Showing papers in "Journal of The Optical Society of America A-optics Image Science and Vision in 1999"
TL;DR: In this article, the theoretical performance of blazed binary diffractive elements composed of pillars carefully arranged on a two-dimensional grid whose period is smaller than the structural cutoff was investigated through electromagnetic theories.
Abstract: We report here on the theoretical performance of blazed binary diffractive elements composed of pillars carefully arranged on a two-dimensional grid whose period is smaller than the structural cutoff. These diffractive elements operate under unpolarized light. For a given grating geometry, the structural cutoff is a period value above which the grating no longer behaves like a homogeneous thin film. Because the grid period is smaller than this value, effective-medium theories can be fully exploited for the design, and straightforward procedures are obtained. The theoretical performance of the blazed binary elements is investigated through electromagnetic theories. It is found that these elements substantially outperform standard blazed echelette diffractive elements in the resonance domain. The increase in efficiency is explained by a decrease of the shadowing effect and by an unexpected sampling effect. The theoretical analysis is confirmed by experimental evidence obtained for a 3λ-period prismlike grating operating at 633 nm and for a 20°-off-axis diffractive lens operating at 860 nm.
346 citations
TL;DR: In this article, a heuristic model of irradiance fluctuations for a propagating optical wave in a weakly inhomogeneous medium is developed under the assumption that small-scale irradiance fluctuation fluctuations are modulated by large-scale fluctuations of the wave.
Abstract: A heuristic model of irradiance fluctuations for a propagating optical wave in a weakly inhomogeneous medium is developed under the assumption that small-scale irradiance fluctuations are modulated by large-scale irradiance fluctuations of the wave. The upper bound for small turbulent cells is defined by the smallest cell size between the Fresnel zone and the transverse spatial coherence radius of the optical wave. A lower bound for large turbulent cells is defined by the largest cell size between the Fresnel zone and the scattering disk. In moderate-to-strong irradiance fluctuations, cell sizes between those defined by the spatial coherence radius and the scattering disk are eliminated through spatial-frequency filtering as a consequence of the propagation process. The resulting scintillation index from this theory has the form σI2=σx2+σy2+σx2σy2, where σx2 denotes large-scale scintillation and σy2 denotes small-scale scintillation. By means of a modification of the Rytov method that incorporates an amplitude spatial-frequency filter function under strong-fluctuation conditions, tractable expressions are developed for the scintillation index of a plane wave and a spherical wave that are valid under moderate-to-strong irradiance fluctuations. In many cases the models also compare well with conventional results in weak-fluctuation regimes. Inner-scale effects are taken into account by use of a modified atmospheric spectrum that exhibits a bump at large spatial frequencies. Quantitative values predicted by these models agree well with experimental and simulation data previously published. In addition to the scintillation index, expressions are also developed for the irradiance covariance function of a plane wave and a spherical wave, both of which have the form BI(ρ)=Bx(ρ)+By(ρ)+Bx(ρ)By(ρ), where Bx(ρ) is the covariance function associated with large-scale fluctuations and By(ρ) is the covariance function associated with small-scale fluctuations. In strong turbulence the derived covariance shows the characteristic two-scale behavior, in which the correlation length is determined by the spatial coherence radius of the field and the width of the long residual correlation tail is determined by the scattering disk.
331 citations
TL;DR: In this paper, the authors apply a Bayesian method for inferring an optimal basis to the problem of finding efficient image codes for natural scenes and demonstrate that the results obtained by applying the learned bases to these problems are improved over those obtained with traditional techniques.
Abstract: We apply a Bayesian method for inferring an optimal basis to the problem of finding efficient image codes for natural scenes. The basis functions learned by the algorithm are oriented and localized in both space and frequency, bearing a resemblance to two-dimensional Gabor functions, and increasing the number of basis functions results in a greater sampling density in position, orientation, and scale. These properties also resemble the spatial receptive fields of neurons in the primary visual cortex of mammals, suggesting that the receptive-field structure of these neurons can be accounted for by a general efficient coding principle. The probabilistic framework provides a method for comparing the coding efficiency of different bases objectively by calculating their probability given the observed data or by measuring the entropy of the basis function coefficients. The learned bases are shown to have better coding efficiency than traditional Fourier and wavelet bases. This framework also provides a Bayesian solution to the problems of image denoising and filling in of missing pixels. We demonstrate that the results obtained by applying the learned bases to these problems are improved over those obtained with traditional techniques.
