Showing papers in "Applied Optics in 1996"
TL;DR: This work critically reviewed recent research at the National Physical Laboratory, the International Bureau of Weights and Measures, and elsewhere that has led to revised formulas and data for the dispersion and density of the major components of the atmosphere to yield a set of equations that match recently reported measurements to within the experimental error.
Abstract: The precision of modern length interferometry and geodetic surveying far exceeds the accuracy, which is ultimately limited by the inadequacy of currently used equations for the refractive index of the atmosphere. I have critically reviewed recent research at the National Physical Laboratory, the International Bureau of Weights and Measures, and elsewhere that has led to revised formulas and data for the dispersion and density of the major components of the atmosphere. I have combined selected formulas from these sources to yield a set of equations that match recently reported measurements to within the experimental error, and that are expected to be reliable over very wide ranges of atmospheric parameters and wavelength.
1,209 citations
TL;DR: Avalanche photodiodes, which operate above the breakdown voltage in Geiger mode connected with avalanche-quenching circuits, can be used to detect single photons and are therefore called singlephoton avalanche diodes SPAD's.
Abstract: Avalanche photodiodes, which operate above the breakdown voltage in Geiger mode connected with avalanche-quenching circuits, can be used to detect single photons and are therefore called singlephoton avalanche diodes SPAD's. Circuit configurations suitable for this operation mode are critically analyzed and their relative merits in photon counting and timing applications are assessed. Simple passive-quenching circuits (PQC's), which are useful for SPAD device testing and selection, have fairly limited application. Suitably designed active-quenching circuits (AQC's) make it possible to exploit the best performance of SPAD's. Thick silicon SPAD's that operate at high voltages (250-450 V) have photon detection efficiency higher than 50% from 540- to 850-nm wavelength and still ~3% at 1064 nm. Thin silicon SPAD's that operate at low voltages (10-50 V) have 45% efficiency at 500 nm, declining to 10% at 830 nm and to as little as 0.1% at 1064 nm. The time resolution achieved in photon timing is 20 ps FWHM with thin SPAD's; it ranges from 350 to 150 ps FWHM with thick SPAD's. The achieved minimum counting dead time and maximum counting rate are 40 ns and 10 Mcps with thick silicon SPAD's, 10 ns and 40 Mcps with thin SPAD's. Germanium and III-V compound semiconductor SPAD's extend the range of photon-counting techniques in the near-infrared region to at least 1600-nm wavelength.
1,175 citations
TL;DR: A system is described for making remote measurements of spatially resolved absolute diffuse reflectance and hence noninvasive, noncontact estimates of the tissue optical properties, and it is shown that an often used solution of the diffusion equation cannot be applied for these measurements.
Abstract: The absorption and transport scattering coefficients of biological tissues determine the radial dependence of the diffuse reflectance that is due to a point source. A system is described for making remote measurements of spatially resolved absolute diffuse reflectance and hence noninvasive, noncontact estimates of the tissue optical properties. The system incorporated a laser source and a CCD camera. Deflection of the incident beam into the camera allowed characterization of the source for absolute reflectance measurements. It is shown that an often used solution of the diffusion equation cannot be applied for these measurements. Instead, a neural network, trained on the results of Monte Carlo simulations, was used to estimate the absorption and scattering coefficients from the reflectance data. Tests on tissue-simulating phantoms with transport scattering coefficients between 0.5 and 2.0 mm21 and absorption coefficients between 0.002 and 0.1 mm21 showed the rms errors of this technique to be 2.6% for the transport scattering coefficient and 14% for the absorption coefficients. The optical properties of bovine muscle, adipose, and liver tissue, as well as chicken muscle 1breast2, were also measured ex vivo at 633 and 751 nm. For muscle tissue it was found that the Monte Carlo simulation did not agree with experimental measurements of reflectance at distances less than 2 mm from the incident beam.
613 citations
TL;DR: Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed.
