This PDF file contains the editorial “Editorial: Metamaterials and Photonic Crystals in Warsaw” for OE Vol. 44 Issue 11

An integrated and flexible fiber optic endoscope system for diagnostic investigations in medical cavities using specially designed probe distal and proximal ends so as to facilitate simultaneous speckle correlation analysis and fluorescence spectroscopic imaging is presented.

We report an alternative technique to interrogate a long-period fiber grating (LPG) when the grating sensitivity is based on the peak amplitude changes of the resonant wavelength. To read the amplitude changes, a conventional optical domain reflectometer was used. Bend measurements were performed to apply such method and to determine the grating sensitivity for this physical parameter. Reflective measurements, temperature insensitive, and the possibility of multiplexing LPG sensors are some advantages offered by this technique.

A method for preprocessing spectral information produced by chromotomographic imagers combined with a new postprocessing strategy is proposed. The new technique makes it possible to collect both spectral and limited spatial information from a chromotomographic sensor at kilohertz rates with a conventional charge-coupled device (CCD) array. The proposed technique involves forming a vector projection of each 2-D frame collected by the sensor. The vector projections are postprocessed via a reconstruction algorithm that produces 1-D profiles of the scene and a spectrum for each 1-D pixel. This on-chip preprocessing approach results in a large reduction of the bandwidth through the analog to digital converter, while maintaining the desired temporal resolution of the sensor. High-speed CMOS cameras can obtain comparable speeds, but at significant added cost in hardware and postprocessing time. This makes the proposed sensor attractive for applications where large numbers of sensors are required and the need to obtain spectral information quickly is of paramount importance.

This PDF file contains the editorial “Editorial: Solid State Lighting” for OE Vol. 44 Issue 11

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 R_a but also the special color rendering indices R_i, as well as on the CIELAB color difference ΔE^*_ab 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.

In this research, experiments and optical simulations are carried out to study the effect of beveled sidewalls and geometric shapes on the light extraction efficiency of a GaN light-emitting diode (LED) with sapphire substrate. In addition to the conventional rectangular chips, hexagonal chips are experimentally processed for the first time on a novel island-like GaN substrate, on which the beveled sidewalls are naturally formed at each island during GaN epitaxial growth on a sapphire original substrate by hydride vapor phase epitaxy (HVPE) technology. The results from our simulations and experiments show that the output power of a LED with beveled sidewalls is about two times that of a normal LED, and those from hexagonal chips are always better than those from conventional rectangular chips.

Light-emitting diodes (LEDs) are a strong candidate for the next-generation general illumination applications. LEDs are making great strides in brightness performance and reliability; however, the barrier to widespread use in general illumination still remains the cost (dollars per lumen). LED packaging designers are pushing the LED performance to its limits. This is resulting in increased drive currents and thus the need for lower-thermal-resistance packaging. The efficiency and reliability of solid-state lighting devices strongly depends on successful thermal management, because the junction temperature of the chip is the prime driver for effective operation. As the power density continues to increase, the integrity of the package electrical and thermal interconnects becomes extremely important. Experimental results with high-brightness LED packages show that chip attachment defects can cause significant thermal gradients across the LED chips, leading to premature failures. Perfect chip and interconnect structures for highly conductive substrates showed only a 2 K temperature variation over a chip area of approximately 1 mm², while defective chips experienced greater than 40 K temperature variations over an identical area. A further numerical study was also carried out with parametric finite-element models to understand the temperature profile variation of the chip active layer due to the bump defects. Finite-element models were utilized to evaluate the effects of hot spots in the chip active layer. The importance of zero defects in one of the more popular interconnect schemes--the epi-down soldered flip-chip configuration--is investigated and demonstrated.

The authors propose the use of holograms to modify both sides of the substrate of an organic light-emitting diode (OLED) to increase its light output efficiency and to discuss the associated angular, spectral, and near-field effects. A single-source macroscopic phase hologram experiment is used to illustrate the idea. Since the emitted light originates from dynamically varying positions with different directions and has wavelengths longer than the nanometer layer thickness of an OLED, the angular, wavelength, and near-field effects have been analyzed. Diffraction efficiency due to the dynamic density and position variations of the excitons has been discussed. The subwavelength near-field wave propagation effect is described with finite-difference time domain solutions to Maxwell's equations. These effects are not limited to the holographic method, but should exist in other substrate-modification methods proposed for increasing the OLED's external light efficiency.

