Showing papers in "Optical Engineering in 1995"

Optical Methods of Engineering Analysis

Abstract: Introduction 1. Light and interference 2. Classical interferometry 3. Photoelasticity theory 4. Basic applied photoelasticity 5. Photoelasticity methods and applications 6. Geometrical moire theory 7. In-plane motion and strain measurement 8. Moire mapping of slope, contour and displacement 9. Diffraction and Fourier optics 10. Moire with diffraction and Fourier optical processing 11. Procedures of moire analysis with optical processing 12. Principles of moire interferometry 13. A moire interferometer 14. Experimental methods in moire interferometry 15. Holographic interferometry theory 16. Holographic interferometry methods 17. Laser speckle and combinations of speckle fields 18. Speckle photography 19. Speckle correlation interferometry 20. Electronic speckle pattern interferometry 21. Phase shifting to improve interferometry.

Frequency-domain multichannel optical detector for noninvasive tissue spectroscopy and oximetry

Abstract: We have designed a multisource frequency-domain spectrometer for the optical study of biological tissues. Eight multiplexed, intensity-modulated LEDs are employed as the light sources. Four of them emit light at a peak wavelength of 715 nm (λ1); the other four, 850 nm (λ2). The frequency of intensity modulation is 120 MHz. This instrument measures the frequency-domain parameters phase, dc intensity, and ac amplitude at the two wavelengths λ1 and λ2 and for different distances between light source and detector. From these frequency-domain raw data, the absolute values of the absorption and reduced scattering coefficients of tissue at λ1 and λ2 are obtained. The oxy- and deoxyhemoglobin concentrations, and hence the hemoglobin saturation, are then analytically derived from the molar extinction coefficients. Acquisition times as short as hundreds of milliseconds provide real-time monitoring of the measured parameters. We performed a systematic test in vitro to quantify the precision and accuracy of the instrument reading. We also report in vivo measurements. This spectrometer can be packaged as a compact portable unit.

Volume holographic memory systems: techniques and architectures

Abstract: Although the concept of using multiplexed holography for data storage has been considered for some time, recent advances in several critical device technologies along with developments in storage materials have greatly enhanced the likelihood of successful implementations. We review several basic architectural concepts along with various multiplexing options and associated techniques for holographic data storage.

Control of solid state laser dynamics by semiconductor devices

Abstract: We have successfully demonstrated that an appropriately designed semiconductor saturable absorber device, the antiresonant Fabry-Perot saturable absorber, can reliably start and sustain stable mode locking of solid state lasers such as Nd:YAG, Nd:YLF, Nd:Glass, Cr:LiSAF, and Ti:sapphire lasers. Especially for solid state lasers with long upper-state lifetimes, previous attempts to produce self-starting passive mode locking with saturable absorbers was always accompanied by self-Q-switching. We derive criteria that characterize the dynamic behavior of solid state lasers in the important regimes of Q-switching, mode-locked Q-switching, and continuous-wave mode locking in the picosecond and femtosecond range for the pulsewidth. We demonstrate that semiconductor absorbers can be designed to predetermine the dynamic behavior of a laser for a given solid state laser material and present an experimental verification. This allows for the development and design of robust, compact pico- and femtosecond solid state laser sources for scientific and industrial applications.

Mueller matrix imaging polarimetry

Abstract: The design and operation of a Mueller matrix imaging polarimeter is presented. The instrument is configurable to make a wide variety of polarimetric measurements of optical systems and samples. In one configuration, it measures the polarization properties of a set of ray paths through a sample. The sample may comprise a single element, such as a lens, polarizer, retarder, spatial light modulator, or beamsplitter, or an entire optical system containing many elements. In a second configuration, it measures an optical system's point spread matrix, a Mueller matrix relating the polarization state of a point object to the distribution of intensity and polarization across the image. The instrument is described and a number of example measurements are provided that demonstrate the Mueller matrix imaging polarimeter's unique measurement capability.

Three-dimensional microscopy by optical scanning holography

Abstract: We first briefly review a new 3-D imaging technique called optical scanning holography (OSH). We then discuss the technique's 3-D holographic magnification in the context of optical scanning and digital reconstruction. Finally, we demonstrate the 3-D imaging capability of OSH by holographically recording two planar objects at different depths and reconstructing the hologram digitally.

