Showing papers in "Optical Engineering in 2003"

Reconstruction and enhancement of active thermographic image sequences

Abstract: Active thermography has gained broad acceptance as a non- destructive evaluation method for numerous in-service and manufactur- ing applications in the aerospace industry. However, because of the dif- fusive nature of the process, it is subject to blurring and degradation of the signal as one attempts to image deeper subsurface features. Despite this constraint, active thermographic response is deterministic, to the extent that the postexcitation time evolution for a defect-free sample can be accurately predicted using a simple one-dimensional model. In the patented thermal signal reconstruction method, the time history of every pixel in the field of view is compared to such a model in the logarithmic domain, where deviations from ideal behavior are readily identifiable. The process separates temporal and spatial nonuniformity noise compo- nents in the image sequence and significantly reduces temporal noise. Time-derivative images derived from the reconstructed data allow detec- tion of subsurface defects at earlier times in the sequence than conven- tional contrast images, significantly reducing undesirable blurring effects and facilitating detection of low-thermal-contrast features that may not be detectable in the original data sequence. © 2003 Society of Photo-Optical In-

High-speed 3-D shape measurement based on digital fringe projection

Abstract: A high-speed 3-D shape measurement technique based on digital fringe projection has been developed and experimented. This technique uses a computer-generated color fringe pattern whose red, green, and blue channels are sinusoidal fringe patterns with a 120-deg phase shift between neighboring channels. When this color fringe pat- tern is sent to a digital-micromirror-device (DMD) based video projector with no color filter, three gray-scale fringe patterns are repeatedly pro- jected to an object surface in sequence with a cycle time of approxi- mately 10 ms. The three phase-shifted fringe patterns deformed by the object surface are captured by a CCD camera with proper synchroniza- tion between the camera and the projector. The 3-D shape of the object surface is reconstructed by using a phase wrapping and unwrapping algorithm and a phase-coordinate conversion algorithm. Experimental results demonstrated the feasibility of this technique for high-speed 3-D shape measurement with a potential measurement speed up to 100 Hz. © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1525272) Subject terms: fringe projection; phase shifting; digital micromirror device; 3-D shape measurement.

Novel data processing techniques for dispersive white light interferometer

Abstract: White light interferometry has been used in the sensing area for many years. A novel data processing method for demodulating the information from the interference spectrum of a white light system is presented. Compared with traditional algorithms, both high-resolution and large dynamic range have been achieved with a relatively low-cost system. Details of this arithmetic are discussed. A compact white light interferometric system employing this algorithm has been developed, combined with fiber Fabry-Perot sensors. A60.5-nm stability over 48 hours with a dynamic range on the order of tens of microns has been achieved with this system. The temperature dependence of this system has been analyzed, and a self-compensating data processing approach is adopted. Experimental results demonstrated a 61.5-nm shift in the temperature range of 10 to 45°C. © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1613958)

Adaptive anomaly detection using subspace separation for hyperspectral imagery

Abstract: We propose adaptive anomaly detectors that find materials whose spectral characteristics are substantially different from those of the neighboring materials. Target spectral vectors are assumed to have different statistical characteristics from the background vectors. We use a dual rectangular window that separates the local area into two regions—the inner window region (IWR) and outer window region (OWR). The statistical spectral differences between the IWR and OWR are exploited by generating subspace projection vectors onto which the IWR and OWR vectors are projected. Anomalies are detected if the pro- jection separation between the IWR and OWR vectors is greater than a predefined threshold. Four different methods are used to produce the subspace projection vectors. The four proposed anomaly detectors are applied to Hyperspectral Digital Imagery Collection Experiment (HY- DICE) images and the detection performance for each method is evaluated. © 2003 Society of Photo-Optical Instrumentation Engineers.

Calibration of a three-dimensional shape measurement system

Abstract: A 3-D surface shape measurement system based on a digital fringe projection and phase shifting technique is described. In this system, three phase-shifted fringe patterns and a centerline pattern are used to determine the absolute phase map of the object. This phase map is then converted to the x , y , and z coordinates of the object surface by a conversion algorithm. To determine the accurate values of the system parameters as required by the conversion algorithm, a two-step calibration procedure was developed. The parameters were first measured to determine their approximate values, then a calibration plate was measured by the system at various positions, and an iteration algorithm used to estimate the system parameters. Measurement results of several objects are presented. The standard deviation of the measurement error was found to be 0.23 mm.

