Showing papers in "Optical Engineering in 2005"
TL;DR: Multichip and phosphor-type white LED models were analyzed by simulation of their color characteristics and luminous efficacy of radiation, compared with those of conventional light sources for general lighting.
Abstract: White LED spectra for general lighting should be designed for high luminous efficacy as well as good color rendering. Multichip and phosphor-type white LED models were analyzed by simulation of their color characteristics and luminous efficacy of radiation, compared with those of conventional light sources for general lighting. Color rendering characteristics were evaluated based on the CIE Color Rendering Index CRI, examining not only Ra but also the special color rendering indices Ri, as well as on the CIELAB color difference Eab for the 14 color samples defined in CIE 13.3. Several models of three-chip and four-chip white LEDs as well as phosphor-type LEDs are optimized for various parameters, and some guidance is given for designing these white LEDs. The simulation analysis also demonstrated several problems with the current CRI, and the need for improvements is discussed. © 2005
325 citations
TL;DR: In this article, a near-ultraviolet (UV)-based white light-emitting diode (LED) lighting system linked with a semiconductor InGaN LED and compound phosphors for general lighting applications is proposed.
Abstract: A near-ultraviolet (UV)-based white light-emitting diode (LED) lighting system linked with a semiconductor InGaN LED and compound phosphors for general lighting applications is proposed We have developed for the first time a novel type of high-color rendering index (Ra) white LED light source, which is composed of near-UV LED and multiphosphor materials showing orange (O), yellow (Y), green (G), and blue (B) emissions The white LED shows the superior characteristics of luminous efficacy and high Ra to be about 40 lm/W and 93, respectively Luminous and chromaticity characteristics, and their spectral distribution of the present white LED can be evaluated using the multipoint LED light source theory It is revealed that the OYGB white LED can provide better irradiance properties than that of conventional white LEDs Near-UV white LED technologies, in conjunction with phosphor blends, can offer superior color uniformity, high Ra, and excellent light quality Consequently we are carrying out a "white LEDs for medical applications" program in the second phase of this national project from 2004 to 2009
182 citations
Journal Article•
173 citations
TL;DR: A comprehensive system for classification of imaging spectrometers based on two fundamental properties: the method by which they scan the object spatially, and the methodBy which they obtain spectral information.
Abstract: The continuing development of new and fundamentally different classes of imaging spectrometers has increased the complexity of the field of imaging spectrometry. The rapid pace at which new terminology is introduced to describe the new types of imaging spectrometers sometimes leads to confusion, particularly in discussions of the relative merits of the different types. In some cases, multiple different terms are commonly used to describe the same fundamental approach, and it is not always clear when these terms are synonymous. Other terminology in common use is overly broad. When a single term may encompass instruments that operate in fundamentally different ways, important distinctions may be obscured. In the interest of clarifying the terminology used in imaging spectrometry, we present a comprehensive system for classification of imaging spectrometers based on two fundamental properties: the method by which they scan the object spatially, and the method by which they obtain spectral information.
144 citations
TL;DR: In this paper, the physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated, and the measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence.
Abstract: The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2, HfO2, Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.
126 citations
TL;DR: In this article, 3D optical underwater images are computed by applying the method of weighted averages on a sequence of 2D images. But the 3D images are recorded in fresh water and brackish sea water salinity 15 ˚, at 4 to 5 m range.
Abstract: New 3-D optical underwater images are presented. The 3-D images are recorded in fresh water and brackish sea water salinity 15‰, at 4- to 5-m range. For the first time, underwater 3-D images are computed by our new algorithm, which applies the method of weighted averages on a sequence of 2-D images. It is proposed that 3-D gated viewing images can be recorded at any contrast level between 0 and 100%. A novel and dynamic way of measuring the depth of gating is presented. A novel correction for gated viewing 3-D imaging is pre- sented. For the first time, an exact solution of the depth of gating is proposed for a rectangular laser pulse. © 2005 Society of Photo-Optical Instru-
122 citations
TL;DR: This approach is based on a simple nonlinear function, which is deduced by analyzing the geometry of measurement system and hence perfectly describes the mapping between the depth map and the phase-difference distribution.