The current white LED color has a wide range of correlated color temperatures and varying distances to the Planckian locus, which cause different tints in the white color. This color variation keeps a number of illumination and indication applications away from LEDs. This article introduces a passive method, adding correction filters, to correct the color and achieve visual equivalence of LED white light. The efficiency of using correction filters is discussed.

In commercial high-brightness phosphor-coated white LED packages the phosphor is put down on the die at the center of the hemispherical encapsulation, representing a quasi point source that provides convenient optical control in luminaire design. However, specific applications may benefit from other package geometries and beam shapes regarding efficiency, color uniformity and thermal management. In order to examine optical arrangements, a solid model of an InGaN LED die and the optical system including the phosphor were simulated using a Monte Carlo forward ray tracing technique. Photoluminescence was implemented as two separate processes: short-wavelength LED emission and phosphor absorption was traced first, followed by reemission of the down converted radiation by the phosphor layer; optical properties of existing phosphors were used. Output parameters of the two ray traces were combined and evaluated for the geometries examined.

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.

The spectral properties of commercially available halogen and LED-based MR16s were investigated. The metrics used to characterize the MR16s include the total luminous flux (TLF), correlated color temperature (CCT), color-rendering index (CRI), angular variation of CCT, and luminous efficacy. The halogen MR16s were included as a baseline for comparison with LED-based MR16s. Seven LED-based MR16s were investigated, including three constructed from 5-mm LEDs, and four constructed from high-power devices based on a larger die. It is shown that MR16s constructed from white LEDs are not suitable as a direct replacement for existing alternatives at this time, due to their low power efficiency and poor TLF. MR16s constructed with a single phosphor also have poor CRIs and undesirable CCTs. An MR16 with an extra phosphor in the red region of the spectrum shows improvement in CRI and a lower CCT than the other LED-based MR16s. All of the LED-based MR16s had a variation of CCT that was larger than those of the halogen devices.

Although lighting systems powered by photovoltaic (PV) cells have existed for many years, they are not widely used, especially in lighting for buildings, due to their high initial cost and low conversion efficiency. One of the technical challenges facing PV-powered lighting systems has been how to use the dc power generated by the PV module to energize common light sources that are designed to operate efficiently under ac power. Usually, the efficacy of dc light sources is very poor compared to ac light sources. Rapid developments in LED lighting systems have made this technology a potential candidate for PV-powered lighting systems. This study analyzed the efficiency of each component of PV-powered lighting systems to identify optimum system configurations for different applications.

A novel passive autofocus searching algorithm that takes account of human visual system limitations is presented. Comparing to the existing algorithms, the proposed method offers better overall performance in terms of converging speed, accuracy, and anti-backlash ability.

An optical method for shape and deformation measurement of a rotating blade, which is based on Fourier transform profilometry (FTP) and the stroboscopic effect, is proposed and has been verified by experiments. The integrated principles of the method, the process of 3-D phase unwrapping, and the detailed design of the synchronization control unit are discussed. Experimental results on a commercial electric fan prove that FTP can reconstruct the shape of a rotating blade efficiently and reveal its deformation different times clearly by using stroboscopic structured illumination. This method can be used for studying high-speed motion with rapid periodic signals.

The latest time-frequency tool, wavelet analysis, is presented to extract the phase information φ(x,y) from a single spatial carrier fringe pattern. First, the fringe pattern is generated and projected onto an object. The spatial carrier fringe pattern, carrying the 3-D shape of the object, is then captured using a CCD camera. Afterward, the spatial carrier fringe pattern is modulated by two high-frequency waves, which makes the spectrum of the carried phase relatively lower among the spectra of two modulated patterns. Following this, the spectrum of the object is separated into the approximation subband using a wavelet transform. The phase map of the 3-D shape of the object is then extracted from the mixed spectra by reconstructing only the approximation of their wavelet transforms. Finally, the 3-D shape of the object can be calculated from the phase map.