307 citations
TL;DR: In this paper, the design and fabrication of a micropolarizer array for imaging polarimetry is described for the 3-5-µm-wavelength region, where interference lithography is used to generate small grating features through an etch mask layer.
Abstract: The design and fabrication of a micropolarizer array for imaging polarimetry is described for the 3–5-µm-wavelength region. Each micropolarizer consists of a 475-nm-period Mo wire grid in a 16 µm×16 µm aperture. Interference lithography is used to generate the small grating features through an etch mask layer. Arrays of 256×256 micropolarizers at three distinct angular orientations have been fabricated that permit the measurement of the first three Stokes vector components in each pixel of an imaging polarimeter. An imaging system composed of a micropolarizer array integrated directly onto a focal plane array has been assembled, and initial testing has been performed.
300 citations
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TL;DR: Here it is shown how to do the factoring of sensitivity into efficiency and equivalent noise, and the invariances of the two factors are documented.
Abstract: Measuring the dependence of visual sensitivity on parameters of the visual stimulus is a mainstay of vision science. However, it is not widely appreciated that visual sensitivity is a product of two factors that are each invariant with respect to many properties of the stimulus and task. By estimating these two factors, one can isolate visual processes more easily than by using sensitivity measures alone. The underlying idea is that noise limits all forms of communication, including vision. As an empirical matter, it is often useful to measure the human observer’s threshold with and without a noise background added to the display, to disentangle the observer’s ability from the observer’s intrinsic noise. And when we know how much noise there is, it is often useful to calculate ideal performance of the task at hand, as a benchmark for human performance. This strips away the intrinsic difficulty of the task to reveal a pure measure of human ability. Here we show how to do the factoring of sensitivity into efficiency and equivalent noise, and we document the invariances of the two factors.
272 citations
TL;DR: The model has been applied to simulate a variety of experimental methods in which image aberrations are estimated from measurements taken in the object space and the results suggest that for some types of aberration, these methods may yield biased estimates.
Abstract: A schematic eye model based on anatomical data, which had been previously designed to reproduce image quality on axis, has been transformed into a wide-angle model by simply adding a spherical image surface that plays the role of the retina. This model captures the main features of the wide-angle optical design of the human eye with minimum complexity: four conic optical surfaces plus a spherical image surface. Seidel aberrations (spherical aberration, coma, astigmatism, field curvature, and distortion), longitudinal and transverse chromatic aberrations, and overall monochromatic spot diagrams have been computed for this eye model and for field angles ranging from 0 degree to 60 degrees by both finite and third-order ray tracing. The modulation transfer function for each field angle has been computed as well. In each case our results have been compared with average experimental data found in the literature, showing a reasonably good agreement. The agreement between the model and experimental data is better off axis, mainly at moderate (10 degrees-40 degrees) field angles, than on axis. The model has been applied to simulate a variety of experimental methods in which image aberrations are estimated from measurements taken in the object space. Our results suggest that for some types of aberration, these methods may yield biased estimates.
256 citations
TL;DR: In this paper, the authors present a tutorial with presentation of the main ideas and provision of references to the (dispersed) technical literature for the signal detection theory and progress on modeling human noise-limited performance is summarized.
Abstract: In 1946 and 1948, three very important papers by Albert Rose [J. Soc. Motion Pict. Eng. 47, 273 (1946); J. Opt. Soc. Am. 38, 196 (1948); L. Marton, ed. (Academic, New York, 1948)] were published on the role that photon fluctuations have in setting fundamental performance limits for both human vision and electronic imaging systems. The papers were important because Rose demonstrated that the performance of imaging devices can be evaluated with an absolute scale (quantum efficiency). The analysis of human visual signal detection used in these papers (developed before the formal theory of signal detectability) was based on an approach that has come to be known as the Rose model. In spite of its simplicity, the Rose model is a very good approximation of a Bayesian ideal observer for the carefully and narrowly defined conditions that Rose considered. This simple model can be used effectively for back-of-the-envelope calculations, but it needs to be used with care because of its limited range of validity. One important conclusion arising from Rose's investigations is that pixel signal-to-noise ratio is not a good figure of merit for imaging systems or components, even though it is still occasionally used as such by some researchers. In the present study, (1) aspects of signal detection theory are presented, (2) Rose's model is described and discussed, (3) pixel signal-to-noise ratio is discussed, and (4) progress on modeling human noise-limited performance is summarized. This study is intended to be a tutorial with presentation of the main ideas and provision of references to the (dispersed) technical literature.
