Showing papers in "Optical Engineering in 2001"

Mathematical model for the irradiance probability density function of a laser beam propagating through turbulent media

Abstract: We develop a model for the probability density function (pdf) of the irradiance fluctuations of an optical wave propagating through a turbulent medium. The model is a two-parameter distribution that is based on a doubly stochastic theory of scintillation that assumes that small-scale irradiance fluctuations are modulated by large-scale irradi- ance fluctuations of the propagating wave, both governed by indepen- dent gamma distributions. The resulting irradiance pdf takes the form of a generalized K distribution that we term the gamma-gamma distribution. The two parameters of the gamma-gamma pdf are determined using a recently published theory of scintillation, using only values of the refractive-index structure parameter C n (or Rytov variance) and inner scale l 0 provided with the simulation data. This enables us to directly calculate various log-irradiance moments that are necessary in the scaled plots. We make a number of comparisons with published plane wave and spherical wave simulation data over a wide range of turbu- lence conditions (weak to strong) that includes inner scale effects. The gamma-gamma pdf is found to generally provide a good fit to the simu- lation data in nearly all cases tested. © 2001 Society of Photo-Optical Instrumen-

Laser ranging: a critical review of usual techniques for distance measurement

Abstract: We review some usual laser range finding techniques for in- dustrial applications. After outlining the basic principles of triangulation and time of flight (pulsed, phase-shift and frequency modulated continu- ous wave (FMCW)), we discuss their respective fundamental limitations. Selected examples of traditional and new applications are also briefly presented. © 2001 Society of Photo-Optical Instrumentation Engineers.

Subpixel displacement and deformation gradient measurement using digital image/speckle correlation (DISC)

Abstract: Experimental solid mechanics relies heavily on surface dis- placement and deformation gradient measurements. Digital image/ speckle correlation (DISC) uses digital image processing to resolve dis- placement and deformation gradient fields. The practical implementation of DISC involves important challenges such as computation complexity and the discrepancy of the sensitivities and accuracies claimed in previ- ous studies. We develop an iterative, spatial-gradient based algorithm, which uses only first-order spatial derivatives of the images before and after deformation. Simulated images are then used to verify this algo- rithm, as well as to study the impact of speckle size on the accuracy. Based on these simulations, the sensitivity of DISC to displacement and deformation gradient, as well as an optimal speckle size for optimal ac- curacy, is recommended. The algorithm is then calibrated using rigid body translation and rotation, and an application of DISC to thermome- chanical diagnostics of electronic packaging is also presented. © 2001

Optical properties of cyclic olefin copolymers

Abstract: The optical properties of a cyclic olefin copolymer, Topas™, made by Celanese AG, are presented. This class of materials is attractive on account of their high use temperature, excellent optical transmission, low birefringence, and low moisture uptake. These materials are compared with other commonly used thermoplastics, namely, polycarbonate and polymethyl methacrylate. Finally, Topas™ is compared with a number of other optical polymers being studied for optical waveguiding applications. It is found that at 830 nm Topas™ has low losses (less than 0.5 dB/cm), so that it may be useful for datacom applications. At the telecommunication wavelength of 1550 nm the losses are in the range of 0.7 dB/cm.

Large-aperture multiple quantum well modulating retroreflector for free-space optical data transfer on unmanned aerial vehicles

Abstract: We describe progress in the development of a multiple quan- tum well modulating retroreflector, including a description of recent dem- onstrations of an infrared data link between a small rotary-wing un- manned airborne vehicle and a ground-based laser interrogator using the device designed and fabricated at the Naval Research Laboratory (NRL). Modulating retroreflector systems couple an optical retroreflector, such as a corner cube, and an electro-optic shutter to allow two-way optical communications using a laser, telescope, and pointer-tracker on only one platform. The NRL modulating retroreflector uses a semiconductor-based multiple quantum well shutter capable of modula- tion rates greater than 10 Mbps, depending on link characteristics. The technology enables the use of near-infrared frequencies, which is well known to provide covert communications immune to frequency allocation problems. This specific device has the added advantage of being com- pact, lightweight, covert, and requires very low paper. Up to an order of magnitude in onboard power can be saved using a small array of these devices instead of the radio frequency equivalent. In the described dem- onstration, a Mbps optical link to an unmanned aerial vehicle in flight at a range of 100 to 200 feet is shown. Near real-time compressed video was also demonstrated at the Mbps level and is described. © 2001 Society of