The automation of a rapid and simple trapping-force calibration system is desired for optical tweezers to measure biological forces. One simple calibration method, the water-dragging-force method, is to calibrate the trapping force against a given water dragging force with an image-processing technique. However, the conventional image-processing technique is too slow because of the time it takes for image recording, transferring, storing, and retrieving. The pattern recognition technique of our automated calibration method is rapid and simple, because it directly processes the image signal without recording, storing, and retrieving any image. In an experiment, we have combined the dragging force method with the real-time pattern recognition technique to calibrate the relationship between the trapping force at a given laser power and a displacement of a bead. We demonstrate the calibration results of the trapping forces on two target beads 10 and 6.2 µm in diameter, separately, at two laser powers, 5 and 10 mW. Each calibration procedure is finished in 5 min at a pattern recognition rate of 10 Hz with a spatial resolution of 75 nm. We believe that this technique is reliable and rapid enough to be applied to biological force measurement.

Off-axis negative-branch and off-axis positive-branch hybrid resonators, suitable for a laser with large medium cross section and a small output coupling, are explored numerically. The basis of our theory is the Fresnel-Kirchhoff integral equation, and the calculations describe a passive resonator. With respect to mirror tilting, the calculations confirm that in the unstable direction, the off-axis negative-branch resonator is less sensitive to mirror misalignments than the off-axis positive-branch one. In the stable direction, the sensitivity to mirror misalignment is larger than in the unstable direction and is dependent on the mirror curvature. The fabrication of cylindrical mirrors for such hybrid resonators is difficult, and deviations in mirror radius of curvature are possible. The effects of such deviations are addressed. Near the nominal mirror parameters, the off-axis negative-branch unstable resonator is insensitive to mirror curvature variations if the resonator length is appropriately adapted to the mirror curvatures.

Theoretical analyses of stimulated Raman scattering (SRS) in high-power ytterbium-doped double-clad fiber lasers and power amplifiers with bidirectional end pump and distributed pump are comprehensively presented. The Raman effects are quantitatively compared in these schemes with different fiber lengths, core diameters, signal wavelengths, pump absorption, facet reflections, etc., and the related restrictions are also illustrated. Fiber power amplifiers, characteristically with low facet reflections, have the advantage of significantly higher SRS thresholds than fiber laser resonators, which makes master oscillator power amplifiers better candidates for kilowatt single-transverse-mode fiber laser sources.

A model based on rate equations that consider the spatial distribution of photon density and inversion population density, as well as the turnoff time of an acoustic-optical modulator, is developed to perform accurate predictions of the characteristics of a green laser. Numerical solutions are consistent with the experimental results of a diode-pumped acousto optical Q-switched intracavity-frequency-doubled Nd:GdVO₄/KTP green laser.

With a 10-km single-mode fiber and a pair of fiber Bragg gratings (FBGs), we design four different linear cavity structures for supercontinuum (SC) generation using a cw Raman fiber laser (RFL) as pump. The output characteristics of the generated SC in these different constructs are compared when the RFL reaches its maximum output power. The broadest SC spectrum of 100 nm and the highest output power of 1.1 W are obtained when the nested cavity structure is employed. The influences of the feedback FBGs and cavity constructs on the SC generation are analyzed by considering the Raman effect and four-wave mixing.

We demonstrate a simple technique for the interrogation of fiber Bragg grating (FBG) displacement/bending sensors using a self-seeded Fabry-Perot laser diode (FP-LD). The sensing element consists of two almost identical FBGs mounted on the surface of a cantilever beam, which results in a reflection spectrum containing two close peaks with a separation depending on the amount of bending of the beam. The sensing element is made to select a particular mode of a gain-switched FP-LD, so that stable short pulses at the two wavelengths defined by the reflection peaks of the FBGs are generated simultaneously when the modulating frequency of the FP-LD is set properly. The wavelength separation of the dual-wavelength pulses is converted into a time difference by transmitting the pulses through a dispersive fiber, so that the wavelength measurement is replaced by the less expensive time measurement. To multiplex similar sensors that are sufficiently far apart spectrally, it is only necessary to change the modulating frequency of the FP-LD. This technique provides an intensity-referencing-free demodulation scheme, an easy way to interrogate a series of sensors, and an effective means to compensate for drifts due to temperature changes.