244 citations
TL;DR: In this article, the spatial distributions of melanin and hemoglobin in human skin are separated by independent-component analysis of a skin color image, and the separated components are synthesized to simulate the various facial color images by changing the quantities of the two separated pigments.
Abstract: The spatial distributions of melanin and hemoglobin in human skin are separated by independent-component analysis of a skin color image. The analysis is based on the skin color model with three assumptions: (1) Spatial variation of color in the skin is caused by two pigments, melanin and hemoglobin; (2) the quantities of the two pigments are mutually independent spatially; and (3) linearity holds among the quantities and the observed color signals in the optical density domain. The results of the separation agree well with physiological knowledge. The separated components are synthesized to simulate the various facial color images by changing the quantities of the two separated pigments.
232 citations
TL;DR: A fully phase image encryption technique that uses the double random-phase encoding method that shows better performance than amplitude-based encryption with respect to the mean-squared-error metric.
Abstract: A fully phase image encryption technique that uses the double random-phase encoding method is presented. The performance of this fully phase encryption is compared with that of amplitude-based encryption in the presence of noise. Analytic bounds on the mean squared error for the decrypted images are obtained. The accuracy of the theoretical error bounds is confirmed by comparing them with the mean squared errors obtained by using numerical and statistical methods for actual images. Fully phase-based encryption shows better performance than amplitude-based encryption with respect to the mean-squared-error metric.
221 citations
TL;DR: In this paper, the spatially resolved reflectance of turbid media is studied at short source-detector separations (approximately one transport mean free path) with Monte Carlo simulations, and it is shown that the first and second moments of the phase function play a significant role in the reflectance curve, whereas the effect of higher order moments is weak.
Abstract: The spatially resolved reflectance of turbid media is studied at short source–detector separations (approximately one transport mean free path) with Monte Carlo simulations. For such distances we found that the first and second moments of the phase function play a significant role in the reflectance curve, whereas the effect of higher-order moments is weak. Second-order similarity relations are tested and are found efficient at reducing the number of relevant parameters necessary to predict the reflectance. Indeed, only the four following parameters are necessary: the refractive index, the absorption coefficient, the reduced scattering coefficient, and a phase function parameter γ that depends on the first and second moments of the phase function. For media of known γ, the absorption and reduced scattering coefficients can be determined from the intensity and the slope of the log of the reflectance, measured at a single distance. Other empirical properties of the reflectance are derived from the simulations, using short-distance measurements, which provide clues for determining the scattering and absorption properties. In particular, the slope of the square root of the reflectance does not depend on the absorption coefficient but depends on both the reduced scattering coefficient and the phase function parameter γ.
217 citations
TL;DR: In this paper, the authors proposed to extend the traditional analysis by introducing intensity correlation functions g(n) of higher order, which allow both to detect non-Gaussian scattering processes and to extract information not available in g(2) alone.
Abstract: Dynamic light-scattering techniques provide noninvasive probes of diverse media, such as colloidal suspensions, granular materials, or foams. In homodyne photon correlation spectroscopy, the dynamical properties of the medium are extracted from the intensity autocorrelation g(2)(τ) of the scattered light by means of the Siegert relation g(2)(τ)=1+|〈E(0)E*(τ)〉|2/〈EE*〉2. This approach is unfortunately limited to systems where the electric field is a Gaussian random variable and thus breaks down when the scattering sites are few or correlated. We propose to extend the traditional analysis by introducing intensity correlation functions g(n) of higher order, which allow us both to detect non-Gaussian scattering processes and to extract information not available in g(2) alone. The g(n) are experimentally measured by a combination of a commercial correlator and a custom digital delay line. Experimental results for g(3) and g(4) are presented for both Gaussian and non-Gaussian light-scattering processes and compared with theoretical predictions.
