrating sphere. The transmittance of s- and p-polarized infrared light was measured at angles of incidence of 0 to 60 deg. By fitting the transmittance with the Fresnel equations, the absorption coefficient was computed for wavelengths of 8 to 14 µm. Bidirectional transmittance and reflectance distribution functions (optical scatter) were measured at 3.39 and 10.6 µm.

Binary Pseudo-random Grating Standard for Calibration of Surface Profilometers

Abstract: We suggest and describe the use of a binary pseudo-random (BPR) grating as a standard test surface for measurement of the modulation transfer function (MTF) of interferometric microscopes. Knowledge of the MTF of a microscope is absolutely necessary to convert the measured height distribution of a surface undergoing metrology into an accurate power spectral density (PSD) distribution. For an'ideal' microscope with an MTF function independent of spatial frequency out to the Nyquist frequency of the detector array with zero response at higher spatial frequencies, a BPR grating would produce a flat 1D PSD spectrum, independent of spatial frequency. For a'real' instrument, the MTF is found as the square root of the ratio of the PSD spectrum measured with the BPR grating to the'ideal,' spatial frequency independent, PSD spectrum. We present the results from a measurement of the MTF of MicromapTM-570 interferometric microscope demonstrating a high efficiency for the calibration method.

Dyson spectrometers for high-performance infrared applications

Abstract: The Dyson spectrometer form has the potential to deliver good imaging performance, high throughput, and low distortion in a compact configuration suitable for cryogenic infrared applications. The three main requirements for a practical implementation—availability of the required concave diffraction grating, availability of the Dyson lens material, and clearance for slit and focal plane packaging—are now within the state of the art, opening the Dyson form to serious consideration. Several high-performance Dyson designs for the long-wavelength infrared are presented.

Image texture classification using a manifold- distance-based evolutionary clustering method

Abstract: We perform unsupervised image classification based on texture features by using a novel evolutionary clustering method, named manifold evolutionary clustering (MEC). In MEC, the clustering problem is considered from a combinatorial optimization viewpoint. Each individual is a sequence of real integers representing the cluster representatives. Each datum is assigned to a cluster representative according to a novel manifold-distance-based dissimilarity measure, which measures the geodesic distance along the manifold. After extracting texture features from an image, MEC determines partitioning of the feature vectors using evolutionary search. We apply MEC to solve seven benchmark clustering problems on artificial data sets, three artificial texture image classification problems, and two synthetic aperture radar image classification problems. The experimental results show that in terms of cluster quality and robustness, MEC outperforms the K-means algorithm, a modified K-means algorithm using the manifold-distance-based dissimilarity measure, and a genetic-algorithm-based clustering technique in partitioning most of the test problems.

Optical analysis of an 80-W light-emitting-diode street lamp

Abstract: Optical analysis is critical to the evaluation of a light-emitting diode (LED) street lamp, especially when the lamp is still in its early stage of development and applications and when optimization is needed for making use of unique characteristics of LEDs In this study, optical analysis of an 80-W LED street lamp was conducted Experimental research on such a lamp was first undertaken The results demonstrated that the average illumination was about 825 lx and the total uniformity was 0364 for a 20-m-long and 10-m-wide test area at a height of 8 m, which is acceptable for the current standard for a submain road Numerical simulation was also conducted; the feasibility of the numerical model was proven by comparison of the simulations with the experimental data, which will be used for future optimization study and other novel designs of the optical system of street lamps Through the simulations and the corresponding analysis, it was found that the tested 80-W LED street lamp had reasonable performance in average illumination, but multiple shadows existed, which would need to be removed in future designs Improvements are suggested to reduce the number of optical elements, to reduce the lamp's volume, and to enhance the illumination performance Two design methods for LED street lamps are summarized, based on the optical analysis

Sharing secret images using Blakley's concept

Abstract: How to protect secret information is a most important issue in military technology. In this paper, a (p,n) secret sharing method using Blakley's concept is proposed to protect the security of secret images. Even though enemies know p−1 of n shadow images, they cannot obtain any information about the secret image. Only p or more than p shadow images can reconstruct it. Experimental results show that the proposed method has the advantage of smaller shared images, which can greatly reduce the storage space.