Measurement of the thickness of fundus layers by partial coherence tomography

Abstract: In the past few years, a new noninvasive optical ranging technique, partial coherence interferometry, has been developed to measure various intraocular distances. A dual-beam version of this method offers high longitudinal resolution by using laser light with high spatial coherence but short coherence length-15 μm (full width at half maximum)-emitted by a special super luminescent diode. This technique is extended to obtain measurements not only parallel to the vision axis but at arbitrary horizontal and vertical angles to it. This is achieved by a new instrument, a fully computer controlled scanning partial coherence interferometer. By tilting the laser beam in horizontal and vertical directions, this scanning partial coherence interferometer measures the distance from the anterior corneal surface to different points of the retina. These results are then plotted to form topographic images containing information about the contour and the thickness profile of different retinal structures, e.g., the retinal thickness and the retinal nerve fiber layer thickness. Provided that there is no other strong reflection nearby, the absolute position of these retinal layers (respective to the cornea as a reference surface) can be determined in vivo with a precision of 5 μm. Furthermore, the intensities of multiple longitudinal scans at different angles between vision axis and measurement direction can be converted into pixel colors and mounted to form a 2-D false color image. These tomograms show the contour and the structure of different retinal layers.

Optimum Angles for a Polarimeter: Part II

Abstract: The four optimum rotation angles for a polarimeter consisting of a quarter-wave plate in conjunction with a linear polarizer are found. This is done by using the determinant and condition numbers of the system measurement matrix as objective functions in a minimization procedure. The four angles so found result in the polarimeter's estimate of the incident Stokes vector to have a minimum sensitivity with respect to fluctuations in the detected flux and errors in the angular position of the optical components constituting the polarimeter. Four optimal angles are presented. These four angles also retain a simplicity of form for the measurement matrix.

Concept of polarization entropy in optical scattering

Abstract: We consider the application of the general theory of unitary matrices to problems of wave scattering involving polarized waves. Haying outlined useful parameterizations of the low dimensional groups associated with these unitary matrices, we develop a general processing strategy, which we suggest has application in the extraction of physical information from a range of scattering matrices in optics. Examples are presented of applying the unitary matrix structure to problems of single and multiple scattering from a cloud of random particles. The techniques are best suited to characterization of depolarizing systems, where the scattered waves undergo a change of degree as well as polarization state. The degree of disorder of the system is then quantified by a scalar, the polarimetric entropy, defined from the eigenvalues of a scattering matrix that ranges from 0 for systems with zero scattering to 1 for perfect depolarizers. Further, we show that the unitary matrix parameterization can be used to extract important system information from the eigenvectors of this matrix.

Measurement of the modulation transfer function of infrared cameras

Abstract: The performance of starting PtSi infrared cameras is characterized based on estimating their spatial frequency response. Applying a modified knife-edge technique, we arrive at an estimate of the edge spread function (ESF), which is used to obtain a profile through the center of the 2-D modulation transfer function (MTF). Using this technique, the complete system MTF in the horizontal and vertical direction is measured for various imaging systems. The influence of charge transfer efficiency (CTE) on the knife-edge measurement and resulting MTF is also modeled and discussed. An estimate of the OlE can actually be obtained from the shape of the ESF in the horizontal direction. In addition, we demonstrate that this technique can be used as a field measurement. By applying the technique at long range, the MTF of the atmosphere can be measured.

Topical measurement of fundus pulsations

Abstract: An instrument suitable for fundus pulsation measurements at preselected points of the retina is developed. Fundus pulsations are measured with a laser interferometer using a single-mode laser diode. Part of the laser light is reflected from the front side of the cornea and part of it is reflected from the retina. The interferences produced by the two remitted waves are detected with a linear CCD array. Evaluation of the interferogram yields the time course of the fundus pulsations. This laser interferometer is coupled to a fundus camera. The setup enables real-time inspection of the laser spot on the fundus. Measurements on seven healthy young subjects are presented. Fundus pulsations in the macular region are significantly smaller than those in the optical disk, but significantly larger than those in peripheral regions of the retina. The association between our results and the subretinal vasculature is discussed and possible future applications of our method are presented.