TL;DR: A summary of some of the more salient features of progress being made in the understanding of the signal-to-noise limitations of medical imaging systems and to place this progress within a historical context is attempted.
Abstract: Over recent decades a quiet revolution has taken place in the application of modern imaging theory to many fields of applied imaging. Nowhere has this movement been more dramatic than within the field of diagnostic medical x-ray imaging, to the extent that there is now a growing consensus around a universal imaging language for the description and inter-comparison of the increasingly diverse range of technologies. This common language owes much to the basic quantum-limited approach pioneered by Rose and his contemporaries. It embodies the fundamentally statistical nature of image signals, and enables scientists and engineers to develop new system designs optimized for the detection of small signals while constraining patient x-ray exposures to tolerable levels. In this paper we attempt to provide a summary of some of the more salient features of progress being made in the understanding of the signal-to-noise limitations of medical imaging systems, and to place this progress within historical context. Reflecting the experiences of both authors, emphasis will be given to medical diagnostics based on x-ray imaging techniques.
TL;DR: This study estimated threshold ratios between multiple performance levels at various external noise contrasts in two different experiments: Gabor orientation identification, and Gabor detection, and found that the observed threshold ratios departed substantially from the d' ratio predicted by the simple noisy linear amplifier model.
Abstract: A widely used method for characterizing and comparing inefficiencies in perceptual processes is the method of equivalent internal noise—the amount of random internal noise necessary to produce the degree of inefficiency exhibited by the perceptual system in processing [J. Opt. Soc. Am.46, 634 (1956)]. One normally estimates the amount of equivalent internal noise by systematically increasing the amount of external noise added to the signal stimulus and observing how threshold—signal stimulus energy required for an observer to maintain a given performance level—depends on the amount of external noise. In a variety of perceptual tasks, a simple noisy linear amplifier model [ D. Pelli , Ph.D. dissertation (University of Cambridge, Cambridge, UK1981)] has been utilized to estimate the equivalent internal noise Ninternal by fitting of the relation between threshold contrast cτ and external noise Next at a single (d′) performance level: cτ2=(d′/β)2(Next2+N internal2). This model makes a strong prediction: Independent of observer and external noise contrast, the ratio between two thresholds at each external noise level is equal to the ratio of the two corresponding d′ values. To our knowledge, this potential test for the internal consistency of the model had never been examined previously. In this study we estimated threshold ratios between multiple performance levels at various external noise contrasts in two different experiments: Gabor orientation identification, and Gabor detection. We found that, in both identification and detection, the observed threshold ratios between different performance levels departed substantially from the d′ ratio predicted by the simple noisy linear amplifier model. An elaborated perceptual template model [Vision Res.38, 1183 (1998)] with nonlinear transducer functions and multiplicative noise in addition to the additive noise in the simple linear amplifier model leads to a substantially better description of the data and suggests a reinterpretation of earlier results that relied on the simple noisy linear amplifier model. The relationship of our model and method to other recent parallel and independent developments [J. Opt. Soc. Am. A14, 2406 (1997)] is discussed.
TL;DR: In this article, the lamellar grating problem is reformulated through the concept of adaptive spatial resolution, and a new coordinate system is introduced such that spatial resolution is increased around the discontinuities of the permittivity function.
Abstract: The lamellar grating problem is reformulated through the concept of adaptive spatial resolution. We introduce a new coordinate system such that spatial resolution is increased around the discontinuities of the permittivity function. We derive a new eigenproblem that we solve by using Fourier expansions for both the field and the coefficients of Maxwell’s equations. We provide numerical evidence that highly improved convergence rates can be obtained.
TL;DR: In this article, the authors measured the contrast sensitivity of older subjects through natural pupils and compared the results with those from a group of younger subjects using a crossed-cylinder aberroscope and calculated modulation transfer functions (MTFs) and root-mean-squared (RMS) wave-front aberrations for fixed pupil diameters of 4 mm and 6 mm.
Abstract: We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and compared the results with those from a group of younger subjects. We also measured each subject’s monochromatic ocular wave-front aberrations using a crossed-cylinder aberroscope and calculated their modulation transfer functions (MTF’s) and root-mean-squared (RMS) wave-front aberrations for fixed pupil diameters of 4 mm and 6 mm and for a natural pupil diameter. The CS at a natural pupil diameter and the MTF computed for a fixed pupil diameter were found to be significantly poorer for the older group than for the younger group. However, the older group showed very similar MTF’s and significantly smaller RMS wave-front aberrations compared with the younger group at their natural pupil diameters, owing to the effects of age-related miosis. These results suggest that although monochromatic ocular wave-front aberrations for a given pupil size increase with age, the reduction in CS with age is not due to this increase.