Accurate metrology for laser damage measurements in nonlinear crystals

Abstract: Accurate laser damage measurements are more difficult to perform in nonlinear optical crystals than in glasses due to several effects proper to these materials or greatly enhanced in these materials. Before discussing these effects, we address the topic of error bar determination for probability measurements. Error bars for the measured damage probabilities are especially important when testing small and expensive samples like nonlinear crystals, where only few sites are used for each measurement. The mathematical basics for the numerical calculation of probability error bars corresponding to a chosen confidence level are presented. Effects that possibly modify the maximum light intensity obtained by focusing into a biaxial nonlinear crystal are mainly the focusing aberrations and self-focusing. Depending on focusing conditions, position of the focal point in the crystal, beam propagation direction, and polarization, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor based on former theoretical work is proposed for this effect. The characteristics of self-focusing are quickly reviewed for the sake of completeness, and a note on parasitic second harmonic generation is added at the end.

Index-guiding liquid-core photonic crystal fiber for solution measurement using normal and surface-enhanced Raman scattering

Abstract: We have explored the use of index-guiding liquid-core (LC) photonic crystal fiber (PCF) as a robust platform for measurements of solutions of trace volume using normal and surface-enhanced Raman scattering (SERS). The LC PCF was fabricated by selectively sealing the cladding air channels at the distal ends of a hollow-core PCF while leaving the center core open, using a fusion splicer. Utilizing a 30-cm-long LC PCF with the entire center core filled with the ~0.1-µL solution of interest, we have obtained normal Raman spectra of water, ethanol, and 1 vol% ethanol in water. Sensitive and reproducible SERS detection of 1.7×10−7 M thiocyanate anions (14 ppb of NaSCN) in water has also been achieved.

Unpolarized calibration and nonuniformity correction for long-wave infrared microgrid imaging polarimeters

Abstract: Recent developments for long-wave infrared (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array. Inherent advantages over other classes of polarimeters include rugged packaging, inherent alignment of the optomechanical system, and temporal synchronization that facilitates instantaneous acquisition of both thermal and polarimetric information. On the other hand, the pixel-to-pixel instantaneous field-of-view error that is inherent in the microgrid strategy leads to false polarization signatures. Because of this error, residual pixel-to-pixel variations in the gain-corrected responsivity, the noise-equivalent input, and variations in the pixel-to-pixel micropolarizer performance are extremely important. The degree of linear polarization is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. We explore the unpolarized calibration issues associated with this class of LWIR polarimeters and discuss the resulting false polarization signature for thermally flat test scenes.

Effect of oil vapor contamination on the performance of porous silica sol-gel antireflection-coated optics in vacuum spatial filters of high-power neodymium glass laser

Abstract: The effect of oil vapor contamination on sol-gel porous silica antireflection (AR) coatings deposited on optics used in vacuum spatial filters of high-power lasers is experimentally studied. The contamination results in the reduction of laser-induced damage threshold (LIDT) and the increase in reflectivity of the AR coatings. In comparison, the silica coatings treated with ammonia and hexamethyl-disilazane (HMDS) show almost no increase of reflectivity, and their LIDT value reduces only marginally at the wavelength of laser operation. It is found that the decrease in LIDT of the coatings exposed to oil vapors occurs due to higher absorption of the laser light, as evidenced from the increase in the imaginary part of the refractive index of the coatings.

Free-space optical communication link across 16 kilometers over the Chesapeake Bay to a modulated retroreflector array

Abstract: This paper presents the results of a successful bidirectional free-space optical link across 16 km to a modulated retroreflector array. The link was implemented at the Naval Research Laboratory's Chesapeake Bay Detachment laser test range. A 6-W cw 1550-nm class 1 M interrogation beam was used to illuminate an array of three modulated cat's-eye retroreflectors located on a tower across the Chesapeake Bay on Tilghman Island. The modulated retroreflectors had a diameter of 16 mm and were arranged in a triangular pattern with a spacing of 30 cm. The interrogating terminal employed a 100-µrad divergence and a high-speed pointing and tracking system to maintain link alignment. Link testing occurred over 12 days in the months of September, October, and November of 2006. Topics presented in this paper include the link scenario for the 16-km free-space optical link, the link budget, and terminal designs, as well as link acquisition and performance. Link performance results presented include data transmission throughput, scintillation data, and pointing and tracking results.