Response of a fiber Bragg grating ultrasonic sensor

Abstract: A fiber Bragg grating (FBG) sensor for detection of ultrasonic waves in liquids and solid structures is investigated. The sensor contains a fiber probe with a FBG, a tunable narrowband laser source, and a photodetector. When ultrasonic waves impinge on the fiber probe, the pitch of the grating is modulated through the elasto-optic effect. The corresponding changes in reflectivity of the grating are detected by a photodetector. The sensitivity of the sensor and its frequency and directional response in the 0.5 to 3.0 MHz range are measured. Some anomalous features of the FBG ultrasound sensor response are noted. It is shown that these features result from a combination of the sensor's response to the direct ultrasonic waves propagating through the medium and the signal generated by axially guided ultrasonic waves propagating through the fiber probe.

Fully phase encryption using fractional Fourier transform

Abstract: We implement a fully phase encryption system, using fractional Fourier transform to encrypt and decrypt a 2-D phase image obtained from an amplitude image. The encrypted image is holographically recorded in a barium titanate crystal and is then decrypted by generating through phase conjugation, a conjugate of the encrypted image. The decrypted phase image is converted into an amplitude image by the phase contrast technique using an electrically addressed spatial light modulator. Experimental results in support of the proposed idea are presented.

Miro: Complete modeling and software for pulse amplification and propagation in high-power laser systems

Abstract: This paper presents some of the most important features of the MirO code, which is a laser propagation program for high-peak-power laser setups. The main propagation models are discussed: the paraxial nonlinear Schrodinger approximation, broadband computation including dispersion effects, and spatial and temporal Talanov transforms are described, as well as the main optical components: a four-level multipass amplifier including both saturation and narrowing effects; broadband and noncollinear frequency conversion; the grating, including high-order dispersion effects; and laser damage effects. Some elementary examples of MirO computation are also provided.

Parameters for designing autostereoscopic imaging systems based on lenticular, parallax barrier, and integral photography plates

Abstract: In designing an autostereoscopic imaging system based on either a lenticular, parallax barrier, or integral photography (IP) plate, geometrical parameters relating the viewing zone and image display panel combined with the plate are the first things to be determined for the system. To define parameters such as the size of the panel, multiview image arrangement, pitch and focal length of the plate, viewing zone distance from the panel, and others, an optical configuration composed of an image mask aligned in front of a point light source array is used. With this configuration, the geometrical parameters are calculated and their relationships are found. In addition, the concept of pixel cells is introduced to arrange multiview images in the display panel for full par- allax image generation. © 2003 Society of Photo-Optical Instrumentation Engineers.

Strain measurement by three-dimensional electronic speckle pattern interferometry: potentials, limitations, and applications

Abstract: Principles and new developments of 3-D electronic speckle pattern interferometry (3-D ESPI) for strain and stress analysis are presented. The potentials and limitations are discussed, and some applications used in industry are shown. The newly developed 3-D ESPI technique allows rapid measurement of both the shape and the 3-D deformation of complex industrial components. The combination of shape and deformation provides all the necessary data for the accurate and quantitative determination of true strain and thus stress on nearly any industrial component. Based on a lightweight compact sensor design and new glass fiber concepts, the new device has been greatly miniaturized and can be easily attached to the components during testing. It is well suited for the harsh environmental conditions often found in the real test world.

Beam shaping with a plano-aspheric lens pair

Abstract: A pair of plano-aspheric lenses can be used to transform a collimated, radially symmetric, Gaussian beam to a radially symmetric flat-top beam. Diffraction of the output beam due to the choice of irradiance profile, as well as the finite aperture of the optics, must be considered if a propagating beam is required. Choosing both lenses to be positive, one can show that the aspheric surfaces are strictly convex, which facilitates fabrication by magnetorheological figuring. A fused silica lens pair is demonstrated, which can be used at any wavelength from 250 to 1550 nm to transform a Gaussian to a flat-top beam. Measurements of both the irradiance profile and phase of the output beam are presented and compared to the ideal design. These optics transform 78% of the total input beam power into the flat-top region of the output beam, which is uniform to better than 5% rms. For applications requiring uniform illumination, this represents a fourfold improvement in power utilization over the Gaussian input. The output wavefront is flat to a quarter wave at 514 nm, resulting in a beam that propagates approximately 0.5 m without significant change in profile.