Abstract: This paper presents a novel calibration approach for determining the mapping relationship between the depth map and the phase difference in fringe projection profilometry. This approach is based on a simple nonlinear function, which is deduced by analyzing the geometry of measurement system and hence perfectly describes the mapping between the depth map and the phase-difference distribution. The calibration is implemented by translating a target plane to a sequence of given positions with known depths, and measuring its phase distributions. A least-squares estimation algorithm with linear computation is deduced to retrieve the related parameters and to reconstruct the mapping function. Both computer simulation and experiment are carried out to demonstrate the validity of this technique.
115 citations
TL;DR: A unified, simplified, and concise overview of spectral target detection algorithms for hyperspectral imaging applications is presented, focusing on detection algorithms derived using established statistical techniques and whose performance is predictable under reasonable assumptions about hyperspectRAL imaging data.
Abstract: A unified, simplified, and concise overview of spectral target detection algorithms for hyperspectral imaging applications is presented. We focus on detection algorithms derived using established statistical techniques and whose performance is predictable under reasonable assumptions about hyperspectral imaging data. The emphasis on a signal processing perspective enables us to better understand the strengths and limitations of each algorithm, avoid unrealistic performance expectations, and apply an algorithm properly and sensibly.
111 citations
TL;DR: A color-coded binary fringe projection technique that solves the problem of correctly decoding the fringe patterns and the use of an adaptive threshold scheme enables the extraction of the 3-D information and texture of an object without being affected by the color of the object surface.
Abstract: Color coding has been used for 3-D shape measurement in many recently developed fringe projection techniques. Use of color allows for more information to be coded in the same number of patterns as compared to the black-and-white techniques. However, one major problem of using color is that the appearance of the color fringe patterns projected onto the object can be affected by the color of the object surface itself. Thus, correctly decoding the fringe patterns can be difficult and sometimes even impossible. We describe a color-coded binary fringe projection technique that solves this problem. The use of an adaptive threshold scheme enables the extraction of the 3-D information and texture of an object without being affected by the color of the object surface. The development of a color gray-code concept, which is an extension of the gray-code technique, further reduces decoding errors. In addition, this technique can be used to measure objects with discontinuous features. The system has small digitizing errors and its measurement accuracy is hardly affected by system noise and nonlinearity errors. The system setup, color pattern design, shape reconstruction, and experimental results are presented
103 citations
TL;DR: In this paper, a 1550-nm free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors.
Abstract: A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.
97 citations
TL;DR: Numerical experimental results show the algorithm has a fast response for finding the best-in-focus image among the captured images, compared with related autofocus techniques pre- sented in the past.
Abstract: We present a new algorithm to determine, quickly and accu- rately, the best-in-focus image of biological particles. The algorithm is based on a one-dimensional Fourier transform and on the Pearson cor- relation for automated microscopes along the Z axis. We captured a set of several images at different Z distances from a biological sample. The algorithm uses the Fourier transform to obtain and extract the image frequency content of a vector pattern previously specified to be sought in each captured image; comparing these frequency vectors with the fre- quency vector of a reference image (usually the first image that we cap- ture or the most out-of-focus image), we find the best-in-focus image via the Pearson correlation. Numerical experimental results show the algo- rithm has a fast response for finding the best-in-focus image among the captured images, compared with related autofocus techniques pre- sented in the past. The algorithm can be implemented in real-time sys- tems with fast response, accuracy, and robustness; it can be used to get focused images in bright and dark fields; and it offers the prospect of being extended to include fusion techniques to construct multifocus final images. © 2005 Society of Photo-Optical Instrumentation Engineers.