Abstract: The software code SKYEAD.pack for retrieval of aerosol size distribution and optical thickness from data of direct and diffuse solar radiation is described; measurements are carried out with sky radiometers in the wavelength range 0.369-1.048 µm. The treatment of the radiative transfer problem concerning the optical quantities is mainly based on the IMS (improved multiple and single scattering) method, which uses the delta-M approximation for the truncation of the aerosol phase function and corrects the solution for the first- and second-order scattering. Both linear and nonlinear inversion methods can be used for retrieving the size distribution. Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed. The results can be summarized as follows: (a) the SKYRAD.pack code can retrieve the columnar aerosol features with accuracy and efficiency in several environmental situations, provided the input parameters are correctly given; (b) when data of both direct and diffuse solar radiation are used, the detectable radius interval for aerosol particles is approximately from 0.03 to 10 µm; (c) besides the retrieval of the aerosol features, the data-analysis procedure also permits the determination of average values for three input parameters (real and imaginary aerosol refractive index, ground albedo) from the optical data; (d) absolute calibrations for the sky radiometer are not needed, and calibrations for direct and diffuse radiation can be carried out with field data; (e) the nonlinear inversion gives satisfactory results in a larger radius interval, without the unrealistic humps that occur with the linear inversion, but the results strongly depend on the first-guess spectrum; (f) aerosol features retrieved from simulated data showed a better agreement with the given data for the linear inversion than for the nonlinear inversion.
504 citations
TL;DR: A method is proposed that permits the derivation of algorithms that satisfy both requirements of phase-stepping methods, up to any arbitrary order, based on a one-to-one correspondence between an algorithm and a polynomial.
Abstract: If the best phase measurements are to be achieved, phase-stepping methods need algorithms that are 112 insensitive to the harmonic content of the sampled waveform and 122 insensitive to phase-shift miscalibration. A method is proposed that permits the derivation of algorithms that satisfy both requirements, up to any arbitrary order. It is based on a one-to-one correspondence between an algorithm and a polynomial. Simple rules are given to permit the generation of the polynomial that corresponds to the algorithm having the prescribed properties. These rules deal with the location and multiplicity of the roots of the polynomial. As a consequence, it can be calculated from the expansion of the products of monomials involving the roots. Novel algorithms are proposed, e.g., a six-sample one to eliminate the effects of the second harmonic and a 10-sample one to eliminate the effects of harmonics up to the fourth order. Finally, the general form of a self-calibrating algorithm that is insensitive to harmonics up to an arbitrary order is given.
485 citations
TL;DR: It is demonstrated that the novel geometric-optics method can be applied to the computation of the extinction cross section and single-scattering albedo for ice crystals with size parameters along the minimum dimension as small as ~6 and overall agreement has been obtained for the phase function.
Abstract: A new geometric-optics model has been developed for the calculation of the single-scattering and polarization properties for arbitrarily oriented hexagonal ice crystals. The model uses the ray-tracing technique to solve the near field on the ice crystal surface, which is then transformed to the far field on the basis of the electromagnetic equivalence theorem. From comparisons with the results computed by the finite-difference time domain method, we show that the novel geometric-optics method can be applied to the computation of the extinction cross section and single-scattering albedo for ice crystals with size parameters along the minimum dimension as small as ~6. Overall agreement has also been obtained for the phase function when size parameters along the minimum dimension are larger than ~20. We demonstrate that the present model converges to the conventional ray-tracing method for large size parameters and produces single-scattering results close to those computed by the finite-difference time domain method for size parameters along the minimum dimension smaller than ~20. The present geometric-optics method can therefore bridge the gap between the conventional ray-tracing and the exact numerical methods that are applicable to large and small size parameters, respectively.
457 citations
TL;DR: This work aims to quantify systematically the variations of spectral radiances that emerge from an ocean with varying Chl when the authors change the geometric conditions, namely, the zenith-Sun angle, the viewing angle, and the azimuth difference between the solar and observational vertical planes.