Two algorithms to estimate fractal dimension of gray-level images

Abstract: The fractal dimension is a fascinating feature highly correlated with the human perception of surface roughness, and has been successfully applied to texture analysis, segmentation, and classification. Several approaches have been developed to estimate the fractal dimension. Among them, the box-counting (BC) method is nonstochastic and popular in estimating the fractal dimension of a two-tone image. The differential box-counting (DBC) method, a generalization of the classical BC method, was proposed to compute the fractal dimension for a 2-D gray-level image. However, the classical BC and the DBC methods have several major drawbacks, such as overcounting and undercounting the number of boxes. Hence, the real value of the fractal dimension cannot be reached. In this work, two algorithms that can obtain more accurate estimates of the fractal dimension are proposed. The first one, a modified algorithm of the DBC method, is called the shifting DBC (SDBC) algorithm, and the second one is called the scanning BC (SBC) algorithm. We theoretically prove that the SDBC algorithm approaches the estimated value closer to the exact fractal dimension than the DBC method. Simulation results show that the two proposed algorithms can resolve the drawbacks that the BC and the DBC methods possess. Compared to the DBC method, the two proposed algorithms consistently give more satisfactory results on synthetic and natural textured images.

Experimental characterization of phenanthrenequinone-doped poly(methyl methacrylate) photopolymer for volume holographic storage

Abstract: We present an experimental study on the photopolymer poly- (methyl methacrylate) doped with phenathrenequinone molecules. Mate- rial characteristics for holographic data storage, including optical charac- teristics, scattering effect, relation between diffraction efficiency and number of holograms, and exposure schedule for multiple storage, are investigated. An experimental demonstration of multiple storage of digital data pages in a polymer cube is presented. © 2003 Society of Photo-Optical

Geometric optics-based design of laser beam shapers

Abstract: When diffraction effects are not important, geometrical optics (ray tracing, conservation of energy within a bundle of rays, and the constant optical path length condition) can be used to design laser beam shapers by solving beam shaping equations or by optimizing a beam shaping merit function for the configurations, including aspheric ele- ments or spherical-surface gradient-index lenses, which are required to change the input irradiance and phase profile into a more useful form. Geometrical optics-based methods are presented for shaping both rota- tionally and rectangular symmetric laser beam profiles. Solutions of the beam shaping equations are presented for a two-plano-aspheric lens system for shaping a circular symmetric Gaussian beam into a top-hat output beam profile and a two-mirror system with no central obscuration for shaping an elliptical Gaussian input beam into a Fermi-Dirac output beam profile. © 2003 Society of Photo-Optical Instrumentation Engineers.

Automatic calibration of low-cost digital cameras

Abstract: Recent developments of digital cameras in terms of the size of charge-coupled device (CCD) arrays and reduced costs are leading to their applications in traditional as well as new photogrammetric, surveying, and mapping functions. Digital cameras, intended to replace conventional film-based mapping cameras, are becoming available along with many smaller formats capable of precise measurement applications. All such cameras require careful calibration to determine their metric characteristics, which are essential to carrying out photogrammetric activities. We introduce a new approach for incorporating straight lines in a bundle adjustment for calibrating off-the-shelf, low-cost digital cameras. The optimal configuration for successfully deriving the distortion parameters is considered when establishing the required test field. Moreover, a framework for automatic extraction of the straight lines in the images is presented and tested. The developed calibration procedure can be used as an efficient tool to investigate the most appropriate model that compensates for various distortions associated with the camera being calibrated. Experiments performed to compare line-based with traditional point-based self-calibration methods prove the feasibility of the suggested approach.

Camera calibration from road lane markings

Abstract: Three-dimensional computer vision techniques have been ac- tively studied for the purpose of visual traffic surveillance. To determine the 3-D environment, camera calibration is a crucial step to resolve the relationship between the 3-D world coordinates and their corresponding image coordinates. A novel camera calibration using the geometry prop- erties of road lane markings is proposed. A set of equations that com- putes the camera parameters from the image coordinates of the road lane markings and lane width is derived. The camera parameters include pan angle, tilt angle, swing angle, focal length, and camera distance. Our results show that the proposed method outperforms the others in terms of accuracy and noise sensitivity. The proposed method accurately de- termines camera parameters using the appropriate camera model and it is insensitive to perturbation of noise on the calibration pattern. © 2003

Ultrafast laser-induced crystallization of amorphous silicon films

Abstract: Ultrashort pulsed laser irradiation is used to crystallize 100-nm amorphous-silicon (a-Si) films. The crystallization process is ob- served by time-resolved pump-and-probe reflection imaging in the range of 0.2 ps to 100 ns. The in-situ images, in conjunction with postpro- cessed scanning electron microscopy (SEM) and atomic force micros- copy (AFM) mapping of the crystallized structure, provide evidence for nonthermal ultra-fast phase transition and subsequent surface-initiated crystallization. © 2003 Society of Photo-Optical Instrumentation Engineers.