We present details of numerical techniques developed to compensate the effects of hysteresis experienced by a hybrid piezoelectric fiber optic voltage sensor. The techniques, implemented using a real-time signal processing system, are tested and their effectiveness evaluated experimentally. The best of the proposed algorithms provides phase error compensation from approximately 7 to nearly 0 deg, and allows us to perform sensor calibration to achieve accuracy better than 0.5% (full scale output).

We demonstrate an efficient prism coupling method to couple nonplanar infrared waves into a 2-D planar photonic-crystal (PhC) device, such as a PhC waveguide (PhCW). A coupling efficiency of 31.1% is obtained experimentally. Results from finite-difference time-domain (FDTD) modeling are shown to closely match the results measured in a fabricated sample.

We demonstrate nondefect mid-infrared waveguides based on the self-collimation effect in photonic crystals (PhCs). Due to the spatial dispersion properties serving to confine and route light, propagation loss as low as 2.56 dB/mm is obtained in a silicon-on-insulator (SOI)-based waveguide. The efficiency, together with their ability of arbitrary and structureless routing of electromagnetic beams, of these self-collimation devices provide a potential candidate for miniaturizing devices.

We experimentally investigate an optically gain-clamped (GC) L-band (1570 to 1600 nm) erbium-doped fiber amplifier (EDFA). Two lasing wavelengths in the optical feedback loop, 1568 and 1600 nm, are separately used for gain clamping with different loop attenuations. The characteristics of the EDFA with single-channel and simulated multichannel operations are examined and compared. We find that the 1568-nm lasing light is a better selection for an L-band GC EDFA to offer good channel gain with satisfactory noise figure characteristics.

The performance of a spectral-phase-encoded (SPE) optical code-division multiple-access (OCDMA) system is analyzed. Regarding the incorrectly decoded signal (IDS) as a nonstationary random process, we derive a novel probability distribution for it. The probability distribution of the IDS is considered a chi-squared distribution with degrees of freedom r=1, which is more reasonable and accurate than in previous work. The bit error rate (BER) of an SPE OCDMA system under multiple-access interference is evaluated. Numerical results show that the system can sustain very low BER even when there are multiple simultaneous users, and as the code length becomes longer or the initial pulse becomes shorter, the system performs better.

A wavelength interleaver structure with asymmetrical output spectra, an important yet seldom investigated device, is presented. The interleaver has become an important fiber optics communication component. Numerous interleaver designs have been proposed and studied. However, most interleavers are designed for symmetrical spectrum splicing, so they cannot be very useful in applications where only a few channels among multiples need to be added or dropped. As a result, the design capability of network engineers is severely limited by the nature of the symmetrical interleaver. In this work, an asymmetrical interleaver structure based on the modified Michelson interferometer is proposed and investigated. The asymmetrical interleaver implemented with the lattice filter approach is also investigated for comparison. The proposed structure has better performance compared to other asymmetrical interleaver structures such as the lattice filter approach, and it also has advantages of compact size and low material costs. With an asymmetrical interleaver, network engineers could enjoy great design flexibility they would not possess otherwise.

Some N-bit programmable photonic time delay structures are described for use in the compensation and the emulation of polarization-mode dispersion. Our key idea is to have one switchable differential group delay (DGD) module, which specifically contains the zero DGD and the minimum DGD resolution step of τ, connected to the rest of the variable photonic DGD bit modules that offer DGD values from +2τ to +2^N–2τ. Our proposed variable photonic DGD architectures use 2×2 polarization-maintaining optical switches or 90-deg polarization rotators to choose the desired DGDs. When a set of all variable photonic DGD bit modules is attached to the polarization rotator, 2^N DGD values ranging from –(2^N–1–1)τ to (2^N–1)τ are obtained. Our simulation study points out that compact programmable photonic DGD modules can be achieved by using materials with strong birefringence and our reflective configuration. In addition, materials with low wavelength sensitivity and weak birefringence are preferred for use in broadband wavelength and low-polarization-dependence systems, respectively. Mirror-based programmable photonic DGD structures are also introduced.