TL;DR: In this paper, a high-speed multichannel ellipsometer with frequency-coupled rotating compensators and a fixed polarizer and analyzer was proposed for real-time Mueller matrix spectroscopy of surface modification and thin-film growth.
Abstract: We describe the design of a high-speed multichannel ellipsometer in the optical configuration PC1r(ω1)SC2r(ω2)A having frequency-coupled rotating compensators (C1r and C2r) and a fixed polarizer and analyzer (P and A) symmetrically placed about the sample (S) on the polarization generation and detection arms of the instrument. For this instrument the frequency-coupled compensators rotate continuously at ω1= 5ω and ω2=3ω, where π/ω is the fundamental optical period. Although the dual rotating-compensator configuration has been proposed and demonstrated earlier, we focus on its extension to real-time Mueller matrix spectroscopy of surface modification and thin-film growth utilizing high-speed multichannel detection with a wide spectral range. The proposed instrument design provides the capability of extracting all 16 elements of the unnormalized Mueller matrix of an evolving sample at 1024 points from 1.5 to 6.5 eV with potential acquisition and repetition times of 0.2 s. Techniques of data acquisition, data reduction, and instrument calibration are described for the general case of arbitrary compensator retardances and polarizer and analyzer angles. We expect that the proposed instrument will have important applications in studies of surfaces and thin films that exhibit anisotropy and inhomogeneity.
TL;DR: In this paper, an efficient finite-difference time-domain algorithm for the analysis of axially symmetric diffractive optical elements is proposed. But the algorithm is not suitable for focusing planar and Gaussian beams.
Abstract: We formulate and apply an efficient finite-difference time-domain algorithm to the analysis of axially symmetric diffractive optical elements. We discuss aspects relating to minimizing numerical dispersion in the incident field, application of absorbing boundary conditions in the radial direction, convergence to a steady state, and propagation of the steady-state electromagnetic fields from the finite-difference time-domain region to the plane of interest. Incorporation of these aspects into a single finite-difference time-domain algorithm results in an extremely efficient and robust method for diffractive optical element analysis. Application to the analysis of subwavelength and multilevel lenses, both with and without loss, for focusing planar and Gaussian beams is presented.
TL;DR: A method for photographing objects behind glass and digitally removing the reflections from the surface of the glass, leaving the image of the objects behind the glass intact.
Abstract: The image of an object can vary dramatically, depending on lighting, specularities, reflections, and shadows. It is often advantageous to separate these incidental variations from the intrinsic aspects of an image. We describe a method for photographing objects behind glass and digitally removing the reflections from the surface of the glass, leaving the image of the objects behind the glass intact. We describe the details of this method, which employs simple optical techniques and independent component analysis and show its efficacy with several examples.
TL;DR: Two- and three-dimensional models for DIC imaging under partially coherent illumination were derived and tested by using phantom specimens viewed with several conventional DIC microscopes and quasi-monochromatic light to show quantitative and qualitative agreement between model and data for several imaging conditions.
Abstract: Imaging models for differential-interference-contrast (DIC) microscopy are presented Two- and three-dimensional models for DIC imaging under partially coherent illumination were derived and tested by using phantom specimens viewed with several conventional DIC microscopes and quasi-monochromatic light DIC images recorded with a CCD camera were compared with model predictions that were generated by using theoretical point-spread functions, computer-generated phantoms, and estimated imaging parameters such as bias and shear Results show quantitative and qualitative agreement between model and data for several imaging conditions
TL;DR: In this paper, the principles of RBS were mathematically modeled to facilitate a better understanding of the origins of the signals obtained, and the foveal area was modeled as a radially symmetric birefringent medium.