Laser-induced fluorescence spectroscopy at endoscopy: tissue optics, Monte Carlo modeling, and in vivo measurements

Abstract: The optical properties (absorption coefficient, scattering coefficient and the anisotropic factor of scattering) and fluorescence characteristics of normal and abnormal bronchial tissue were measured in vitro. After adding additional blood optical properties to in vitro optical properties of tissue, the in vivo bronchial fluorescence was simulated and analyzed by Monte Carlo modeling. The Monte Carlo simulation results showed that with an appropriate illumination and fluorescence collection geometry, the distortion of in vivo fluorescence spectra of tissue caused by variations of optical properties at different wavelengths could be much reduced. Based on these results, a spectrofluorometry system was developed for the collection of in vivo laser-induced fluorescence spectra of tissue during endoscopy. In comparing the in vivo fluorescence spectral shape of bronchial tissue collected by this system with the intrinsic one obtained in vitro, we found no obvious distortion in the in vivo spectra. This was completely consistent with the analysis of Monte Carlo modeling. The in vivo measurement results demonstrated that significant differences in fluorescence intensity between normal and diseased bronchial tissue (dysplasia, carcinoma in situ) can be used to differentiate them from each other. Also, changes in fluorescence intensity are more robust for detecting abnormal tissues than the differences in spectral characteristics.

Attentive mechanisms for dynamic and static scene analysis

Abstract: Attention mechanisms extract regions of interest from image data to reduce the amount of information to be analyzed by time-consuming processes such as image transmission, robot navigation, and object recognition. Two such mechanisms are described. The first one is an alerting system that extracts moving objects in a sequence through the use of multiresolution representations. The second one detects regions in still images that are likely to contain objects of interest. Two types of cues are used and integrated to compute the measure of interest. First, bottom-up cues result from the decomposition of the input image into a number of feature and conspicuity maps. The second type of cues is top-down, and is obtained from a priori knowledge about target objects, represented through invariant models. Results are reported for both the alerting and the attention mechanisms using cluttered and noisy scenes.

Two-dimensional Fourier transform profilometry for the automatic measurement of three-dimensional object shapes

Abstract: Two-dimensional Fourier transform profilometry (2-D FTP) for the automatic measurement of three-dimensional (3-D) object shapes is presented and verified by the experiment in this paper. In some applications, in which there are specklelike structures and discontinuity in the observed fringe pattern, 2-D FTP provides a better separation of the height information from noise.

Perceptual-based hyperspectral image fusion using multiresolution analysis

Abstract: A perceptual-based multiresolution image fusion technique is demonstrated using the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) hyperspectral sensor data. The AVIRIS sensor, which simultaneously collects information in 224 spectral bands that range from 0.4 to 2.5 μm in approximately 10-nm increments, produces 224 images, each representing a single spectral band. The fusion algorithm consists of three stages. First, a Daubechies orthogonal wavelet basis set is used to perform a multiresolution decomposition of each spectral image. Next, the coefficients from each image are combined using a perceptual-based weighting. The weighting of each coefficient, from a given spectral band image, is determined by the spatial-frequency response (contrast sensitivity) of the human visual system. The spectral image with the higher saliency value, where saliency is based on a perceptual energy, will receive the larger weight. Finally, the fused coefficients are used for reconstruction to obtain the fused image. The image fusion algorithm is analyzed using test images with known image characteristics and image data from the AVIRIS hyperspectral sensor. By analyzing the signal-to-noise ratios and visual aesthetics of the fused images, contrast-sensitivity-based fusion is shown to provide excellent fusion results and to outperform previous fusion methods.

Temporal and spatial properties of the time-varying speckles of botanical specimens

Abstract: Techniques for measuring time-varying biospeckle of botanical specimens are investigated. Experimental evidence on the probability density function is presented. Several applicable techniques, such as power spectral density and correlation, are used for measuring and analyzing the temporal speckle intensity variations. These techniques are shown to exhibit information about the shelf life and aging of some botanical specimens used in our study. We also present a new phenomena related to the spatial properties of time-varying speckles. Theoretical and experimental results on the size of a speckle in a speckle pattern and its relation to the temporal intensity variation are also detailed.

Highly efficient photorefractive polymers for dynamic holography

Abstract: The performance of recently developed highly efficient photoref_ ract1ve polyme_rs .base~ on the photoconducting polymer host poly( N-vinylcarbazole) 1s 1nvest1gated, and the use of these materials as recording media in dynamic holography and other applications is evaluated.- A diffraction efficiency 'rJ = 86% (limited only by absorption and reflection losses), a two-beam coupling net gain coefficient r = 200 cm - 1 and light-induced refractive index modulations as high as lin=7X 10-3 are demonstrated. Hologram growth rates of the order of 500 ms are observed with. recording light intensities <10 mW /cm2 , using either lowpower laser diodes (675 nm) or a HeNe laser (633 nm). The materials show good sensitivity in this part of the spectrum. Applications such as dynamic time-average interferometry are demonstrated.