TL;DR: A novel iterative method of generating three-plane, phase-only computer-generated holograms using a numerical Fresnel propagation algorithm based on the iterative Fresnel ping-pong two-plane algorithm is presented.
Abstract: A novel iterative method of generating three-plane, phase-only computer-generated holograms is presented. It is based on the iterative Fresnel ping-pong two-plane algorithm. A modification is introduced to extend the method for three planes, i.e., two object planes and a hologram itself. The described method enables the design of low-noise and high-efficiency phase-only holograms using a numerical Fresnel propagation algorithm. The source method is described, followed by the modified algorithm. Numerical simulation results and algorithm parameters are discussed, followed by a discussion of the method limitations.
TL;DR: A novel copyright protection scheme for digital images based on visual cryptography and statistics is proposed that can register multiple secret images without altering the host image and can identify the rightful ownership without resorting to the original image.
Abstract: A novel copyright protection scheme for digital images based on visual cryptography and statistics is proposed. The proposed method employs sampling distribution of means and visual cryptography to achieve the requirements of robustness and security. Our method can register multiple secret images without altering the host image and can identify the rightful ownership without resorting to the original image. Moreover, the proposed method enables the secret images to be of any size regardless of the size of the host image. Finally, experimental re- sults show that the proposed scheme can resist several common attacks. © 2005 Society of Photo-Optical Instrumentation Engineers.
TL;DR: A novel structured light method, namely a trap- ezoidal phase-shifting method, for 3-D shape measurement, which can be done in real time at a frame rate of 40 fps and a resolution of 532500 points, and which is significantly less sensitive to the defocusing effect of the captured images.
Abstract: We propose a novel structured light method, namely a trap- ezoidal phase-shifting method, for 3-D shape measurement. This method uses three patterns coded with phase-shifted, trapezoidal- shaped gray levels. The 3-D information of the object is extracted by direct calculation of an intensity ratio. Compared to traditional intensity- ratio-based methods, the vertical or depth resolution is six times better. Also, this new method is significantly less sensitive to the defocusing effect of the captured images, which makes large-depth 3-D shape mea- surement possible. If compared to sinusoidal phase-shifting methods, the resolution is similar, but the data processing speed is at least 4.5 times faster. The feasibility of this method is demonstrated in a previ- ously developed real-time 3-D shape measurement system. The recon- structed 3-D results show similar quality to those obtained by the sinu- soidal phase-shifting method. However, since the data processing speed is much faster 4.6 ms per frame, both image acquisition and 3-D re- construction can be done in real time at a frame rate of 40 fps and a resolution of 532500 points. This real-time capability allows us to mea- sure dynamically changing objects, such as human faces. The potential applications of this new method include industrial inspection, reverse engineering, robotic vision, computer graphics, medical diagnosis, etc. © 2005 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.2147311 Subject terms: 3-D shape measurement; structured light; trapezoidal phase shifting; intensity ratio.
TL;DR: In this paper, a spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and is being constructed at the National Institute of Standards and Technology.
Abstract: A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and is being constructed at the National Institute of Standards and Technology. The source is designed to have a capability of producing any spectral distribution, mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as a photometric and colorimetric standard. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of display colorimeters. The results indicate that the errors can be reduced by an order of magnitude when the tunable source is used to calibrate the colorimeters, compared with measurement errors when the colorimeters are calibrated against Illuminant A. The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.
TL;DR: In this paper, a 17-month aerosol column absorption monitoring experiment conducted in Greenbelt, Maryland, where the imaginary part of effective refractive index k was inferred from the measurements of direct and diffuse atmospheric transmittances by a UV-multifilter rotating shadow-band radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network).