Bibliographical review for reflectance of diffusing media

Abstract: When light travels through a diffusing medium, the incident radiation is scattered and a part of it is reemitted. The reflected propor- tion is defined as the reflectance and provides the quantification of color sensations. Many authors have tried to derive reflectance values by vari- ous means. This bibliographical synthesis reviews the main theories on the subject, comparing them and especially their assumptions and deri- vations. First, the widely used theory of Kubelka and Munk—which has been proved to be a particular case of Schuster's formula—is described, according to its terms, limits, improvements, and applications. Next, the well-known electromagnetic approach by Mie is presented, along with methods derived from radiative transfer theory, multilayer methods (the ''pile of plates'' due to Stokes), multiflux methods (developed by Mudgett and Richards or Volz for instance), and the corpuscular treatment pro- posed by Silvy. Finally, new methods are listed that allow the calculation of reflectance values, such as Monte Carlo simulations, expert systems, or neural networks. A chronological organization chart is also given to place each work or advance relative to the others. © 2001 Society of Photo-

Quantitative prediction of the modulation behavior of twisted nematic liquid crystal displays based on a simple physical model

Abstract: A method to perform a predictive search for a given amplitude and phase modulation response in twisted nematic liquid crystal displays is presented. The algorithm is based on a simple physical model that we recently proposed and that considers the effect of liquid-crystal layers located in the vicinity of the edges, which are not able to tilt. This model was demonstrated to explain accurately the experimental transmittance modulation curves. Here the model is applied to perform a predictive search for an optimized modulation by changing the input and output polarization configurations. A generalized configuration to generate and detect elliptically polarized light is proposed. The method is applied for seeking two different configurations useful for optical image processors: phase-only modulation and amplitude-only modulation. The excellent agreement with the experimental measurements validates the potential of the proposed method.

Optical feedback interferometry for sensing application

Abstract: We review laser diode feedback interferometry as a general tool for sensing applications. After outlining the basic principles and the theoretical approaches used to describe the phenomenon, we present a few selected examples of applications in interferometry, as developed by various groups in recent years, such as a displacement sensor, a veloci- meter or vibration sensor, and an absolute distance meter or range finder and angle sensor. Experimental results are also reported as an illustration. © 2001 Society of Photo-Optical Instrumentation Engineers.

Wavelet analysis for the elimination of striping noise in satellite images

Abstract: A new destriping technique based on wavelet analysis and its application to satellite imagery is presented. The method was tested on a heavily striped Landsat MSS image using Daubechies wavelets of different orders. Qualitative and quantitative analyses were carried out to evaluate the performance for each wavelet, both by visual inspection and by measuring the signal-to-noise ratio of the denoised images. The outcome demonstrates the viability of the wavelet transform as a destriping tool by significantly reducing striping noise. Furthermore, the method proved to defeat some of the problems commonly found with traditional destriping techniques, such as the Gibbs phenomenon, border effects, edge blurring, and the preservation of radiometric level. In addition to its relatively easy implementation, the method is inexpensive in computer time and storage space, as well as suitable for application to problems other than the field of remote sensing.

Hologram reconstruction using a digital micromirror device

Abstract: The aim of this work is an advancement of digital holographic interferometry by using a digital micromirror device (DMD) for real-time display of interferometric fringes. The theory describing the formation of the diffracted image produced by a coherently illuminated DMD is pre- sented. Digital holograms of simple structures are calculated assuming plane and spherical reference waves, respectively. These synthetic ho- lograms are used to control the micromirrors of the DMD, which now acts as a reflective hologram. Experimental holographic reconstructions of synthetic holograms prove the feasibility of DMDs in principle for this aim. Further experimental results show the reconstructed interferogram of fields recorded by digital holography and superposed numerically. This opens a new range of applications in digital holographic interferom- etry. © 2001 Society of Photo-Optical Instrumentation Engineers.