Abstract: The upwelling radiance field beneath the ocean surface and the emerging radiance field are not generally isotropic. Their bidirectional structure depends on the illumination conditions (the Sun's position in particular) and on the optical properties of the water body. In oceanic case 1 waters, these properties can be related, for each wavelength λ, to the chlorophyll (Chl) concentration. We aim to quantify systematically the variations of spectral radiances that emerge from an ocean with varying Chl when we change the geometric conditions, namely, the zenith-Sun angle, the viewing angle, and the azimuth difference between the solar and observational vertical planes. The consequences of these important variations on the interpretation of marine signals, as detected by a satelliteborne ocean color sensor, are analyzed. In particular, the derivation of radiometric quantities, such as R (λ), the spectral reflectance, or [ L(w)(λ)](N), the normalized water-leaving radiance that is free from directional effects, is examined, as well as the retrieval of Chl. We propose a practical method that is based on the use of precomputed lookup tables to provide values of the f/Q ratio in all the necessary conditions[ f relates (R(λ) to the backscattering and absorption coefficients, whereas Q is the ratio of upwelling irradiance to any upwelling radiance]. The f/Q ratio, besides being dependent on the geometric configuration (the three angles mentioned above), also varies with λ and with the bio-optical state, conveniently depicted by Chl. Because Chl is one of the entries for the lookup table, it has to be derived at the beginning of the process, before the radiometric quantities R(λ) or [L(W)(λ)](N) can be produced. The determination of Chl can be made through an iterative process, computationally fast, using the information at two wavelengths. In this attempt to remove the bidirectional effect, the commonly accepted view relative to the data-processing strategy is somewhat modified, i.e., reversed, as the Chl index becomes a prerequisite parameter that must be identified prior to the derivation of the fundamental radiometric quantities at all wavelengths.
398 citations
TL;DR: A new analysis procedure was developed, modeling atmospheric spectra with the absorption structures of the individual trace gases, to determine their concentrations and corrects differences in the wavelength-pixel mapping of these spectra.
Abstract: Differential optical absorption spectroscopy (DOAS) has become a widely used method to measure trace gases in the atmosphere. Their concentration is retrieved by a numerical analysis of the atmospheric absorption spectra, which often are a combination of overlapping absorption structures of several trace gases. A new analysis procedure was developed, modeling atmospheric spectra with the absorption structures of the individual trace gases, to determine their concentrations. The procedure also corrects differences in the wavelength-pixel mapping of these spectra. A new method to estimate the error of the concentrations considers the uncertainty of this correction and the influence of random residual structures in the absorption spectra.
328 citations
TL;DR: The authors' experience with the application of the needle optimization technique to the design of optical coating is summarized and a physical interpretation of the technique is provided.
Abstract: The authors' experience with the application of the needle optimization technique to the design of optical coating is summarized. A physical interpretation of the technique is provided, and its main features are identified. Guidelines on the application of the needle optimization technique to various types of design problems are given.
309 citations
TL;DR: The results show that, when compared with conventional imaging, PDI yields a factor of 2-3 increase in the distance at which certain target features can be detected.
Abstract: Polarization-difference imaging (PDI) was recently presented by us as a method of imaging through scattering media [Opt. Lett. 20, 608 (1995)]. Here, PDI is compared with conventional, polarizationblind imaging systems under a variety of conditions not previously studied. Through visual and numerical comparison of polarization-difference and polarization-sum images of metallic targets suspended in scattering media, target features initially visible in both types of images are shown to disappear in polarization-sum images as the scatterer concentration is increased, whereas these features remain visible in polarization-difference images. Target features producing an observed degree of linear polarization of less than 1% are visible in polarization-difference images. The ability of PDI to suppress partially polarized background variations selectively is demonstrated, and discrimination of target features on the basis of polarization information is discussed. Our results show that, when compared with conventional imaging, PDI yields a factor of 2-3 increase in the distance at which certain target features can be detected.
294 citations
TL;DR: The celestial objects were observed for 1.5 h as they passed through the meridian and the shape of the reflecting Schmidt plate has to be changed with each different declination δ and in the tracking process.
Abstract: A special reflecting Schmidt telescope is used to observe celestial objects The telescope has an aperture of 4m, f ratio of 5, and a 5° field of view Its optical axis is fixed and tilted 25° to the horizontal that runs from south to north The celestial objects were observed for 15 h as they passed through the meridian The shape of the reflecting Schmidt plate has to be changed with each different declination δ and in the tracking process This is achieved with active optics The sky area to be observed is -10° ≤ δ ≤ +90° There are plans to place ~4000 optical fibers on the telescope focal surface that will lead to a dozen spectrographs
TL;DR: The MIPAS (Michelson interferometer for passive atmospheric sounding) balloon-borne and space-based experiments apply the Fourier-transform infrared spectrometers to limb-emission sounding by the use of cryogenic instrumentation to study ozone trends and periods of significant ozone depletion.