Abstract: Retinal birefringence scanning (RBS) is a new technique that is used to detect the fixation of the eye remotely and noninvasively. The method is based on analysis of polarization changes induced by the retina. In this study, the principles of RBS were mathematically modeled to facilitate a better understanding of the origins of the signals obtained. Stokes vector analysis and Mueller matrix multiplication were augmented with Poincare sphere representation. The cornea was modeled as a linear retarder. The foveal area was modeled as a radially symmetric birefringent medium. The model accurately predicted the frequency and phase of RBS signals obtained during central and paracentral fixation. The signal that indicates central fixation during RBS likely results from a combination of the radial birefringence of the Henle fibers and the overlying corneal birefringence.
TL;DR: In this article, the lowest-order Gaussian beam mode is considered in a high-aperture theory based on a new variation of the complex source point model, where combinations of sources and sinks are assumed.
Abstract: The lowest-order Gaussian beam mode is considered in a high-aperture theory based on a new variation of the complex source point model. To avoid singularities, combinations of sources and sinks are assumed. The resultant beam is a rigorous solution of Maxwell’s equations for all space that reduces to the conventional Gaussian beam in the paraxial limit and that is physically realizable. The field in the region of the waist and far from the waist is explored. It is demonstrated that direct rigorous evaluation of the field is feasible.
TL;DR: A theoretical study of one-dimensional blazed-binary gratings shows that the reason for this high efficiency is a waveguiding effect as mentioned in this paper. But this is not the case with standard echelette diffractive elements.
Abstract: Recent experimental and numerical results clearly show that blazed-binary diffractive elements outperform their standard blazed-echelette counterparts in the resonance domain. A theoretical study of one-dimensional blazed-binary gratings shows that the reason for this high efficiency is a waveguiding effect. The electromagnetic study supports the idea that, through waveguiding, a reduction of the shadowing zone is achieved, and thus the efficiency is increased. This is intrinsic to high-frequency binary structures and cannot be achieved with standard echelette diffractive elements.
TL;DR: Spectrally resolved white-light interferometry was used to measure the wavelength dependence of refractive index (i.e., dispersion) for various ocular components and verified its efficacy by accurate measurement of the dispersive properties of water and fused silica.
Abstract: Spectrally resolved white-light interferometry was used to measure the wavelength dependence of refractive index (i.e., dispersion) for various ocular components. Verification of the technique's efficacy was substantiated by accurate measurement of the dispersive properties of water and fused silica, which have both been well-characterized in the past by single-wavelength measurement of the refractive index. The dispersion of bovine and rabbit aqueous and vitreous humors was measured from 400 to 1100 nm. In addition, the dispersion was measured from 400 to 700 nm for aqueous and vitreous humors extracted from goat and rhesus monkey eyes. An unsuccessful attempt was also made to use the technique for dispersion measurement of bovine cornea and lens. The principles of white-light interferometry, including image analysis, measurement accuracy, and limitations of the technique, are discussed. In addition, alternate techniques and previous measurements of ocular dispersion are reviewed.
TL;DR: In this article, an approximate analysis for the propagation of Bessel, Bessel-Gauss, and Gaussian beams with a finite aperture is derived, based on the fact that the circ function can be expanded into an approximate sum of complex Gaussian functions, so that these three beams are typically expressed as a combination of a set of infinite-aperture Bessel Gauss beams.
Abstract: An approximate analysis is derived for the propagation of Bessel, Bessel–Gauss, and Gaussian beams with a finite aperture. This treatment is based on the fact that the circ function can be expanded into an approximate sum of complex Gaussian functions, so that these three beams are typically expressed as a combination of a set of infinite-aperture Bessel–Gauss beams. Correspondingly, the evaluation of the diffracted field distribution of the beams is reduced to the summation of Bessel–Gauss functions. From analytical results, the present approach provides a good description of the diffracted beams in the region far (greater than a factor of the Fresnel distance) from the aperture. A possible extension of this method to other apertured beams is also discussed.
TL;DR: In this paper, a localized approximation of beam shape coefficients in the generalized Lorenz-Mie theory for arbitrary shaped beams is presented. But this approximation is not suitable for the case of Gaussian beams.
Abstract: A so-called localized approximation, allowing one to speed up the evaluation of beam shape coefficients in the generalized Lorenz–Mie theory for spheres, has been previously introduced and, in the case of Gaussian beams, rigorously justified. We examine and demonstrate the validity of this approximation for arbitrary shaped beams.