Visual attention and target detection in cluttered natural scenes

Abstract: Rather than attempting to fully interpret visual scenes in a parallel fashion, biological systems appear to employ a serial strategy by which an attentional spotlight rapidly selects circumscribed regions in the scene for further analysis The spatiotemporal deployment of attention has been shown to be controlled by both bottom-up (image-based) and top-down (volitional) cues We describe a detailed neuromimetic computer implementation of a bottom-up scheme for the control of visual attention, focusing on the problem of combining information across modalities (orientation, intensity, and color information) in a purely stimulusdriven manner We have applied this model to a wide range of target detection tasks, using synthetic and natural stimuli Performance has, however, remained difficult to objectively evaluate on natural scenes, because no objective reference was available for comparison We present predicted search times for our model on the Search–2 database of rural scenes containing a military vehicle Overall, we found a poor correlation between human and model search times Further analysis, however, revealed that in 75% of the images, the model appeared to detect the target faster than humans (for comparison, we calibrated the model’s arbitrary internal time frame such that 2 to 4 image locations were visited per second) It seems that this model, which had originally been designed not to find small, hidden military vehicles, but rather to find the few most obviously conspicuous objects in an image, performed as an efficient target detector on the Search–2 dataset Further developments of the model are finally explored, in particular through a more formal treatment of the difficult problem of extracting suitable low-level features to be fed into the saliency map

Effect of conductivity and dielectric constant on the modulation voltage for nonlinear optic polymer based opto-electronic devices

Abstract: Presented is the effect of using various cladding materials with different conductivities and dielectric constants on the applied voltage for optoelectronic (OE) devices based on nonlinear optical (NLO) polymers. Using a conductive polymer, we have demonstrated a 3 to 13 times increase in the effective electro-optic (EO) coefficient of electrode- poled NLO polymers, compared to using passive polymer claddings. We have achieved the lowest poling voltage to date for maximum EO coefficient, 300 V, for a two-layer waveguide structure consisting of a 2-?m- thick NLO polymer layer and a 2-?m-thick conductive cladding layer. The dielectric constants of both the NLO polymer core and passive polymer cladding materials used for conventional polymer-based integrated optic devices are typically very similar in magnitude. This suggests that only a small fraction of the applied modulation voltage is reaching the NLO polymer core layer, requiring 4 to 5 times higher modulation voltage than the desired V?. We have demonstrated a factor-of-2 decrease in the modulation voltage using the same conductive polymer, due to its possessing a much higher dielectric constant than the core material at the modulation frequency tested. The results show promise for shorter, lower-operating-voltage devices.

Comparison of SNR image quality metrics for remote sensing systems

Abstract: Different definitions of the signal-to-noise ratio (SNR) are be- ing used as metrics to describe the image quality of remote sensing systems. It is usually not clear which SNR definition is being used and what the image quality of the system is when an SNR value is quoted. This paper looks at several SNR metrics used in the remote sensing community. Image simulations of the Kodak Space Remote Sensing Camera, Model 1000, were produced at different signal levels to give insight into the image quality that corresponds with the different SNR metric values. The change in image quality of each simulation at different signal levels is also quantified using the National Imagery Interpretability Rating Scale (NIIRS) and related to the SNR metrics to better under- stand the relationship between the metric and image interpretability. An analysis shows that the loss in image interpretability, measured as DNI- IRS, can be modeled as a linear relationship with the noise-equivalent change in reflection (NE Dr). This relationship is used to predict the val- ues that the various SNR metrics must exceed to prevent a loss in the interpretability of the image from the noise. © 2001 Society of Photo-Optical

Airborne lidar for fisheries applications

Abstract: An airborne lidar using commercial, off-the-shelf components was developed by the National Oceanic and Atmospheric Administration for fisheries' surveys. We make several test flights of the system and develop several signal processing techniques to discriminate between the returns from fish and from small particles in the water. We use these techniques to produce several products of use to fisheries' biologists. Examples are presented of gray-scale images similar to those produced by acoustic echo sounders, of maps of spatial distributions of fish, and of vertical profiles of fish biomass. © 2001 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1348000)

Comparative study of wavelet thresholding methods for denoising electronic speckle pattern interferometry fringes

Abstract: This paper presents a comparative study of different thresholding methods for speckle noise reduction in electronic speckle pattern interferometry fringes using several wavelet bases. An approach based on the removal of the wavelet subbands of the transformed image is presented. The performance of this method is analyzed and compared with the results obtained with the denoising techniques that use wavelet shrinkage. It is shown that the wavelet subband removal method reduces speckle noise and maintains image features more effectively than the wavelet shrinkage techniques based on soft and hard thresholding.