Abstract: A deeper understanding of long-term ozone trends and periods of significant ozone depletion as well as of the anthropogenic greenhouse effect requires the concerted actions of experimenters and modelers. With respect to observations, atmospheric constituents need to be measured simultaneously and on a global basis. Fourier-transform infrared spectrometers are especially suited for this measurement task. Apromising and challenging branch of Fourier-transform infrared spectroscopy is its application to limb-emission sounding by the use of cryogenic instrumentation. This method allows the measurements to be made independently of the time of the day. The MIPAS (Michelson interferometer for passive atmospheric sounding) balloon-borne (MIPAS-B) and space-based (MIPAS-S) experiments apply this technique. While the MIPAS-B instrument has already been used for several years for stratospheric process studies, the MIPAS-S instrument is in development for the European Space Agency's ENVISAT mission. Instrumental aspects of these MIPAS experiments are highlighted, the most important results in ozone research achieved with MIPAS-B are reviewed, and a brief overview of the scientific capabilities of the MIPAS space experiment is given.
TL;DR: A general theory for fiber-optic, evanescent-wave spectroscopy and sensors is presented for straight, uncladded, step-index, multimode fibers and an three-dimensional model is formulated within the framework of geometric optics.
Abstract: A general theory for fiber-optic, evanescent-wave spectroscopy and sensors is presented for straight, uncladded, step-index, multimode fibers. A three-dimensional model is formulated within the framework of geometric optics. The model includes various launching conditions, input and output end-face Fresnel transmission losses, multiple Fresnel reflections, bulk absorption, and evanescent-wave absorption. An evanescent-wave sensor response is analyzed as a function of externally controlled parameters such as coupling angle, f number, fiber length, and diameter. Conclusions are drawn for several experimental apparatuses.
TL;DR: The performance of shift-multiplexed memories is characterized by using a spherical wave as the reference beam and the shift selectivity, the cross talk, the exposure schedule, and the storage density of the method are derived.
Abstract: Shift multiplexing is a holographic storage method particularly suitable for the implementation of holographic disks. We characterize the performance of shift-multiplexed memories by using a spherical wave as the reference beam. We derive the shift selectivity, the cross talk, the exposure schedule, and the storage density of the method. We give experimental results to verify the theoretical predictions.
TL;DR: The results suggested that the RDG-FA theory is a reasonable approximation for optics of a wide range of fractal aggregates, considerably extending its domain of applicability.
Abstract: The range of validity of the Rayleigh–Debye–Gans approximation for the optical cross sections of fractal aggregates (RDG-FA) that are formed by uniform small particles was evaluated in comparison with the integral equation formulation for scattering (IEFS), which accounts for the effects of multiple scattering and self-interaction. Numerical simulations were performed to create aggregates that exhibit mass fractallike characteristics with a wide range of particle and aggregate sizes and morphologies, including xp = 0.01–1.0, |m − 1| = 0.1–2.0, N = 16–256, and Df = 1.0–3.0. The percent differences between both scattering theories were presented as error contour charts in the |m − 1|xp domains for various size aggregates, emphasizing fractal properties representative of diffusion-limited cluster–cluster aggregation. These charts conveniently identified the regions in which the differences were less than 10%, between 10% and 30%, and more than 30% for easy to use general guidelines for suitability of the RDG-FA theory in any scattering applications of interest, such as laser-based particulate diagnostics. Various types of aggregate geometry ranging from straight chains (Df ≈ 1.0) to compact clusters (Df ≈ 3.0) were also considered for generalization of the findings. For the present computational conditions, the RDG-FA theory yielded accurate predictions to within 10% for |m − 1| to approximately 1 or more as long as the primary particles in aggregates were within the Rayleigh scattering limit (xp ≤ 0.3). Additionally, the effect of fractal dimension on the performance of the RDG-FA was generally found to be insignificant. The results suggested that the RDG-FA theory is a reasonable approximation for optics of a wide range of fractal aggregates, considerably extending its domain of applicability.