Wide-angle achromatic prism beam steering for infrared countermeasure applications

Abstract: The design and analysis of achromatic doublet prisms for use in laser beam steering is presented. The geometric relationships describ- ing the maximum steering angle are given, as are discussions of first- and second-order dispersion reduction. Infrared (IR) material alterna- tives and optimum IR material characteristics for wide-angle achromatic prism beam steering are also investigated. Sixteen materials in 120 dif- ferent combinations have been examined to date. For midwave IR appli- cations it is shown that the minimum dispersion currently achievable across the full 2 to 5 mm spectrum is 1.7816 mrad at an average maxi- mum steering angle of 45 deg. This is accomplished using LiF/ZnS dou- blet prisms. Several issues related to the azimuth and elevation angles into which light is steered as a function of prism rotation angles are also presented. © 2003 Society of Photo-Optical Instrumentation Engineers.

Advances in silica-based integrated optics

Abstract: Recent advances within the realization of silica-based planar waveguide circuitry are presented. This ranges from the production methods for planar waveguides, including a novel method based on the utilization of focused UV-laser beams for direct waveguide imprinting, to the functionalities that are embedded into the glass materials and wave- guide circuitry. Planar waveguide amplifiers, lasers, and the pursuit to obtain highly nonlinear materials to realize purely glass-based switches, modulators, and wavelength converters are also presented. Further- more, microring resonators are discussed, and finally the latest results

Toward integrated single-photon-counting microarrays

Abstract: Silicon, shallow junction, Geiger-mode avalanche photo- diodes (APDs) can be manufactured with complementary metal-oxide semiconductor (CMOS) compatible processing steps and provide single- photon-counting sensitivity. As we move toward providing integrated de- tection of increasingly nanoscopic-sized emissions, small-area detectors and arrays that can be easily integrated into marketable systems will be required. Geiger-mode diodes with diameters of 10, 15, and 20 mm are manufactured and the dark counts measured at 10 V above breakdown are 9, 95, and 990, respectively, at room temperature. The simulated and measured optical crosstalk is found to be significantly reduced for detec- tor pixel pitches beyond 300 mm. The activation energy of the dark count with temperature is found to be 0.58 eV, representing an order of mag- nitude drop in dark count for every 27°C decrease in temperature. The responsivity of the detectors, without antireflection coatings, is found to peak between 550 and 650 nm with a photon detection probability of 43% at 10 V above the breakdown voltage. The low dark counts of the detectors and high photon detection probability highlight the potential these detectors have for fluorescence decay experiments and also in future integrated photonic detection systems. © 2003 Society of Photo-Optical

Efficient gamut clipping for color image processing using LHS and YIQ

Abstract: Out-of-gamut problems exist in color image processing and color reproduction in different devices. Extensive research has been done on transforming out-of-gamut colors to the inside of the output device's color gamut for color reproduction. We focus on the out-of- gamut problem in color image processing. Luminance, hue, and satura- tion (LHS) and YIQ color coordinate systems are investigated. Efficient techniques for luminance and saturation processing and gamut clippings are presented. The efficient technique of luminance clipping for LHS is developed based on scaling, and the efficient technique of luminance clipping for YIQ is developed based on shifting. However, the efficient technique of saturation clipping for both LHS and YIQ is a combination of scaling and shifting. Such efficient techniques reduce the complexity of forward and backward transformations between the color representing the color coordinate system (RGB) and the processing color coordinate system (LHS or YIQ) significantly. Illustrations show the result of RGB, luminance, and saturation clippings. Saturation clipping is the best in terms of maintaining the hue and contrast of the original image. Experi- ments are also conducted to measure the efficiency of the proposed efficient techniques. It shows that the processing time is reduced by 27% comparing with the conventional technique using forward and backward color transformations. © 2003 Society of Photo-Optical Instrumentation Engineers.

Formation of orthoscopic three-dimensional real images in direct pickup one-step integral imaging

Abstract: We show that three-dimensional orthoscopic real images can be produced in direct pickup one-step integral imaging. Impairment of the image resolution in direct pickup two-step integral imaging can be avoided with this approach.

Unified approach for holography and shearography in surface deformation measurement and nondestructive testing

Abstract: Holography and shearography are two useful whole-field non- contacting optical tools for nondestructive flaw detection and precision measurements. Holography serves as a displacement transducer since it gives direct measurements on displacements whereas shearography serves as a strain gage since it gives direct measurements on displace- ment gradients. This paper views holography and shearography and their variations as a single optical technique having the same basic mathematical formulation and instrumentation. A key optical component used in both techniques is a doubly-refractive prism that combines two angularly separated laser rays to interfere at near collinearity, thereby permitting the use of a low-resolution CCD camera for recording the interference pattern. Shearography uses a doubly-refractive prism with small image shearing so that two neighboring points on the test surface are brought to interfere at the image plane of the camera, whereas ho- lography, on the other hand, uses a doubly-refractive prism with large image shearing so that light scattered from two different objects—a test object and a reference surface (serving as a reference beam)—are brought to interfere at the image plane of the camera. Hence, testing and measurements made using holography may also be made using shearography, and vice versa. © 2003 Society of Photo-Optical Instrumentation