Multi-image gradient-based algorithms for motion estimation

Abstract: Image registration algorithms based on gradient methods provide quantitative motion measurements from sequences of video im- ages. Although such measurements can be degraded by image noise, larger degradations typically result from systematic bias in the algorithms that is present even if the images are noise-free. To improve the accu- racy of motion measurements, we develop a new class of multi-image algorithms based on multidimensional digital filters. The new algorithms provide better estimates of spatial and temporal gradients and also com- pensate for motion blur caused by the nonzero acquisition time of the imager. We optimize filters to measure arbitrary motions, and we illus- trate the results when those filters are used to estimate constant velocity movements. We also show results for filters that are optimized for har- monic analysis of periodic motions. Using these algorithms, systematic bias in the amplitude of sinusoidal motion is less than 0.001 pixels for motions smaller than 1 pixel in amplitude. This represents a hundredfold decrease in bias compared to existing methods. © 2001 Society of Photo-

Mean-quantization-based fragile watermarking for image authentication

Abstract: The authors propose an image authentication scheme, which is able to detect malicious tampering while tolerating some incidental distortions By modeling the magnitude changes caused by incidental distortion and malicious tampering as Gaussian distributions with small and large variances, respectively, they propose to embed a watermark by using a mean-quantization technique in the wavelet domain The pro- posed scheme is superior to the conventional quantization-based ap- proaches in credibility of authentication Statistical analysis is conducted to show that the probabilities of watermark errors caused by malicious tampering and incidental distortion will be, respectively, maximized and minimized when the new scheme is applied Experimental results dem- onstrate that the credibility of the method is superior to that of the con- ventional quantization-based methods under malicious attack followed by an incidental modification, such as JPEG compression, sharpening or blurring © 2001 Society of Photo-Optical Instrumentation Engineers

Optically interrogated MEMS pressure sensors for propulsion applications

Abstract: Pressure sensors suitable for propulsion applications that uti- lize interrogation by fiber optics are described. To be suitable for many propulsion applications, sensors should have fast response, have a con- figuration that can be readily incorporated into sensor arrays, and be able to survive harsh environments. Microelectromechanical systems (MEMS) technology is utilized here for sensor fabrication. Optically inter- rogated MEMS devices are expected to eventually be more suitable than electrically interrogated MEMS devices for many propulsion applications involving harsh environments. Pressure-sensor elements are formed by etching shallow cavities in glass substrates followed by anodic bonding of silicon onto the glass over the cavity. The silicon is subsequently etched to form the pressure-sensitive diaphragm. Light emerging from a fiber is then used to interferometrically detect diaphragm deflection due to external pressure. Experimental results for static and dynamic pres- sure tests carried out in a shock tube demonstrate reasonable linearity, sensitivity, and time response. © 2001 Society of Photo-Optical Instrumentation

Image dataset for testing search and detection models

Abstract: The TNO Human Factors Searchu2 image dataset consists of: a set of 44 high-resolution digital color images of different complex natural scenes, the ground truth corresponding to each of these scenes, and the results of psychophysical experiments on each of these images. The images in the Searchu2 dataset are a subset of a larger set that has been used in a visual search and detection experiment. Each scene (image) contains a single military vehicle that serves as a search target. The image dataset, an Excel file with the ground truth and observer data, and an extensive report describing the dataset are available on CD-ROM (requests by email to the first author). The dataset can be used to develop and validate digital metrics that compute the visual distinctness (contrast, conspicuity, or saliency) of targets in complex scenes, and models of human visual search and detection. The dataset has already been used in more than ten different studies in the literature, ranging from studies evaluating target detectability metrics to eye movement studies and attempts to model the human visual system. In addition to this work, eight other articles in this special section address the Searchu2 dataset.

Microlens systems for fluorescence detection in chemical microsystems

Abstract: Micro-optical systems based on refractive microlenses are in- vestigated. These systems are integrated on a chemical chip. They focus an excitation beam into the detection volume (microliter or even submi- croliter scale) and collect the emitted light from fluorescent molecules. The fluorescence must be carefully separated by spatial and spectral filtering from the excitation. This paper presents the ray tracing simula- tion, fabrication, and measurement of three illumination systems. The measurements show that an adroit placement and combination of micro- fabricated lenses and stops can increase the separation between the excitation light and the fluorescence light. Moreover we present the suc- cessful detection of a 20 nM Cy5™ (Amersham Life Science Ltd.) solu- tion in a 100-mm-wide and 50-mm-deep microchannel (excitation volume '250 pL) using one of these illumination systems. The microchemical chip with the micro-optical system has a thickness of less than 2 mm. © 2001 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1359522)

Security optical systems based on a joint transform correlator with significant output images