Abstract: Compared to the visible spectral region, very little is known about aerosol absorption in the UV. Without such information it is impos- sible to quantify the causes of the observed discrepancy between mod- eled and measured UV irradiances and photolysis rates. We report re- sults of a 17-month aerosol column absorption monitoring experiment conducted in Greenbelt, Maryland, where the imaginary part of effective refractive index k was inferred from the measurements of direct and diffuse atmospheric transmittances by a UV-multifilter rotating shadow- band radiometer (UV-MFRSR, U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network). Colocated ancillary measure- ments of aerosol effective particle size distribution and refractive index in the visible wavelengths (by CIMEL sun-sky radiometers, National Aero- nautics and Space Administration (NASA) Aerosol Robotic Network (AERONET)), column ozone, surface pressure, and albedo constrain the forward radiative transfer model input, so that a unique solution for k is obtained independently in each UV-MFRSR spectral channel. Inferred values of k are systematically larger in the UV than in the visible wave- lengths. The inferred k values enable calculation of the single scattering albedo v, which is compared with AERONET inversions in the visible
TL;DR: In this article, a doubled Ti:sapphire femtosecond laser is used to ablate passive microfluidic devices (channels, reservoirs, through-holes) in polymers [polymethyl methacrylate (PMMA), polyimide (PI, Kapton), and glass (Pyrex).
Abstract: We present some applications of femtosecond laser ablation for microfluidics. A doubled Ti:sapphire femtosecond laser (=400 nm; pulse width, 90 fs; pulse energy up to 350 µJ; pulse repetition rate, 1 kHz) is used to microstructure passive microfluidic devices (channels, reservoirs, through-holes) in polymers [polymethyl methacrylate (PMMA), polyimide (PI, Kapton)] and glass (Pyrex). These materials are selected because of their extended use in the fabrication of microfluidic chips. In all cases, channels of some tenths of micrometers are obtained with a good-quality finishing for fluid transport. In the same sense, reservoirs and holes are produced. These latter elements are fabricated in larger dimensions to combine them with channels in some presented prototypes. The well-known feature of ultrashort pulses is that no edge effects are observed because the absence of thermal effects enables a good sealing. For PMMA, polymer bonding technologies are used. For Pyrex, the well-known silicon sealing by anodic bonding is chosen. In both cases, the fabricated prototypes work properly with a good flow behavior and no leakage is observed.
TL;DR: An objective measurement framework for signal-level image fusion performance, based on a direct comparison of visual information in the fused and input images, is proposed to model and predict subjective fusion performance results otherwise obtained through extremely time- and resource-consuming perceptual evaluation.
Abstract: An objective measurement framework for signal-level image fusion performance, based on a direct comparison of visual information in the fused and input images, is proposed. The aim is to model and predict subjective fusion performance results otherwise obtained through extremely time- and resource-consuming perceptual evaluation proce- dures. The measure associates visual information with edge, or gradient, information that is initially parametrized at all locations of the inputs and the fused image. A perceptual-information preservation model is then used to quantify the success of information fusion as the accuracy with which local gradient information is transferred from the inputs to the fused image. By considering the perceptual importance of different im- age regions, such local fusion success estimates are integrated into a single, numerical fusion performance score between 0 total information loss and 1 ideal fusion. The proposed metric is optimized and vali- dated using extensive subjective test results and validation procedures. The results clearly indicate that the proposed metric is perceptually meaningful in that it corresponds well with the results of perceptual fu- sion evaluation. Finally, an application of the proposed evaluation ap- proach to fusion algorithm selection and fusion parameter optimization demonstrates its general usefulness. © 2005 Society of Photo-Optical Instrumen-
TL;DR: In this article, a fiber drawing process with a single pulse of a 248-nm excimer laser applied during the fiber drawing of single mode fibers was used to construct DTG arrays with a reflectivity of up to 40% and type II DTGs with reflectivity near 100%.