TL;DR: The performance specifications of a holographic three-dimensional disk system are experimentally characterized and a surface density of 10 bits/µm(2) is experimentally demonstrated with a 100-µM-thick photopolymer as the recording medium.
Abstract: The performance specifications of a holographic three-dimensional disk system are experimentally characterized. A surface density of 10 bits/μm^2 is experimentally demonstrated with a 100-μm-thick photopolymer as the recording medium.
TL;DR: The adaptive digital filter is developed, which not only permits us to use this converter to obtain measurements at arbitrary times but as a bonus shows us how to move much of the complexity of an interferometric-control and data-acquisition system from hardware to software.
Abstract: Laser fringes have long been used to establish the x axis in interferometric spectrometry, but solutions for the intensity axis have been less satisfactory. Now we are seeing the rapid commercial development of low-cost, medium-speed, sigma–delta analog-to-digital converters developed for stereo audio applications. A single chip provides two channels of 20-bit precision at 50 kHz, a significant improvement over many current systems of much greater cost and complexity. But while the laser works in the spatial domain, this converter operates strictly in the time domain; it cannot be triggered. I have developed a bridge between these two domains, the adaptive digital filter, which not only permits us to use this converter to obtain measurements at arbitrary times but as a bonus shows us how to move much of the complexity of an interferometric-control and data-acquisition system from hardware to software. For example, flexible fringe subdivision (to increase the free spectral range) is easily obtained with a simple and efficient algorithm, completely free of laser ghosts. Compensation for drive velocity variation is also possible, requiring only a modest increase in computer memory.
TL;DR: An encrypted holographic data-storage system that combines orthogonal-phase-code multiplexing with a random-phase key that retains the low cross-talk performance and the minimum code storage requirements typical in an orthogonic- phase-code-multiplexing system is described.
Abstract: An encryption method and apparatus for holographic data storage are disclosed. In a system using orthogonal phase-code multiplexing, data is encrypted by modulating the reference beam using an encryption key K represented by a unitary operator. In practice, the encryption key K corresponds to a diffuser or other phase-modulating element placed in the reference beam path, or to shuffling the correspondence between the codes of an orthogonal phase function and the corresponding pixels of a phase spatial light modulator. Because of the lack of Bragg selectivity in the vertical direction, the phase functions used for phase-code multiplexing are preferably one dimensional. Such phase functions can be one-dimensional Walsh functions. The encryption method preserves the orthogonality of reference beams, and thus does not lead to a degradation in crosstalk performance.
TL;DR: Monte Carlo radiative transfer simulation of light scattering in planetary atmospheres is not a simple problem, especially the study of angular distribution of light intensity.
Abstract: Monte Carlo radiative transfer simulation of light scattering in planetary atmospheres is not a simple problem, especially the study of angular distribution of light intensity. Approximate phase functions such as Henyey-Greenstein, modified Henyey-Greenstein, or Legendre polynomial decomposition are often used to simulate the Mie phase function. An alternative solution using an exact calculation alleviates these approximations.
TL;DR: The InGaAs/lnP avalanche photodiodes, designed for optical receivers and range finders, can be operated biased above the breakdown voltage, achieving single-photon sensitivity, and the performance of state-of-the-art detectors in the near IR is compared.
Abstract: Commercially available InGaAs/InP avalanche photodiodes, designed for optical receivers and range finders, can be operated biased above the breakdown voltage, achieving single-photon sensitivity. We describe in detail how to select the device for photon-counting applications among commercial samples. Because of the high dark-counting rate the detector must be cooled to below 100 K and operated in a gated mode. We achieved a noise equivalent power of 3 × 10−16 W/Hz1/2 to a 1.55-μm wavelength and a time resolution well below 1 ns with a best value of 200-ps FWHM. Finally we compare these figures with the performance of state-of-the-art detectors in the near IR, and we highlight the potentials of properly designed InGaAs/InP avalanche photodiodes in single-photon detection.