Abstract: An improved optical security system based on correlation between two phase-only computer generated masks is proposed The two phase masks are placed together at the input plane of a joint transform correlator A priori known output image is obtained in the system output only if one mask is the right key for the other mask In addition to a simple verification, our security system is capable of identifying the type of an input mask according to the corresponding output image that it generates The two phase masks are designed using an iterative optimization algorithm with constraints in the input and output domains Computer simulations are presented with the resultant images formed by the two phase-only elements

Polymeric waveguide prism-based electro-optic beam deflector

Abstract: Beam steering devices without moving parts are highly desir- able for their potential application in emerging optical technologies such as holographic optical storage systems, all optical networks, and optical switches. We demonstrate a thin-film waveguide beam deflector device that consists of an electro-optic prism array within a polymer waveguide. An electrode structure defines the prism array within the planar wave- guide. The deflection efficiency of 28 mrad/kV and the maximum deflec- tion angle of 68.4 mrad at 6300 V are obtained for this demonstration device. Further optimization of electrode-field poling and processing is likely to improve these results by at least an order of magnitude. © 2001

Color features for quality control in ceramic tile industry

Abstract: We study visual quality control in the ceramics industry. In tile manufacturing, it is important that in each set of tiles, every single tile looks similar. Currently, the estimation is usually done by human vision. Our goal is to design a machine vision system that can estimate the sufficient similarity, or same appearance, to the human eye. Our main approach is to use accurate spectral representation of color, and com- pare spectral features to the RGB color features. A laboratory system for color measurements is built. Experimentations with five classes of brown tiles are presented and discussed. In addition to the k-nearest neighbor (k-NN) classifier, a neural network called the self-organizing map (SOM) is used to provide understanding of the spectral features. Every single spectrum in each tile of a training set is used as input to a 2-D SOM. The SOM is analyzed to understand how spectra are clustered. As a result, tiles are classified using a trained 2-D SOM. It is also of interest to know whether the order of spectral colors can be determined. In our approach, all spectra are clustered in a 1-D SOM, and each pixel (spectrum) is presented by pseudocolors according to the trained nodes. Finally, the results are compared to experiments with human vision. © 2001 Society of

Algorithm for expanding the dynamic range of a Shack-Hartmann sensor by using a spatial light modulator

Abstract: Normally, the dynamic range of a Shack-Hartmann sensor is limited by the foci leaving their respective subapertures, thus a definite attachment of the foci to their subapertures is difficult. By using an array of spatial light modulators in front of the microlenses of the sensor to switch on and off the subapertures, a definite assignment of the spots to their subapertures is possible. We present a coding algorithm that needs only log2 N11 frames to assign N spots unequivocal to their subaper- tures. © 2001 Society of Photo-Optical Instrumentation Engineers.

Real-time identification of smoke images by clustering motions on a fractal curve with a temporal embedding method

Abstract: Automatic forest fire detection with CCD cameras requires a landscape image analysis in two stages: first the tracking of local dynamic envelopes of pixels, and second the discrimination between the various natural phenomena that may cause such envelopes. For this second process, we have to deal with restrictive conditions: lack of spatial information, complexity of motions, and real-time constraints on detection. We present here a fast algorithm adapted to the extraction of complex motions in small spatial envelopes. The principle of the method is to extract local motions from cluster analysis of points in a multidimensional temporal embedding space. We detail the four successive steps of this method: temporal embedding of gray-levels, fractal indexing of points, chaining points into a linked list, and motion extraction from point sequences of the linked list.

Efficient intensity-hue-saturation-based image fusion with saturation compensation

Abstract: The intensity-hue-saturation (IHS) and Brovey transforms are two widespread image fusion methods in the remote sensing community. However, both are sensitive to the characteristics of the analyzed area. Restated, they often face color distortion problems with fused images. This work investigates models representing the color space in images. Schemes governing saturation change during fusion are also evaluated to derive the correct saturation values for each pixel in a fused image. In addition, algorithms that compensate distorted saturation values for dis- tinct image fusion methods are derived and discussed. Methods to ex- tend current image fusion methods to any arbitrary order are also out- lined. Finally, an illustrative example of a fused SPOT image demonstrates the color distortion problem while an IKONOS image is adopted to evaluate various contemporary image fusion methods. © 2001 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1355956) Subject terms: image fusion; intensity-hue-saturation (IHS); Brovey transform; IHS with saturation compensation.