Imaging Stokes polarimetry with piezoelastic modulators and charge-coupled-device image sensors

Abstract: A new type of 2-D polarimeter is developed for use in high-resolution observations of solar magnetic fields. Two piezoelastic polarization modulators are used in combination with charge-coupled-device (CCD) image sensors to simultaneously record all four Stokes parameters. Demodulation of the fast 50 and 100 kHz intensity modulations produced by the piezoelastic modulators is achieved by CCD sensors used as synchronous integrators sensitive to a single frequency. The temporary buffer storage needed to separate the charges generated during the two modulation half periods is obtained by covering every second row of the CCD sensors with an opaque mask. The charges are shifted back and forth between the photosensitive uncovered and the adjacent storage rows in phase with the modulation. The polarization signal is calculated from the difference between the charges accumulated in two adjacent rows. A separate CCD sensor is needed for each normalized Stokes parameter Q/I, U/I, or V/I. Because the high modulation frequency lies well above the seeing frequencies occurring in solar observations, precision polarimetry becomes possible. We have demonstrated the capability of this new type of instrument to achieve a polarimetric sensitivity below 10-3 in a single frame. By frame averaging the noise level in the fractional polarization can be reduced to the order of 10-5.

Design and analysis of apposition compound eye optical sensors

Abstract: Methods are presented for designing and estimating the performance of artificial apposition compound eye optical sensors Apposition compound eyes have been investigated for a wide range of applications from robotics to smart weapons While artificial apposition compound eyes have been constructed and demonstrated, optical design issues and performance prediction for these systems have never been adequately addressed in the literature Apposition compound eyes are a useful paradigm for applications where wide field of view is critical but high spatial resolution is not required Natural arthropod compound eyes and their biological models are first scrutinized to give insight into designing and modeling artificial apposition compound eye optical sensors Such sensors are shown to have comparable sensitivity to traditional single-aperture sensors for extended sources A method for enhancing resolution over the extended-source case is demonstrated for single point sources For the first time, the frequency response of an artificial apposition compound eye is addressed while taking into account the differences between artificial and natural ommatidia Both a numercal integration technique for determining the contrast transfer function and a Fourier-transform modulation-transfer-function method are presented and validated with experimental results

General algorithm of phase-shifting interferometry by iterative least-squares fitting

Abstract: A new numerical phase-measuring algorithm for digital phases-hifting interferometry is presented. In this algorithm, the actual values of phase shifts are taken as unknowns together with the wavefront phases ultimately to be determined, and they are computed through iterative spatial and serial least-squares fittings. The algorithm is more general than existing methods in that any arbitrary number of unequal phase shifts may be adopted with no prescriptive restrictions on their spacings. The algorithm is therefore found inherently free from any errors of phase shifts, allowing accurate measurement even in harsh environmental conditions with vibratory disturbances.

Temperature-dependent refractive index models for BaF2, CaF2, MgF2, SrF2, LiF, NaF, KCl, ZnS, and ZnSe

Abstract: A method for developing an accurate temperature-dependent dispersion model is described. It combines a room-temperature Sellmeier dispersion model with temperature-dependent refractive index measurements at a few wavelengths to predict the refractive index over a wide temperature and wavelength region. The method is applied to nine well-characterized materials to give dispersion over a wide temperature and wavelength region.

Aliasing reduction in staring infrared imagers utilizing subpixel techniques

Abstract: We introduce and analyze techniques for the reduction of aliased signal energy in a staring infrared imaging system. A standard staring system uses a fixed two-dimensional detector array that corresponds to a fixed spatial sampling frequency determined by the detector pitch or spacing. Aliasing will occur when sampling a scene containing spatial frequencies exceeding half the sampling frequency. This aliasing can significantly degrade the image quality. The aliasing reduction schemes presented here, referred to as microscanning, exploit subpixel shifts between time frames of an image sequence. These multiple images are used to reconstruct a single frame with reduced aliasing. If the shifts are controlled, using a mirror or beam steerer for example, one can obtain a uniformly sampled microscanned image. The reconstruction in this case can be accomplished by a straightforward interlacing of the time frames. If the shifts are uncontrolled, the effective sampling may be nonuniform and reconstruction becomes more complex. A sampling model is developed and the aliased signal energy is analyzed for the microscanning techniques. Finally, a number of experimental results are presented that illustrate the perlormance of the microscanning methods.