An extensive investigation has been carried out, by computer simulation, to evaluate the impact of self phase modulation (SPM) on residual dispersion for a wavelength-division multiplexed (WDM) system at a bit rate of 10 Gbit/s. Degradation of the eye opening of the transmitted pulses at the output of the transmission fiber due to interplay of the SPM and the group-velocity dispersion effects is investigated for post- and bi-end compensation configurations, including residual dispersion. Hence the eye-opening penalty and the threshold power limit at 3-dB eye-opening penalty are determined for both the configurations. It is found that the bi-end compensation configuration offers the best performance in respect of the SPM effect and that for a WDM system positive residual dispersion should be used.

We experimentally find that, for a communication system with many polarization dependent loss (PDL) and polarization mode dispersion (PMD) components, the total PDL and the product of the maximum and minimum transmission coefficients (T_maxT_min) are wavelength dependent, even though they are treated as wavelength independent in most of the simulations. At a given wavelength, however, the PDL and PMD elements are concatenated, and the total T_maxT_min of the system is equal to the products of the T_maxT_min of each comprising element.

The application of a position-sensing detector to optical logic operations has been introduced and experimentally demonstrated for the first time. We describe some possible architectures for all 16 two-input Boolean gates as well as for multi-input gates.

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 presented. For the first time, an exact solution of the depth of gating is proposed for a rectangular laser pulse.

Range-gated imaging can improve the signal to backscattering noise ratio (SBR) in turbid media. This is achieved by synchronizing a short duration, high intensity pulse with precise camera gating. It is well known that shorter pulse length and shorter camera gate duration can enhance the SBR. However, there is no analytical model of the backscattering noise (as a function of the pulse length and gate timing) that can be used to minimize backscattering noise within the camera-captured signal. We propose a formulation (a modification of Falk's lidar equation) that models the backscattering noise as a convolution with a fixed upper limit. This formulation predicts a variation of backscattering noise within the returning signal. In particular, the model predicts higher SBR toward the tail region of the target-reflected irradiance. It confirms the experimental results reported by other authors. Additionally, the model explains experimentally observed SBR improvement for shorter pulses and shorter gating intervals (if adequately positioned within the returning pulse).

Atmospheric turbulence effects greatly reduce the resolution that can be obtained by systems that must form images through the atmosphere. Postdetection image reconstruction techniques, such as speckle imaging, and deconvolution from wavefront sensing provide a means of overcoming some of these effects by postprocessing sets of short-exposure image measurements. Previous work has shown that using image quality metrics to select the best subset of an ensemble of measured images to process can yield better results than processing all the measured data. In this paper we extend this idea to select a subset of frames using metrics derived from wavefront sensor (WFS) measurements made simultaneously with the image measurements. This approach to using WFS data may allow the amount of data that is saved to be reduced, or automate the process of sifting the data for the best subsets to process. Our results indicate that the WFS-based metrics are consistent with the image-quality-based metrics.

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 adjustment 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 proposed 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 combined approaches can achieve significant improvement over other current approaches.

The linear matched filter is widely used in hyperspectral target detection applications. Its detection performance is typically evaluated based on Gaussian (or normal) probability density models. However, it is well known that hyperspectral backgrounds exhibit "long-tail" behavior, which cannot be accurately modeled by normal distributions. In this work, we model the distribution of hyperspectral backgrounds using a more accurate model based on elliptically contoured multivariate t distributions. For the target class, we still use a normal distribution model, because the body of the distribution is important for detection applications. We show that the tails of the background distribution deteriorate the performance of the matched filter. In this sense, performance predictions based exclusively on normal models, which ignore the long tails, can be inaccurate and overly optimistic. The proposed technique, which allows for an easy control of the tail's heaviness, can be a very useful tool for evaluating target detection performance for backgrounds with both light and heavy tails.