Abstract: Fiber Bragg gratings (FBG) were manufactured during the fiber drawing process [draw tower grating (DTG)] with excellent reflectivity values. This was done in the region of 1550 nm by single pulses of a 248-nm excimer laser applied during the fiber drawing process of single mode fibers. An improved setup for the writing process and special photosensitive fibers enable the manufacture of type I DTG arrays with a reflectivity of up to 40% and type II DTGs with a reflectivity near 100%. Details of the setup and results of the DTG arrays and DTGs of type II are reported.
TL;DR: In this paper, the point spread functions and modulation transfer functions of axicons and LSOEs are analyzed experimentally and numerically in detail, and the obtained results correspond to different defocusing parameters.
Abstract: We present the abilities of diffractive elements for imaging with extended depth of focus. The elements of interest belong to the class of diffractive structures focusing incident light into a segment of the optical axis. We describe the imaging properties of the two following elements of this kind: the annular axicon and the light sword optical element (LSOE). In particular, the point spread functions and the modulation transfer functions of axicons and LSOEs are analyzed experimentally and numerically in detail. The obtained results correspond to different defocusing parameters. The performed experiments confirm the usefulness of axicons and LSOEs for imaging with extended depth of focus.
TL;DR: Experimental results demonstrate that the proposed adjustable IHS-BT approach with spectral adjust- ment approaches can achieve significant improvement over other approaches, and a simple procedure of dynamic range adjustment DRA is presented.
Abstract: Among various image fusion methods, intensity-hue- saturation IHS and Brovey transforms BT can quickly merge huge amounts of IKONOS/QuickBird imagery. However, spectral degradation often appears in the fused images. Moreover, IHS and BT suffer from individual color distortion on saturation compression and saturation stretching, respectively. To balance these two saturation changes during the fusion process, an adjustable IHS-BT approach with spectral adjust- ment is proposed. Furthermore, to solve the typical bright target recovery BTR problems, a simple procedure of dynamic range adjustment DRA is also presented. By adopting different DRA techniques, the pro- posed IHS-BT method is divided into two different fusion approaches: the model of preserving spectral information and the model of enhancing spatial details. Experimental results demonstrate that the proposed com- bined approaches can achieve significant improvement over other cur- rent approaches. © 2005 Society of Photo-Optical Instrumentation Engineers.
TL;DR: Fast as discussed by the authors is a simulation tool that computes downward surface UV doses, UV indices, and irradiances in the spectral range 290 to 400 nm with a resolution as small as 0.05 nm.
Abstract: FastRT is a fast, yet accurate, UV simulation tool that computes downward surface UV doses, UV indices, and irradiances in the spectral range 290 to 400 nm with a resolution as small as 0.05 nm. It computes a full UV spectrum within a few milliseconds on a standard PC, and enables the user to convolve the spectrum with user-defined and built-in spectral response functions including the International Commission on Illumination (CIE) erythemal response function used for UV index calculations. The program accounts for the main radiative input parameters, i.e., instrumental characteristics, solar zenith angle, ozone column, aerosol loading, clouds, surface albedo, and surface altitude. FastRT is based on look-up tables of carefully selected entries of atmospheric transmittances and spherical albedos, and exploits the smoothness of these quantities with respect to atmospheric, surface, geometrical, and spectral parameters. An interactive site, http://nadir.nilu.no/~olaeng/fastrt/fastrt.html, enables the public to run the FastRT program with most input options. This page also contains updated information about FastRT and links to freely downloadable source codes and binaries.
TL;DR: In this paper, the authors reported experiments on the ablation of arsenic trisulphide and silicon using high-repetition-rate (megahertz) trains of picosecond pulses.
Abstract: We report experiments on the ablation of arsenic trisulphide and silicon using high-repetition-rate (megahertz) trains of picosecond pulses. In the case of arsenic trisulphide, the average single pulse fluence at ablation threshold is found to be >100 times lower when pulses are delivered as a 76-MHz train compared with the case of a solitary pulse. For silicon, however, the threshold for a 4.1-MHz train equals the value for a solitary pulse. A model of irradiation by high-repetition-rate pulse trains demonstrates that for arsenic trisulphide energy accumulates in the target surface from several hundred successive pulses, lowering the ablation threshold and causing a change from the laser-solid to laser-plasma mode as the surface temperature increases.