TL;DR: In this paper, an inversion algorithm is formulated in a Bayesian framework and an efficient optimization technique is presented for calculating the maximum a posteriori image, where the data are modeled as complex Gaussian random vector with shot-noise statistics, and the unknown image is modeled as a generalized Gaussian Markov random field.
Abstract: Frequency-domain diffusion imaging uses the magnitude and phase of modulated light propagating through a highly scattering medium to reconstruct an image of the spatially dependent scattering or absorption coefficients in the medium. An inversion algorithm is formulated in a Bayesian framework and an efficient optimization technique is presented for calculating the maximum a posteriori image. In this framework the data are modeled as a complex Gaussian random vector with shot-noise statistics, and the unknown image is modeled as a generalized Gaussian Markov random field. The shot-noise statistics provide correct weighting for the measurement, and the generalized Gaussian Markov random field prior enhances the reconstruction quality and retains edges in the reconstruction. A localized relaxation algorithm, the iterative-coordinate-descent algorithm, is employed as a computationally efficient optimization technique. Numerical results for two-dimensional images show that the Bayesian framework with the new optimization scheme outperforms conventional approaches in both speed and reconstruction quality.
TL;DR: It is shown that analyses based on both analysis from transport theory and transport-theory-based Monte Carlo calculations, that the diffusion coefficient for photon migration should be taken to be independent of absorption are flawed.
Abstract: A number of investigators have recently claimed, based on both analysis from transport theory and transport-theory-based Monte Carlo calculations, that the diffusion coefficient for photon migration should be taken to be independent of absorption. We show that these analyses are flawed and that the correct way of extracting diffusion theory from transport theory gives an absorption-dependent diffusion coefficient. Experiments by two different sets of investigators give conflicting results concerning whether the diffusion coefficient depends on absorption. The discrepancy between theory and the earlier set of experiments poses an interesting challenge.
TL;DR: In this paper, the power density in the vicinity of a tapered fiber is calculated, with the vectorial model of step-index circular waveguides, for the fundamental HE11 mode carrying a power of 1 Watt, and it is possible to obtain theoretical densities in the range of 108 W/cm2 at the fiber surface.
Abstract: The power density in the vicinity of a tapered fiber is calculated, with the vectorial model of step-index circular waveguides. For the fundamental HE11 mode carrying a power of 1 Watt, we show that it is possible to obtain theoretical densities in the range of 108 W/cm2 at the fiber surface. The promising use of such intense evanescent fields as “atomic mirrors” is considered, and the feasibility of these guides is investigated.
TL;DR: In this paper, a technique was developed to obtain three-dimensional structural information on a length scale well below the Rayleigh length with conventional far-field optics by spectrally selecting a single molecule with high-resolution laser spectroscopy and using a CCD camera to register the spatial distribution of the emitted photons in three dimensions.
Abstract: A technique has been developed to obtain three-dimensional structural information on a length scale well below the Rayleigh length with conventional far-field optics. By spectrally selecting a single molecule with high-resolution laser spectroscopy and using a CCD camera to register the spatial distribution of the emitted photons in three dimensions, one can determine the position of a molecule with unprecedented accuracy. One can resolve details in the specimen with sub-diffraction-limited resolution in three dimensions by applying this procedure to as many molecules as are present in the same diffraction-limited volume and obtaining their mutual positions. The feasibility of this technique is demonstrated for the system of pentacene in p-terphenyl at cryogenic temperatures for which molecules were localized with an accuracy of better than 40 nm in the lateral and 100 nm in the axial directions.
TL;DR: The results demonstrate that age-related neural changes play a major role in the decline in contrast sensitivity with age, and efficiencies were lower for old than for young observers.
Abstract: Contrast sensitivity under photopic conditions declines with age; however, the cause of this decline remains unknown. To address this issue, we measured detection thresholds for sine wave gratings in noise, under various conditions of spatial-frequency uncertainty, and estimated observers' internal noise and calculation efficiency. Statistical analyses revealed that efficiencies were lower for old (median age at 68 years) than for young (median age at 22 years) observers; no significant differences in internal noise were found. A control experiment ruled out the possibility that reduced retinal illuminance causes the decline in efficiency with age. Our results demonstrate that age-related neural changes play a major role in the decline in contrast sensitivity with age. Possible contributing mechanisms are considered.