Error compensation for a three-dimensional shape measurement system

Abstract: We present an error compensation method for a full-field 3-D shape measurement system based on a digital fringe projection and phase shifting technique. The error map of the system is first established by comparing the measured coordinates with the coordinates defined by a coordinates measuring machine (CMM) at preselected sample points within the measurement volume. An eight-point interpolation algorithm based on the Shepard's method is then used to compensate for the errors in the measured coordinates. Experimental results showed that the accuracy of the system was improved by more than 60% after error compensation.

Mobile shearography system for the inspection of aircraft and automotive components

Abstract: Shearography is an approved and powerful tool for the nondestructive investigation of technical components with respect to material faults and structural imperfections. An application field of high interest is the inspection of aircraft and automotive components. Both material and structural imperfections can be recognized as typical fringe patterns in the shearogram. However, a reliable fault detection requires a detectable fault response to the applied load at the surface and a sufficient quality of the fringe pattern as well. Therefore, the testing of large-scale technical objects having in general noncooperative surfaces needs a careful design of the following system components: the shearography sensor, the illumination unit, the loading system, and the evaluation software. We report a mobile shearography system that was especially designed for the inspection of aircraft and automotive parts under industrial boundary conditions. Several examples illustrate the efficiency of the system.

Measurement of surface shape and deformation by phase-shifting image digital holography

Abstract: Measurement of shape and deformation of diffusely reflecting surfaces by phase-shifting digital holography is presented that uses an imaging setup. The difference of the reconstructed phases before and after tilt of the object illumination beam provides the contour lines of the surface height, while before and after object deformation delivers those of object displacement. This method enables measurements of both the surface shape and deformation of 3-D objects of various sizes with the same optical system and processing software. Suppression of speckle noise is also discussed. Although the setup is the same as electronic speckle pattern interferometry with a phase-shifted reference beam, the present method is more flexible because phase information can also be used for numerical reconstruction of the defocused region of a 3-D object.

Structured microlens arrays for beam shaping

Abstract: Structured microlens arrays are used for beam shaping, diffusion, and homogenization, providing the ability to perform complex beam shaping tasks, both intensity control and spatial energy distribution, with high efficiency (mostly limited by surface losses) and without the presence of artifacts such as zero order. Applications include excimer laser beam shaping, display screens, light-control films, and illumination.

Public-key-based optical image cryptosystem based on data embedding techniques

Abstract: We propose a public-key-based optical image cryptosystem for practical secure communications since conventional optical crypto- systems that use a symmetric algorithm are confronted by the problem of key delivery. The proposed system employs a hybrid architecture in which a double-random-phase encoding is used to cipher and decipher an image and an asymmetric algorithm is used for ciphering and deci- phering the session key. To solve this problem of key delivery, the double-random-phase encoding algorithm is analyzed to establish a co- vert channel and therefore elucidate the phenomenon that the amplitude part of the ciphered image is less sensitive to quantization error than the other parts in the Fourier and output planes. The session key is then hidden in the covert channel setup in the ciphered image and extracted at the receiver side. Experimental results demonstrate that the amplitude parts in the Fourier and output planes are better suited to convey the session key because they enable reconstructed images with a higher visual quality to be obtained. © 2003 Society of Photo-Optical Instrumentation En- gineers. (DOI: 10.1117/1.1588660)

Image multiplying and high-frequency oscillations effects in the Fresnel region light propagation simulation

Abstract: A new effect during light propagation simulation in the Fresnel region is discovered. Image multiplying and high-frequency noise effects are caused by the sampling of the complex light field. Image multiplying is observed in the case of the standard integral calculation. High-frequency noise is observed in the convolution approach. These effects are observed in a computer simulation only and do not exist in nature. The analysis of these effects enable us to define the range of the numerical algorithms application.

Determination of strains from fringe patterns using space-frequency representations

Abstract: A new approach to the retrieval of fringe pattern frequency information is introduced. In this approach, strain information is extracted from fringe patterns by using either short-time Fourier transforms or wavelets. Space-frequency representation of the fringe signals is introduced as a tool to get the space localization of the strains. The idea of spectral energy density maxima to define the instantaneous frequency (ridge points) is presented. Examples of application are given. Results show that this approach provides an excellent tool for pattern analysis.