TL;DR: A new method for image encryption and decryption in which the image is broken up into bit planes and each bit plane undergoes a jigsaw transform, which increases the robustness of the encryption system by several orders of magnitude.
Abstract: We propose a new method for image encryption and decryption in which the image is broken up into bit planes. Each bit plane undergoes a jigsaw transform. The transformed bit planes are combined together and then encrypted using random phase masks and fractional Fourier transforms. The different fractional parameters, the random phase codes, and the jigsaw transform index form the key to the encrypted data. This increases the robustness of the encryption system by several orders of magnitude. Different variations of the juxtaposition of the pieces of the image are also considered. These include the rotated version of the jigsaw pieces. The computational complexity of the bit-plane-based jigsaw algorithm is further improved using the third dimension (i.e., along different bit planes) for scrambling as well. The results of computer simulation are presented to verify the proposed idea and analyze the performance of the proposed techniques.
TL;DR: In this paper, the femtosecond laser pulses are used to produce submicrometer holes and periodic structures in metals, semiconductors, and dielectrics on arbitrarily shaped surfaces.
Abstract: Direct-write micro- and nanostructuring laser technologies are very important for the fabrication of new materials and multifunctional devices. Using tightly focused femtosecond laser pulses one can produce submicrometer holes and periodic structures in metals, semiconductors, and dielectrics on arbitrarily shaped surfaces. The achievable structure size is not restricted by the diffraction limit. It is determined by material properties and the laser pulse stability. We report investigations of possibilities to use femtosecond laser pulses for nanostructuring of different materials.
TL;DR: In this article, a new method of developing fiber temperature sensors using a fiber Bragg grating-loop ringdown scheme is introduced, where temperature measurements are converted to measuring time constants.
Abstract: A new method of developing fiber temperature sensors using a fiber Bragg grating-loop ringdown scheme is introduced. With this new technique, temperature measurements are converted to measuring time constants. Temperature sensing up to 593°C has been demonstrated using a proof-of-concept device. The sensor's stability, repeatability, sensitivity, and dynamic range are also explored.
TL;DR: In this article, the main characteristics of germanium photodetectors integrated in silicon-on-insulator optical waveguides for intrachip optical interconnects are presented.
Abstract: The main characteristics of germanium photodetectors integrated in silicon-on-insulator optical waveguides for intrachip optical interconnects are presented. The epitaxial Ge layers are grown on Si(001) by reduced-pressure chemical vapor deposition. The optical absorption of Ge layers is recorded from 1.2 to 1.7 µm and linked to the layer strain. The responsivity of an interdigitated metal-semiconductor-metal Ge photodetector has been measured. Light coupling from a slightly etched submicron rib silicon-on-insulator waveguide to a Ge photodetector is studied for two configurations: butt coupling and vertical coupling.
TL;DR: In this paper, a pointwise digital image correlation (DIC) technique was proposed to determine displacement fields with subpixel accuracy at each pixel location in a digital image of a deformed object using DIC.
Abstract: The theoretical foundation is developed for a new technique to determine displacements with subpixel accuracy at each pixel location in a digital image of a deformed object using digital image correlation (DIC). This technique, known as pointwise DIC, is evaluated using ideal sinusoidal images for the cases of rigid body translation, extensional strain, and rigid body rotation. Displacement fields obtained using objective correlation functions with and without intensity gradients are compared. Both bilinear and bicubic interpolation schemes are investigated for reconstructing the subpixel intensity and intensity gradient values in undeformed and deformed images. The effects of transforming the Eulerian description of the intensity gradients in the deformed images to the Lagrangian description in the undeformed images are also investigated. The optimal correlation value and the calculated displacement fields of the two interpolation schemes are compared. Theoretical results demonstrate that pointwise DIC can accurately determine displacement fields. To demonstrate the advantages of pointwise DIC over conventional DIC techniques, an ideal image simulating a twinning deformation is correlated, indicating the pointwise technique is up to two orders of magnitude more accurate at determining discontinuous displacements than the conventional technique. Experiments are also conducted on a polycarbonate dogbone specimen that validate pointwise DIC on real images and determine the inherent accuracy in the digital image acquisition.