Measured and predicted light attenuation in dense coastal upslope fog at 650, 850, and 950 nm for free-space optics applications

Abstract: Free-space optics FSO has gained considerable importance in this decade of demand for high-bandwidth transmission capabilities. FSO can provide the last mile solution, but the availability and reliability issues concerned with it have received increasing attention and need thorough investigation. In this work, we present our results on fog attenu- ation measurement and prediction at wavelengths 650, 850, and 950 nm with peak values up to 500 dB/km. For the attenuation measurement, optical wavelengths are transmitted over the same path of fog in free air to a receiver, measuring the power at every wavelength. The measure- ment of fog attenuation was performed at the France Telecom RD free-space optics; atmospheric propagation; fog attenu- ation prediction; meteorological range; availability.

Spectral reflectance and emissivity of man-made surfaces contaminated with environmental effects

Abstract: Spectral remote sensing has evolved considerably from the early days of airborne scanners of the 1960's and the first Landsat mul- tispectral satellite sensors of the 1970's. Today, airborne and satellite hyperspectral sensors provide images in hundreds of contiguous narrow spectral channels at spatial resolutions down to meter scale and span- ning the optical spectral range of 0.4 to 14 m. Spectral reflectance and emissivity databases find use not only in interpreting these images but also during simulation and modeling efforts. However, nearly all existing databases have measurements of materials under pristine conditions. The work presented extends these measurements to nonpristine condi- tions, including materials contaminated with sand and rain water. In par- ticular, high resolution spectral reflectance and emissivity curves are pre- sented for several man-made surfaces asphalt, concrete, roofing shingles, and vehicles under varying amounts of sand and water. The relationship between reflectance and area coverage of the contaminant is reported and found to be linear or nonlinear, depending on the mate- rials and spectral region. In addition, new measurement techniques are presented that overcome limitations of existing instrumentation and labo- ratory settings. These measurements enable simulation of optical im- ages with objects in the scene attributed with realistic surface reflectance and emissivity spectra. © 2008 Society of Photo-Optical Instrumentation

Experimental results from copropagating analog channels of same wavelength over 600-meter-long standard step index multimode fiber

Abstract: Spatial domain multiplexing is a new dimension in fiber optic multiplexing with the potential to greatly increase the data-carrying capacity of optical fiber communication systems. Spatially multiplexed channels follow helical paths inside the fiber and do not interfere with each other as optical energy from individual channels is distributed in the radial direction at different distances from the origin of the fiber. The resultant output appears as concentric circles when projected on a screen. The experimental setup and results for two spatially modulated analog channels of the same wavelength over approximately 600 m of standard 62.5/125-µm multimode fiber, using 635-nm pigtail laser sources and 30-MHz sinusoidal modulation, are reported here.

Fiber-reinforced polymer-packaged optical fiber sensors based on Brillouin optical time-domain analysis

Abstract: Brillouin optical time-domain reflectometry (BOTDR) and Brillouin optical time-domain analysis (BOTDA) are considered to be promising and practical sensing techniques for large structures. However, there is still a major obstacle to applying BOTDR or BOTDA on large-scales; the high cost and unreliability associated with sensor installation and failure. We report a novel, low-cost, and highly reliable BOTD sensor using a rebar consisting of a bare optical fiber (OF) packaged in fiber-reinforced polymer (FRP) and named BOTD-FRP-OF. We investigate the surface bonding and its mechanical strength scanning-electron-microscope and intensity experiments. Considering the strain difference between OF and host matrix, which may result in measurement error, the strain transfer from host to OF has been studied theoretically. Furthermore, the sensing properties of glass FRP-OFs for strain and temperature at different gauge lengths were tested under different spatial and readout resolutions using commercial BOTDA. Finally, an absolute dual-BOTD-FRP-OF temperature compensation method is proposed and has been tested. This novel FRP-OF rebar shows both high strength and good sensing properties, which can be used in long-term structural health monitoring for civil infrastructure.