Through focus image quality of eyes implanted with monofocal and multifocal intraocular lenses

Abstract: A double-pass method is applied to determine the retinal image quality of eyes implanted with intraocular lenses (IOLs). The effect of focus on image quality was measured in two groups of patients that had been implanted with either monofocal or multifocal IOLs. The results show that the overall retinal image quality is reduced in eyes with multifocal lenses with respect to that implanted with monofocal IOLs. Although the depth of focus is larger in multifocal lOLs (4 to 5 D) than in the monofocal IOLs (2 to 3 D), some patients implanted with monofocal IOLs have higher image quality than those implanted with multifocal IOLs in a range of about 4 D around the best focus. In eyes implanted with monofocal IOLs, astigmatism plays a major role to reduce the retinal contrast, but also increases the depth of focus. These " in vivo " measurements show that there is considerable variability in image quality among individuals with the same type of monofocal IOLs. The main factors causing this variability seem to be age and astigmatism produced by surgery.

Surface immobilization of biomolecules by light

Abstract: Biomolecules performing specific biological functions on material surfaces are progressively employed in the development of miniaturized bioassays, biosensors, bioelectronic devices, and medical equipment. Device performance is improved with covalently immobilized bioconstituents. The unique advantages of using light-controlled reactions to achieve biomolecule immobilization on surfaces are addressed. On activation of introduced light-sensitive reagents, biomolecules are covalently linked to material surfaces. Procedures leading to light-dependent engineering of surfaces are exceptionally facile. Immobilization by light is compatible with biological functions, enabling surface patterning and molecular coating of materials. Current strategies and protocols are illustrated with selected examples of biomolecule photoimmobilization.

Random transparency targets for modulation transfer function measurement in the visible and infrared regions

Abstract: Measurements of modulation transfer function (MTF), particularly for staring imager systems, are affected by the position of the test target with respect to the rows and columns of the detector array. We demonstrate that random transparency targets of known spatial-frequency content allow shift-invariant MTF measurement in the visible, 3- to 5-μm, and 8- to 12-μm bands. Design criteria and verification procedures for the targets are presented.

High-accuracy distance measurements with multiple-wavelength interferometry

Abstract: Multiple-wavelength interferometry is, like classical interferometry, a coherent method, but it offers great flexibility in sensitivity by an appropriate choice of the different wavelengths and it can be operated on rough surfaces. The accuracy of this method depends essentially on the signal processing and on the properties of the source. Practical considerations for distances in the range of several meters with micrometer resolution are given.

Scalable optical architecture for electronic holography

Abstract: Synthetic-aperture holography represents an attractive approach to the problem of three-dimensional data visualization. However, scaling the technology beyond a proof of concept requires a modification of the original architecture to allow for an increased hologram space-bandwidth product. It is demonstrated how the Fourier domain of a scanned aperture display can be segmented in small domains, each being processed by a different scanning element. The behavior of a display exhibiting such a segmentation is described, and the conditions under which images can be produced without incurring significant degradation are derived. A large-scale display based on the implementation of these concepts is demonstrated.

Twyman-Green interferometer for testing microspheres

Abstract: Testing of spherical surfaces using the Twyman-Green interferometer with a laser source suffers from the presence of spurious interference fringes and dust diffraction. By reducing the spatial coherence while maintaining the temporal coherence a tremendous improvement of the fringe quality can be obtained. To conserve sufficient contrast, the reference mirror must be positioned to suitable locations relative to the test arm of the interferometer. These conditions are described and examples are presented.

Real-time visualization of retinal microcirculation by laser flowgraphy

Abstract: A new real-time measuring system is developed to visualize the blood flow map of the human retina using dynamic laser speckle effects and a hardware logic circuit. The retina is illuminated with a diode laser spot through a retinal camera, and the speckle field at the image plane of the spot is scanned by an area sensor. The output signal is analog-to-digital (AID) converted and the blur rate of the intensity at each pixel point is evaluated through a hardware logic circuit specially devel- oped for this system. The results are displayed in 2-D color maps suc- cessively at a rate of 16 frames/s, which allows the real-time observation of retinal blood flow.

Porous media heat exchangers for cooling of high-power optical components

Abstract: Technologies based on porous media can be used in several classes of heat exchangers that can be used to meet the cooling needs of high heat load optical components as well as other high heat flux applications. These include mechanically pumped single-phase and two-phase porous media heat exchangers, as well as capillary pumped (heat pipe) two-phase designs. A brief overview of each of these classes of heat exchangers is given, and several applications representative of the state of the art are described. Various specific technologies are discussed that have demonstrated the capability to dissipate heat fluxes greater than 1000 W/cm2 over areas of interest in optics applications. Finally, an assessment is given of the capabilities of each approach to meet the needs of specific applications.