TL;DR: A novel three-dimensional double directional filtering 3DDDF algorithm for detecting and tracking weak moving dim targets against a complex cluttered background in infrared IR image sequences is assessed and the performance comparisons of the proposed method and 3DDF are compared.
Abstract: We assess the performance of a novel three-dimensional double directional filtering 3DDDF algorithm for detecting and tracking weak moving dim targets against a complex cluttered background in infrared IR image sequences. This proposed method increases the tar- get energy accumulation ability further than the three-dimensional direc- tional filter 3DDF method. Prior to the filtering, a new prewhitening method termed a three-dimensional spatialtemporal adaptive prediction filter TDSTAPF is used to suppress the cluttered background. Exten- sive experiment results demonstrate the proposed algorithms' ability to detect weak dim point targets against a complex cloud-cluttered back- ground in real IR image sequence and the performance comparisons of the proposed method and 3DDF. © 2005 Society of Photo-Optical Instrumentation
TL;DR: In this article, an improved on-site calibration of a shadowband radiometer (UV-MFRSR) is presented, augmented by AERONET-CIMEL measurements of aerosol ex-tinction optical thickness interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals.
Abstract: Radiative transfer calculations of UV irradiance from total ozone mapping spectrometer (TOMS) satellite data are frequently over- estimated compared to ground-based measurements because of the presence of undetected absorbing aerosols in the planetary boundary layer. To reduce these uncertainties, an aerosol UV absorption closure experiment has been conducted at the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) site in Greenbelt, Maryland, using 17 months of data from a shadowband radi- ometer (UV-multifilter rotating shadowband radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network) colocated with a group of three sun-sky CIMEL radiometers (rotating reference instruments of the NASA Aerosol Robotic Network (AERONET)). We describe an improved UV-MFRSR on-site calibration method augmented by AERONET-CIMEL measurements of aerosol ex- tinction optical thickness (t a) interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals. The estimated t a is used as input to a UV-MFRSR spectral-band model, along with indepen- dent column ozone and surface pressure measurements, to estimate zero air mass voltages V0 in three longer wavelength UV-MFRSR chan- nels (325, 332, 368 nm). Daily mean ^V0&, estimates and standard de- viations are obtained for cloud-free conditions and compared with the on-site UV-MFRSR Langley plot calibration method. By repeating the calibrations on clear days, relatively good stability (62% in ^V0& )i s found in summer, with larger relative changes in fall-winter seasons. The changes include systematic day-to-day ^V0& decline for extended peri- ods along with step jump changes after major precipitation periods (rain or snow) that affected the diffuser transmission. When daily ^V0& values are used to calculate t a for individual 3-min UV-MFRSR measurements on the same days, the results compare well with interpolated AERONET t a measurements (at 368 nm most daily 1s root mean square (rms) differences were within 0.01). When intercalibrated against an AERO- NET sunphotometer, the UV-MFRSR is proven reliable to retrieve t a , and hence can be used to retrieve aerosol column absorption in the UV. The advantage of the shadowband technique is that the calibration ob- tained for direct-sun voltage can then be applied to diffuse-radiance volt- age to obtain total and diffuse atmospheric transmittances. These trans- mittances, in combination with accurate t a data, provide the basis for estimating aerosol column absorption at many locations of the USDA UV-B Monitoring and Research network and for correction of satellite estimations of surface UV irradiance. © 2005 Society of Photo-Optical Instru-