An extension of the 2-D discrete Teager-Kaiser energy operator and the 1-D higher-order energy operators to the 2-D continuous case is proposed. These 2-D continuous operators are flexible enough to apply a large class of image gradient filters, and consequently different discrete energy operators are derived. Particularly, the proposed model takes into account the diagonal directions, through the partial derivatives. The obtained operators are computationally very simple, like the classical 2D Teager-Kaiser operator, and are well suited for image-processing applications such as image demodulation or image contrast enhancement. Results of demodulation of synthetic and real images, to estimate envelope information, are presented to show the feasibility of the proposed operators.

Human visual systems are insensitive to slight saturation changes in a color image. This paper presents a novel watermarking approach by hiding grayscale logos in the saturation component of color images. The embedding procedures are first performed in the DWT domains of the achromatic component. Then a proposed technique of saturation adjustment is applied to redirect the marks into the saturation component. To improve the detection robustness, we propose an adaptive weighting scheme to fuse the extracted watermarks in the extraction stage. Experimental results demonstrate that the proposed approach can successfully make the watermark perceptually invisible and robust to image-processing operations such as low-pass filtering, JPEG, scaling and cropping under StirMark, and JPEG-2000. Robustness can be provided in the applications of copy protection and digital archives.

Fractal theory has been widely applied in the field of image compression due to its advantages of resolution independence, fast decoding, and high compression ratio. However, it has a fatal shortcoming of intolerant encoding time for every range block to find its corresponding best matched domain block. In this work, an algorithm is proposed to improve this time-consuming encoding drawback by an adaptive searching window, partial distortion elimination (PDE), and characteristic exclusion algorithms. The proposed methods efficiently decrease the encoding time. In addition, the compression ratio is also raised due to the reduced searching window. While conventional full search fractal encoding to compress a 512×512 image needs to search 247,009 domain blocks for every range block, our experimental results show that our proposed method only needs to search 122 domain blocks, which is only 0.04939% compared to a conventional fractal encoder for every range block to encode a Lena 512×512 8-bit gray image at a bit rate of 0.2706 bits per pixel (bpp) while maintaining almost the same decoded quality in visual evaluation. In addition, the visual decoded quality of the proposed method is better than the most widely used JPEG compressor.

A copyright protection scheme based on chaos and secret sharing techniques is proposed. Instead of modifying the original image to embed a watermark in it, the proposed scheme extracts a feature from the image first. Then, the extracted feature and the watermark are scrambled by a chaos technique. Finally, the secret sharing technique is used to construct a shadow image. The watermark can be retrieved by performing an XOR operation between the shadow images. The proposed scheme has the following advantages. Firstly, the watermark retrieval does not need the original image. Secondly, the scheme does not need to modify the original image for embedding the watermark. Thirdly, compared with several schemes, the scheme is secure and robust in resisting various attacks.

A new approach combining a discrete iterative reconstruction-reprojection algorithm (DIRR) with a finite impulse response (FIR) low-pass filter is proposed for reconstructing images comprising opaque objects in optical computed tomography (CT). This filter, employing a rectangular window function and various bandwidths, is adopted to smooth the reprojection data between iterative reconstruction and reprojection stages. Compared reconstruction results of the traditional iterative reconstruction-reprojection (IRR) algorithm, projection space iteration reconstruction-reprojection (PSIRR) algorithm, and the new DIRR algorithm for an asymmetrical four-peak and a single-peak test image, including a circle round opaque object, are studied. The results show that the reconstruction precision of the new algorithm is related to the value of the bandwidth of the finite impulse response (FIR) filter, and the optimal bandwidth increases when the space frequency of the reconstructed image ascends. Furthermore, the DIRR algorithm with an optimal bandwidth has an obviously higher reconstruction precision than IRR and PSIRR algorithms, and has a potential application of reconstructing images, including obstacle objects with limited views.

This PDF file contains the errata for “OE Vol. 44 Issue 11 Paper 2139099” for OE Vol. 44 Issue 11

This PDF file contains the errata for “OE Vol. 44 Issue 11 Paper 2159010” for OE Vol. 44 Issue 11