TL;DR: It is shown that polarization holography may be used for real-time simultaneous measurement of photoinduced linear and circular birefringence in thin phase polarization holographic gratings recorded with two waves with orthogonal linear polarizations in materials in which illumination with linearly/ Circularly polarized light gives rise to linear/circular bireFringence.
Abstract: We investigate thin phase polarization holographic gratings recorded with two waves with orthogonal linear polarizations in materials in which illumination with linearly/circularly polarized light gives rise to linear/circular birefringence. The theoretical analysis shows that the presence of circular photoanisotropy changes significantly the diffraction characteristics of the gratings. The intensities of the waves diffracted in the +1 and −1 orders of diffraction and their ratio depend substantially on the reconstructing-wave polarization. Experiments with films of side-chain liquid-crystalline azobenzene polyester that is a photoanisotropic material of the considered type confirm the unusual polarization properties. It is shown that polarization holography may be used for real-time simultaneous measurement of photoinduced linear and circular birefringence.
TL;DR: Experiments with fluorescent tissue phantoms demonstrated that intrinsic fluorescence line shape could be recovered and that fluorophore concentration could be estimated within ±15%, over a wide range of optical properties.
Abstract: We present a method for recovering the intrinsic fluorescence coefficient, defined as the product of the fluorophore absorption coefficient and the fluorescence energy yield, of an optically thick, homogeneous, turbid medium from a surface measurement of fluorescence and from knowledge of medium optical properties The measured fluorescence signal is related to the intrinsic fluorescence coefficient by an optical property dependent path-length factor A simple expression was developed for the path-length factor, which characterizes the penetration of excitation light and the escape of fluorescence from the medium Experiments with fluorescent tissue phantoms demonstrated that intrinsic fluorescence line shape could be recovered and that fluorophore concentration could be estimated within ±15%, over a wide range of optical properties
TL;DR: If either the test part or the reference surface in an interferometric test is rotated to N equally spaced positions about the optical axis and the resulting wave fronts are averaged, then errors in the rotated member with angular orders that are not integer multiples of the number of positions will be removed.
Abstract: Wave-front or surface errors may be divided into rotationally symmetric and nonrotationally symmetric terms. It is shown that if either the test part or the reference surface in an interferometric test is rotated to N equally spaced positions about the optical axis and the resulting wave fronts are averaged, then errors in the rotated member with angular orders that are not integer multiples of the number of positions will be removed. Thus if the test piece is rotated to N equally spaced positions and the data rotated back to a common orientation in software, all nonrotationally symmetric errors of the interferometer except those of angular order kNθ are completely removed. It is also shown how this method may be applied in an absolute test, giving both rotationally symmetric and nonsymmetric components of the surface. A general proof is given that assumes only that the surface or wave-front information can be described by some arbitrary set of orthognal polynomials in a radial coordinate r and terms in sin θ and cos θ. A simulation, using Zernike polynomials, is also presented.
TL;DR: The design of the instrument is described and methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements are discussed, which have permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters.
Abstract: A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-µm and 8-14-µm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 µm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-µm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-µm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-µm and 8-14-µm atmospheric windows.
TL;DR: Three different image-processing methods based on the time-averaged technique were compared by the electronic speckle pattern interferometry (ESPI) technique for vibration measurement to increase the fringe pattern's visibility without additional image and/or signal processes.
Abstract: Three different image-processing methods based on the time-averaged technique were compared by the electronic speckle pattern interferometry (ESPI) technique for vibration measurement The three methods are the video-signal-addition method, the video-signal-subtraction method, and the amplitude-fluctuation method Also, errors introduced by using the zero-order Bessel function directly into the analysis of the fringe pattern were investigated The video-signal-addition method has been the most generally used ESPI technique for vibration measurement However, without additional image and/or signal-processing procedures, the fringe pattern obtained directly by the video-signal-addition method is rather difficult to observe The reason for poor visibility of the experimentally obtained fringe pattern with this method is explained To increase the fringe pattern's visibility without additional image and/or signal processes, we tried two video-signal-subtraction methods One of the two methods is the video-signal-subtraction method that has normally been used in the static applications The other method, called the amplitude-fluctuation method, and its associated theory are reported here
TL;DR: Theoretical and experimental aspects of an extrinsic optical-fiber ultrasound sensor based on a thin transparent polymer film acting as a low-finesse Fabry-Perot cavity that is mounted at the end of a multimode optical fiber suggest that it may find application as an alternative to piezoelectric devices for the detection and measurement of ultrasound.
Abstract: Theoretical and experimental aspects of an extrinsic optical-fiber ultrasound sensor are described. The sensor is based on a thin transparent polymer film acting as a low-finesse Fabry-Perot cavity that is mounted at the end of a multimode optical fiber. Performance was found to be comparable with that of a piezoelectric polyvinylidene dinuoride-membrane (PVDP) hydrophone with a sensitivity of 61 mV/MPa, an acoustic noise floor of 2.3 KPa over a 25-MHz bandwidth, and a frequency response to 25 MHz. The wideband-sensitive response and design flexibility of the concept suggests that it may find application as an alternative to piezoelectric devices for the detection and measurement of ultrasound.
TL;DR: Investigation of how the internal three-dimensional structure of the tissues and the optical properties of cell constituents control the reflectance and transmittance of the leaf finds that convex cells in the epidermis focus light on the palisade parenchyma and increase the absorption of radiation.
Abstract: The propagation of light in a typical dicotyledon leaf is investigated with a new Monte Carlo ray-tracing model. The three-dimensional internal cellular structure of the various leaf tissues, including the epidermis, the palisade parenchyma, and the spongy mesophyll, is explicitly described. Cells of different tissues are assigned appropriate morphologies and contain realistic amounts of water and chlorophyll. Each cell constituent is characterized by an index of refraction and an absorption coefficient. The objective of this study is to investigate how the internal three-dimensional structure of the tissues and the optical properties of cell constituents control the reflectance and transmittance of the leaf. Model results compare favorably with laboratory observations. The influence of the roughness of the epidermis on the reflection and absorption of light is investigated, and simulation results confirm that convex cells in the epidermis focus light on the palisade parenchyma and increase the absorption of radiation.
TL;DR: The wavelengths of 19 spectral lines in the region 253-579 nm emitted by Hg pencil-type lamps were measured by Fourier-transform spectroscopy and should be useful as wavelength-calibration standards for moderate-resolution spectrometers at an uncertainty level of 0.0001 nm.
Abstract: The wavelengths of 19 spectral lines in the region 253-579 nm emitted by Hg pencil-type lamps were measured by Fourier-transform spectroscopy. Precise calibration of the spectra was obtained with wavelengths of (198)Hg as external standards. Our recommended values should be useful aswavelength-calibration standards for moderate-resolution spectrometers at an uncertainty level of 0.0001 nm.
TL;DR: Radiation pressure exerted on a spherical particle by one extremely focused Gaussian beam is investigated by the use of generalized Lorenz-Mie theory by comparison with electrostriction predictions and geometric-optics predictions when the particle size is larger than the wavelength.
Abstract: Radiation pressure exerted on a spherical particle by one extremely focused Gaussian beam is investigated by the use of generalized Lorenz–Mie theory (GLMT) Particular attention is devoted to reverse radiation pressure GLMT predictions for different descriptions of the incident beam are compared with electrostriction predictions when the particle size is smaller than the wavelength and with geometric-optics predictions when the particle size is larger than the wavelength
TL;DR: The experimental demonstration and the far-field pattern characterization of an optically controlled phased-array antenna are described, which provides 2(N-1) time-delay values and an analog control of the 0 to 2π phase for each of the p × p signals feeding the antenna.
Abstract: The experimental demonstration and the far-field pattern characterization of an optically controlled phased-array antenna are described. It operates between 2.5 and 3.5 GHz and is made of 16 radiating elements. The optical control uses a two-dimensional architecture based on free-space propagation and on polarization switching by N spatial light modulators of p × p pixels. It provides 2N−1 time-delay values and an analog control of the 0 to 2π phase for each of the p × p signals feeding the antenna (N = 5, p = 4).